Tracking system and marking device to be tracked by tracking system

Abstract

A tracking system is provided for tracking a marking device for attachment to a medical device wherein the marking device comprises a sensing unit comprising a magnetic object that can be excited as a mechanical oscillation of the magnetic object by an external magnetic or electromagnetic excitation field, the tracking system comprises: a field generator for generating a predetermined magnetic excitation field or electromagnetic excitation field for causing a mechanical oscillation of the magnetic object; a transducer for converting a magnetic field or an electromagnetic field generated by the induced mechanical oscillation of the magnetic object into one or more electrical response signals; and a position determination unit for determining a position of the marking device based on the one or more electrical response signals.

Description

[0001] This application is a divisional application of patent application No. 201980099513.3 filed on February 18, 2022, entitled “Tracking System and Marking Device to be Tracked by Tracking System”. Technical Field

[0002] The present invention relates to a tracking system for a marking device attached to a medical device, a corresponding marking device, a corresponding medical device, a tracking method for tracking the marking device, and a tracking computer program. Background Technology

[0003] Invasive, and especially minimally invasive, medical procedures are commonly used tools for the proper assessment and / or management of intravascular conditions in patients.

[0004] Electromagnetic tracking is known for medical devices used in such procedures, especially in minimally invasive procedures. However, this electromagnetic tracking has the following drawbacks: in order to determine not only the location of the medical device but also its orientation, the medical device needs to be equipped with several electromagnetic markers, each of which is suitable for, for example, three-degree-of-freedom (DoF) or five-degree-of-freedom (DoF) positioning.

[0005] Furthermore, known electromagnetic tagging devices are typically much larger than 1 mm. For example, the electromagnetic tagging device used by a tracking system disclosed in the article "Validation of the Calypso Surface Beacon Transponder" by B. Maxwell et al., Journal of Applied Clinical Medical Physics, Vol. 17, pp. 223-234 (2016) has a size of 8 mm.

[0006] As a further problem, electromagnetic tagging devices are generally not read from relatively large distances, such as greater than 30 cm. For example, the system disclosed by B. Maxwell et al. in the article mentioned above allows the tagging device to be read from a distance of about 16 cm. Summary of the Invention

[0007] Therefore, the object of the present invention is to provide an improved tracking system and improved marking device for tracking marking devices, corresponding medical devices, tracking methods, and computer programs. More specifically, the object of the present invention is to provide a small marking device capable of accurately indicating the position of medical devices used during surgery in humans (particularly patients to whom minimally invasive procedures are performed). Another object of the present invention is to provide a tracking system capable of accurately tracking such marking devices.

[0008] According to a first aspect of the invention, a tracking system is provided for tracking a marking device attached to a medical device, and the tracking system is intended for use in surgical procedures. The marking device includes a sensing unit comprising a magnetic object providing a permanent magnetic moment, wherein the sensing unit is configured to convert an external magnetic or electromagnetic excitation field into mechanical oscillations of the magnetic object. The tracking system includes: a field generator for generating a predetermined magnetic or electromagnetic excitation field for inducing mechanical oscillations of the magnetic object; a transducer for converting the magnetic or electromagnetic field generated by the induced mechanical oscillations of the magnetic object into one or more electrical response signals; and a position determination unit for determining the position of the marking device based on the one or more electrical response signals.

[0009] Therefore, a tracking system is provided that can be used to track the position and / or orientation of a medical device using a corresponding marking device attached to the medical device. This tracking system can be used particularly for tracking the position and / or orientation of a medical device during surgery (and even more particularly during minimally invasive surgery) to allow for accurate determination of the position and / or orientation of the medical device using a relatively small marking device.

[0010] For this purpose, the tracking system uses a tagging device comprising a sensing unit of a magnetic object having a permanent magnetic moment. If the sensing unit of the tagging device is then subjected to a predetermined external magnetic or electromagnetic excitation field, the magnetic object begins to oscillate in response to the excitation field. The mechanical oscillation of the magnetic object generates a magnetic or electromagnetic (response) field, which is then converted into one or more electrical response signals by a corresponding transducer. These response signals are then used to derive the position of the tagging device. More specifically, the mechanical oscillation of the magnetic object can typically generate a position-dependent magnetic field change, which can be represented by the response signal and used by the position determination unit to determine the position of the tagging device, and thus the position of the medical device to which the tagging device is attached.

[0011] In this context, the term medical device can specifically refer to a device used in medical procedures. In some embodiments, a medical device can specifically correspond to a device used during surgery (particularly minimally invasive surgery). In some embodiments, a medical device can refer to an interventional tool used in interventional procedures performed on humans (particularly patients).

[0012] Generally, the location determination method using tracking systems and tagging devices as presented herein can be used to perform location determination / locating any medical device to which it is beneficial. Therefore, in some embodiments, the term "medical device" can also be used to locate any other medical device that may be useful. As an example, bandages or patches should be mentioned. For these cases, for safety reasons, such as after surgery, tracking the location and / or orientation of these types of bandages or patches may be important to ensure that all items have been properly placed or removed (if necessary).

[0013] The term "marking device" can be specifically used to refer to any device capable of indicating the position and / or orientation of an object to which the marking device is attached. Specifically, the term "marking device" can refer to a device that includes a magnetic sensing unit, i.e., a sensing unit of a magnetic object that responds to a magnetic or electromagnetic excitation field by performing corresponding mechanical oscillations (particularly rotational oscillations). The tracking system uses these mechanical oscillations to generate an electrical response signal for deriving the position (and orientation) of the marking device.

[0014] The term field generator can specifically refer to a generator of a magnetic or electromagnetic excitation field. In some embodiments, a field generator may include a magnetic field generating array comprising a plurality of generating units. In some embodiments, these generating units may specifically correspond to respective coils arranged in an array of coils. In some embodiments, each coil of the array may be independently controlled. In some embodiments, such independent control may be used to provide a non-uniform magnetic or electromagnetic excitation field, ideally with a constant field gradient within the field's workspace.

[0015] This concept is generally based on the fact that the response of a magnetic object to a magnetic or electromagnetic excitation field (measured by mechanical oscillation) can provide information about the position and / or orientation of a marking device comprising a sensing unit, which includes the magnetic object. This is because the magnetic or electromagnetic excitation field can affect the magnetic object differently depending on its relative position to the field.

[0016] Different possible methods can be used to determine the location of a tagging device and the medical device to which it is attached, i.e., to perform its localization, based on the response of a mechanical oscillator to a magnetic or electromagnetic excitation field. In this context, two specific location determination methods, also known as localization methods, can be used. One method would be to perform location determination based on the coil sensitivity of different coils in the coil array. This method is based on the fact that each coil in the field generator's coil array has a different spatial sensitivity distribution based on its position and orientation within the tracking system. S,i(r). In this case, the magnetic object of the sensing unit will respond to the characteristic mechanical oscillations of each coil, especially to the mechanical object relative to B. S,i The dynamic dipole moment of (r) A defined characteristic amplitude response.

[0017] Another approach would be based on gradient field encoding. This method leverages the fact that the frequency of the marking device can be manipulated to provide independent positional information. For this purpose, a non-uniform magnetic field with an ideal constant field gradient within the workspace can be generated, for example, by applying a low-frequency current to selected coils in a coil array. This non-uniform field can be achieved, for example, by providing the aforementioned independent control of the coils.

[0018] This additional field alters the recovery field B acting on the magnetic object of the sensing unit. rest This changes the frequency of the oscillation. Due to the non-uniform nature of the magnetic or electromagnetic field, the frequency change will depend on the position and orientation of the marking device.

[0019] The details of these positioning methods will be discussed further below. In some embodiments, one such method may be sufficient, while in other embodiments, a combination of two methods may be used to improve accuracy or to identify systematic errors (e.g., strong ferromagnets in the workspace) that may lead to conflicting results between the two methods.

[0020] In some embodiments, the position determination unit may be adapted to determine at least five degrees of freedom for the marking device relative to a coordinate system provided by the tracking system based on the one or more electrical response signals, the at least five degrees of freedom including the position of the marking device relative to the tracking device and at least two orientation angles.

[0021] In some embodiments, the tracking system may define or be provided with a coordinate system, and the tagging device may be positioned relative to said coordinate system. For this purpose, the position determination unit may be adapted to determine at least five degrees of freedom (DoF) of the tagging device based on the one or more response signals. These five degrees of freedom allow the position and orientation (in terms of two orientation angles) of the tagging device relative to the coordinate system of the tracking system to be determined. Thus, by means of this arrangement, it becomes possible to determine the position and orientation of the tagging device using only one tagging device, and thus determine the position and orientation of the medical device to which the tagging device is attached.

[0022] According to some embodiments, the tracking system can be adapted to determine the positions of a plurality of marking devices, each of which includes a corresponding sensing unit. The magnetic object of the corresponding sensing unit can oscillate, particularly rotatably, at different resonant frequencies to generate different magnetic or electromagnetic fields, which can be converted into one or more corresponding electrical response signals specific to the corresponding marking device. The position determination unit can then be provided for determining the position of one or more of the plurality of marking devices based on the corresponding one or more electrical response signals.

[0023] Preferably, the tracking system is adapted to determine the positions of a plurality of marking devices, wherein the magnetic objects of the plurality of marking devices can oscillate at different resonant frequencies, preferably rotatably, such that the sensing signals of different marking devices have different frequencies, wherein the position determination unit is adapted to determine the position of the marking devices based on the generated sensing signals having the different frequencies. The position determination unit is also preferably adapted to determine the orientation of the marking devices based on the generated sensing signals having different frequencies. By using different marking devices with different resonant frequencies, it is possible to distinguish between different marking devices, and to determine the corresponding position and preferably the corresponding orientation for each marking device.

[0024] The plurality of marking devices may be attached to a single medical device, wherein the position determination unit may be adapted to determine the shape and / or position and / or orientation of the medical device based on the determined positions of the plurality of marking devices. Furthermore, the position determination unit may be adapted to determine the shape and / or position and / or orientation of the medical device based on the orientation determined for the plurality of marking devices. While the marking devices may be specifically used to determine the shape and / or position and / or orientation of the medical device, it should be understood that the marking devices may also be used to determine the shape and / or position and / or orientation of other elements (e.g., body tissues) to which the plurality of marking devices are attached. In some embodiments, the plurality of marking devices may also be distributed between the medical device for treating tissue and the corresponding tissue to collect information about the two elements and / or the relationship between the two elements.

[0025] In an embodiment, the position determination unit is adapted to determine the position of a specific marker device relative to the position of another marker device. Furthermore, the orientation of the marker device can be determined relative to the orientation of the other marker device. However, the position and, optionally, the orientation can also be determined relative to another reference. For this purpose, the tracking system may also be provided with a corresponding output unit for outputting the determined position and / or orientation of the marker device.

[0026] In some embodiments, the tracking system position determination unit may be configured to compensate for the temperature dependence of the one or more electrical signals. In some embodiments, the position determination unit may be configured to apply a compensation algorithm to perform such compensation.

[0027] In a preferred embodiment, the tracking system, and particularly the position determination unit, can be configured to compensate for the temperature dependence of one or more electrical signals. For this purpose, the temperature-dependent behavior of the magnetic object, i.e., the temperature dependence of its resonant frequency, is preferably determined experimentally or through corresponding calculations.

[0028] In some embodiments, the tracking system may then be provided with a temperature sensor and / or an input device for inputting temperature. An algorithm may then be provided that takes into account the input temperature and correlates it with a known correlation to the resonant frequency of the magnetic object to compensate for temperature dependence. This allows the temperature effect to be removed from the electrical signal, resulting in a more accurate position determination / location method. That is, in some embodiments, temperature compensation may be performed by means of a compensation algorithm, i.e., implemented in program code.

[0029] Alternatively or additionally, temperature compensation can be achieved through different means, such as physical compensation elements. That is, in some embodiments, the marking device itself (and more specifically, the sensing unit) can be able to compensate for the temperature dependence of the resonant frequency of the mechanical oscillations of a magnetic object. For this purpose, the sensing unit may include a compensation element adapted to modify the resonant frequency in a first frequency direction, opposite to a second frequency direction, according to temperature changes. In the absence of the compensation element being part of the sensing unit, the resonant frequency of the sensing unit would be modified in the second frequency direction according to temperature changes. This arrangement allows for reduction or even elimination of temperature-induced shifts in the resonant frequency. Thus, the first frequency direction may specifically correspond to a direction toward higher or lower frequencies, and the opposite second frequency direction may correspond to a direction toward lower or higher frequencies.

[0030] Preferably, the compensation element comprises a magnetic material whose magnetization changes with temperature, thereby altering the resonant frequency. The magnetic material is selected and arranged within the sensing unit, particularly within the housing of the sensing unit, such that the direction of modification of the resonant frequency is in the direction of a first frequency. The compensating magnetic material is preferably arranged adjacent to the magnetic object and / or to other magnetic objects, as described below. This allows for the design of marking devices that significantly reduce or even eliminate unwanted temperature dependence in a technically relatively simple manner, without requiring excessive space within the housing.

[0031] In some embodiments, the position determination unit may be configured to apply a compensation algorithm to compensate for one or more of the following: a static background field and a dynamic background field.

[0032] In some embodiments, the position determination unit may further apply a compensation algorithm to compensate for static and / or dynamic background fields. The static background field is added to the field of the stationary magnetic object, and thus modulates the recovered field B seen by the oscillating magnetic object. rest Therefore, the resulting resonant frequency is altered, which could be a source of error in using the frequency variation of an oscillating magnetic object to perform position determination.

[0033] In some embodiments, compensation may be performed by a corresponding algorithm implemented in the tracking system, and particularly applied by the position determination unit. For this purpose, the tracking system may be provided with one or more absolute field sensors suitable for measuring the amplitude and orientation of the static background field. Based on the orientation of the marking device, frequency or field corrections can be calculated to achieve correct position and / or orientation values.

[0034] To sense a static background field, any magnetic field sensor with sufficient sensitivity and footprint that can be integrated into the tracking system can be used. As an example, a 3-axis Hall sensor should be mentioned. Alternatively or additionally, a 3-axis array of a temperature-compensated microrobot with a well-defined zero-field frequency can be used. The amplitude and orientation of the background field can be determined based on the changes to its corresponding frequency. Ideally, its resonant frequencies are chosen such that they do not interfere with the frequency of the sensing unit.

[0035] Instead of correcting for the frequency shift during evaluation, coils from a multi-coil tracking system can be used to generate a small offset field to balance the background field and / or even the Earth's magnetic field. If inhomogeneities exist in the field of view due to the presence of ferromagnetic materials, several sets of 3-axis magnetic field sensors can be used to characterize the spatial field variations. Based on the interpolated background field map derived from these measurements, corrections can be calculated for the sensing units at known locations and orientations, or a corresponding corrected offset field can be applied, or a combination of both correction methods can be used.

[0036] According to some embodiments, static and / or dynamic background field effects on the marking device side can also be mitigated. In this case, the sensing unit of the marking device can be designed, for example, to employ two suspended spheres with the same magnetic dipole moment and moment of inertia (or a suitable ratio of the two quantities). Since the reverse oscillation occurs at a single frequency, the first-order effects of the static bias field (such as the Earth's magnetic field) are eliminated.

[0037] In some embodiments, the position determination unit may be configured to apply a compensation algorithm to compensate for nonlinearity caused by different oscillation amplitudes of the mechanical oscillation.

[0038] The position determination unit can be configured to compensate for nonlinearities in the system that may be caused by varying amplitudes of mechanical oscillations from the magnetic object of the sensing unit. In some embodiments, this may specifically include additional optional data processing steps in which an inverse nonlinear filter is applied to reduce the nonlinearity of the tracking system. Thus, the nonlinearity of the tracking system is measured, and the effects of the filter being constructed to reverse the nonlinearity are calculated. This is particularly useful if low-cost components are used, as they tend to exhibit more nonlinear behavior.

[0039] Alternatively, nonlinear filtering can be used as the first processing step. If more than one signal is used, additional signal processing steps exist. If at least one receiving channel does not detect a response from the sensing unit of the tagging device and thus provides a measurement of the background signal, that signal (and all other such signals) is correlated with the received signal, and the correlated component is subtracted from the signal-carrying channel. This subtraction can be performed in the time domain, the frequency domain, or a mixture of both. If no channel exists that does not contain any sensor signal, a data processing strategy sometimes called a “virtual gradiometer” can be used. This decomposes multiple channels in a virtual channel into a linear combination of physical channels to minimize interference from response signals not generated by sensing. The factor of the linear combination can be found by correlating the signals of channels that do not include the signal bands of one or more sensing units.

[0040] In some embodiments, the field generator may include a magnetic field generating array comprising a plurality of generating units arranged in a predetermined spatial configuration. Here, the one or more electrical response signals may indicate characteristic mechanical oscillations of the magnetic object of the sensing unit caused by each of the plurality of generating units, wherein the position determination unit is adapted to determine the position of the marking device based at least in part on the one or more electrical response signals indicating the characteristic mechanical oscillations. In some embodiments, the position determination unit is adapted to determine the amplitude of the characteristic mechanical oscillation of the magnetic object of each of the plurality of generating units based on the one or more electrical response signals.

[0041] In some embodiments, the field generator may include a plurality of generating units arranged in space as a magnetic field generating array. In some embodiments, this spatial arrangement may be two-dimensional. However, a three-dimensional spatial arrangement is also conceivable. In some embodiments, the magnetic field generating array may correspond to a coil array, and the generating unit may correspond to one or more coils. In this case, position estimation / localization can be performed at least in part based on the coil sensitivity of individual coils in the coil array. This method will be described in further detail below.

[0042] In some embodiments, the tracking system may further include a control unit, and the field generator includes a magnetic field generating array or the magnetic field generating array comprising a plurality of generating units arranged in a predetermined spatial arrangement, wherein each of the plurality of generating units is adapted to be controlled by the control unit independently of the remaining generating units, the control unit being adapted to control at least some of the generating units such that at least one spatial excitation field component of the magnetic excitation field or electromagnetic excitation field can be modified by the control, wherein the position determination unit is adapted to determine the position of the marking device based at least in part on the one or more electrical response signals indicating the modification of the at least one spatial excitation field component. In some embodiments, the field generator is adapted to sequentially generate a set of different additional magnetic or electromagnetic coded fields that vary spatially and / or temporally, wherein the position determination unit is adapted to determine the position of the marking device based at least in part on the one or more electrical response signals converted by the transducer based on the magnetic or electromagnetic field, the magnetic or electromagnetic field being generated by mechanical oscillations of the induced magnetic object in response to each of the set of different additional magnetic or electromagnetic coded fields.

[0043] Positioning can also be performed based on gradient field encoding. When coil sensitivity positioning is based on the amplitude distribution picked up by the coil array, the frequency of the markers can be manipulated to provide independent position information. This can be done, for example, by applying a low-frequency current to selected coils of the coil array to generate a non-uniform magnetic field with an ideal constant field gradient within the workspace.

[0044] This additional field alters the recovery field B acting on the oscillating magnetic object. rest This changes the frequency. Due to the non-uniform nature of the field, the frequency variation will depend on the location and orientation of the marker. By sequentially applying several coding fields (e.g., field gradients applied at six different orientations), all three locations of the marker and two of the three orientation parameters can be determined. The remaining angles can be delayed based on the sensor's higher-order response to the external magnetic field, however, at the cost of higher field strengths required to generate sufficient higher-order contributions. The basic coding idea involves gradient coding in MRI; therefore, both frequency coding and phase coding can be performed.

[0045] For frequency coding, a non-uniform field is applied during signal readout to generate the desired frequency shift. For the desired spatial resolution, the applied coding field strength must be suitable for the frequency sensitivity of the marking device and the frequency resolution transmitted by the tracking system.

[0046] For phase coding, a non-uniform coding field is applied before signal readout; that is, the position-related frequency shift is applied only within a short window during which the phase shift of the position-related signal accumulates. When phase resolution is insufficient for precise position determination / localization, the duration and / or amplitude of the phase-coded pulses can be varied in sequential excitation, making it possible to discern ambiguities in phase increments (greater than 2π). Thus, complete spatial information is obtained over several readouts.

[0047] Phase coding using a non-uniform field pattern (e.g., coding a spatial axis) can be combined with frequency coding using another non-uniform field pattern (e.g., coding orthogonal spatial axes) for efficient localization. If a rough marker location is already known according to a sensitivity coding method (which is faster due to its parallel nature), then a few phase coding steps that provide the missing high-resolution (high spatial frequency) components instead of the complete spatial information are sufficient.

[0048] As described in the description, comparing the localization results obtained using gradient coding and sensitivity coding can be used to identify, for example, systematic errors caused by the background field. Furthermore, it should be noted that linear responses to low-frequency external fields using sensing units employing, for example, two suspended magnetic spheres as magnetic objects can be suppressed; in this case, higher-order frequency responses can be used not only for localization but also for integrity checks. However, the field sensitivity of these oscillators is much lower, necessitating a higher gradient field for gradient field coding.

[0049] In another aspect, a marking device for attachment to a medical device is provided. The marking device includes a housing and a sensing unit comprising a magnetic object providing a permanent magnetic moment. The sensing unit is configured to convert an external magnetic or electromagnetic excitation field into mechanical oscillations of the magnetic object, wherein the resulting mechanical oscillations are independent of external pressure experienced by the sensing unit. In some embodiments, the housing may in particular be a rigid housing. In some embodiments, the marking device may have an elongated shape having a maximum dimension less than or equal to 5 mm and a minimum dimension less than or equal to 1 mm. In some embodiments, the magnetic object may be arranged within the housing such that, when the external magnetic or electromagnetic excitation field acts on the magnetic object, the magnetic object can rotate away from its equilibrium orientation. Here, the sensing unit may further include a restoring torque unit for providing a restoring torque to return the magnetic object to its equilibrium orientation if the external magnetic or electromagnetic excitation field has caused the magnetic object to rotate away from its equilibrium orientation to allow the magnetic object to mechanically oscillate at a resonant frequency.

[0050] According to another aspect, a marking device is provided that allows determination of the location and / or orientation of a medical device to which the marking device may be attached. A sensing unit allows conversion of an external magnetic or electromagnetic excitation field generated by a field generator into mechanical oscillations, preferably rotational oscillations, of a magnetic object provided in the sensing unit and having a permanent magnetic moment.

[0051] The sensing unit, including a magnetic object, may specifically include a housing or be provided within a housing. Specifically, the magnetic object may be arranged within the housing. Here, the magnetic object may be specifically arranged within the housing such that it can be rotated away from its equilibrium orientation by an external magnetic torque acting on the magnetic object. The external magnetic torque may be the result of an external magnetic field or electromagnetic field acting on the magnetic object. That is, in some embodiments, the magnetic object rotates away from its equilibrium position by an external magnetic field or electromagnetic field.

[0052] The sensing unit may also include a restoring torque unit for providing a restoring torque to force the magnetic object back to its equilibrium orientation in the event that an external magnetic field or electromagnetic field has caused the magnetic object to rotate away from its equilibrium orientation. This results in rotational oscillations of the magnetic object excited by the external magnetic torque from the external magnetic field or electromagnetic field. These rotational oscillations are thus performed by the magnetic object at a corresponding resonant frequency, which depends on the spatial position and orientation of the sensing unit (and therefore the marking device) in the external magnetic field or electromagnetic field. The resulting magnetic field or electromagnetic field generated by the mechanical rotational oscillations of the magnetic object can then be converted into one or more corresponding response signals. These response signals, in turn, depend on the resonant frequency of the oscillation.

[0053] In some embodiments, rotational oscillations can particularly ultimately result in corresponding induced signals, wherein these induced signals depend on the spatial position and orientation of the tagging device in an external magnetic or electromagnetic field. These induced signals can be generated, specifically, in the excitation and induction signal unit of the tracking system. In particular, the excitation and induction signal unit may include i) a first coil adapted to generate a magnetic torque that provides a magnetic field for rotating a magnetic object of the tracking device away from its equilibrium orientation, thereby exciting rotational oscillations of the magnetic object, and ii) a second coil adapted to generate induced signals dependent on the spatial position and orientation of the tagging device. This allows for the determination of the position and orientation of the tagging device, i.e., six degrees of freedom, making it possible to determine the position and orientation of a medical device equipped with that tagging device using only a single tagging device.

[0054] Additionally, this arrangement allows the tracking system to perform tracking from a relatively large distance, for example, greater than 30 cm, based on the marking device. Furthermore, the marking device can be relatively small, for example, less than 1 mm. For this purpose, in some embodiments, the housing of the marking device can be cylindrical, and the outer diameter of the cylinder is less than 1 mm, more preferably less than 0.5 mm, and even more preferably less than 0.3 mm.

[0055] Preferably, the magnetic object can rotate about a virtual axis of rotation passing through its center, wherein the magnetic object is rotationally symmetrical with respect to the virtual axis of rotation. Specifically, the magnetic object can be a magnetic sphere or a magnetic cylinder. Furthermore, the restoring torque unit can include a torsion spring mechanism for providing the restoring torque. Additionally or alternatively, the restoring torque unit may also include another magnetic object for providing the restoring torque.

[0056] In one embodiment, the magnetic object is attached to one end of an attachment portion (such as a filament), wherein the other end of the attachment portion is attached to a housing. The attachment portion may be adapted to prevent the magnetic object from touching another magnetic object implementing the restoring torque unit due to its magnetic attraction, and to allow the magnetic object to rotate and oscillate. The other magnetic object is preferably attached to the housing statically or fixedly. However, the other magnetic object may also be arranged within the housing such that it can rotate and oscillate relative to the housing. In particular, the other magnetic object may also be attached to one end of another attachment portion (such as a filament), wherein the other end of the attachment portion may be attached to the housing.

[0057] In a preferred embodiment, the other magnetic object can rotate about a virtual axis of rotation passing through it centrally, wherein the other magnetic object is rotationally symmetrical about the virtual axis of rotation. The other magnetic object can also be a magnetic sphere or a magnetic cylinder. Furthermore, the virtual axes of the magnetic objects are preferably aligned with each other.

[0058] These techniques allow for the provision of restoring torque and thus rotational oscillations of the magnetic object, enabling the entire marking device to be relatively small, the resonant frequency of the marking device to be provided as needed, and the construction of the marking device to remain relatively simple.

[0059] Now, in order to perform position determination, the resulting mechanical rotational oscillations of the magnetic object must be independent of any external pressure experienced by the sensing unit. As an example, if the tagging device is used to track medical devices used in invasive procedures, the oscillations of the magnetic object inside the tagging device should not be affected by any external pressures acting on the medical instrument (such as blood pressure or circulatory pressure).

[0060] For this purpose, the marking device is provided with a housing in which a sensing unit is located. This housing may have one or more rigid walls, and in particular, it may be a rigid housing, i.e., a housing with walls that do not change shape when external pressure is applied to it. This means that, since the walls do not bend in response to external pressure, the positioning of the magnetic object within the housing remains largely unaffected by external pressure. This, in turn, results in the distance between the magnetic object and the restoring torque unit, also provided in the housing, remaining constant, independent of pressure acting on the sensing unit from the outside. Therefore, the magnetic force acting between the magnetic object and the restoring torque unit does not change due to distance variations caused by bending of any wall of the housing, and thus is independent of any external pressure acting on the sensing unit. Consequently, the resonant frequency of the rotational oscillation caused by an external magnetic or electromagnetic field acting on the magnetic object is also unaffected by any distance variation. This means that any change in resonant frequency is primarily affected by the position and orientation of the sensing unit, and therefore the marking device, in an external magnetic or electromagnetic field. Therefore, the sensing unit can be used for the positioning / localization of the marking device and thus any medical device to which the marking device is attached.

[0061] By utilizing this magnetic object located within a (rigid) shell, small marking devices can be provided, which can have a remarkably small size, even less than 1 mm. This makes the marking devices particularly suitable for use in tracking systems used to track medical devices during minimally invasive surgery.

[0062] According to another aspect, a medical device for use during surgery is provided, the medical device having a marking device as previously described attached thereto. The marking device is tracked by a tracking system as previously described. In some embodiments, the medical device includes an end end, such as one adapted to attach the marking device thereto. In some embodiments, the medical device may include one or more interventional devices or implants, particularly electrical implants and / or orthopedic implants. In some embodiments, the medical device may particularly include one or more of the following: surgical instruments, imaging probes, endoscopes, bronchoscopes, or ingestible pills. Alternatively or additionally, the medical device may include one or more of the following: catheters, filaments (particularly guidewires), stents, one or more aneurysm coils, one or more vena cava filters, heart valves, shunts, needles, threads, tubes, tubular needles, or radioactive seeds. In some embodiments, the medical device may have a longitudinal shape. The medical device may be adapted to have a plurality of marking devices as described herein attached thereto, wherein the plurality of marking devices may be arranged along the longitudinal axis of the medical device.

[0063] According to another aspect, a tracking method is provided for tracking a marker device as previously described, the marker device being attached to a medical device as described above using a tracking system as described. The tracking system can be used, particularly during surgery. The tracking method includes: generating a magnetic or electromagnetic excitation field for inducing mechanical oscillations of a magnetic object of a sensing unit; converting the magnetic or electromagnetic field generated by the induced mechanical oscillations of the magnetic object of the sensing unit into one or more electrical response signals; and determining the position of the marker device based on the one or more electrical response signals. In yet another aspect, a computer program including program code modules, when run on a computer controlling a previously specified tracking system, the program code modules are used to cause the tracking system to perform the steps of the tracking method described above. Attached Figure Description

[0064] In the following figures:

[0065] Figure 1 An embodiment of the marking device according to the first embodiment is illustrated schematically and exemplary.

[0066] Figure 2 A marking device attached to a medical instrument is illustrated schematically and exemplary.

[0067] Figure 3 Showing according to Figure 2 Different perspective views of labeling devices and medical devices;

[0068] Figure 4 Another embodiment of a tagging device in a tracking system for tracking medical devices is illustrated schematically and exemplary.

[0069] Figure 5A and 5B Another embodiment of the tagging device in a tracking system for tracking medical devices is illustrated schematically and exemplary.

[0070] Figure 6 An embodiment of a marking device and tracking system for determining tumor location is illustrated schematically and exemplary.

[0071] Figure 7 Implementations of multiple marking devices and tracking systems for determining the location and / or orientation and / or shape of regions of interest in patient tissue are illustrated schematically and exemplary.

[0072] Figure 8 An embodiment of a medical device corresponding to a line for treating a cerebral aneurysm, to which a marking device is attached, is schematically and exemplaryly shown.

[0073] Figure 9An embodiment of a medical device corresponding to a liver shunt device to which a marking device is attached is schematically and exemplary.

[0074] Figure 10 An embodiment of a marking device with temperature compensation is illustrated schematically and exemplary.

[0075] Figure 11 and 12 A tracking system for a tracking tag device according to an embodiment of the present invention is illustrated schematically and exemplary.

[0076] Figure 13 The excitation pulse and the generated induced voltage are illustrated schematically and exemplary.

[0077] Figure 14 A multi-coil array integrated into the mattress of a hospital bed with an imaging system is illustrated schematically and exemplary.

[0078] Figure 15 The receiving coil of a tracking system for detecting changes in a magnetic or electromagnetic field caused by mechanical oscillations is illustrated schematically and exemplary.

[0079] Figure 16 The spectrum used to determine the resonant frequency is shown.

[0080] Figure 17 An analog receiving filter is illustrated schematically and exemplary.

[0081] Figure 18 An exemplary illustration shows the bandpass frequency response of the Chebyshev II type.

[0082] Figure 19 The dependence of signal amplitude in different harmonics on the measurement of sensor orientation relative to a single transmit-receive coil is shown, and

[0083] Figure 20 Another embodiment of the marking device is illustrated schematically and exemplary. Detailed Implementation

[0084] Figure 1 An embodiment of a tagging device 501 is illustrated schematically and exemplary for attachment to a medical device for tracking by a tracking system used during surgery (particularly minimally invasive surgery on a person, particularly a patient). The tagging device 501 includes a sensing unit having two magnetic objects 507, 508.

[0085] A magnetic object 508 is suspended on an attachment portion 506, such as a filament, and is therefore free to perform rotational movements about the main axis of the sensing unit. In this embodiment, another magnetic object 507 is fixed. However, in another embodiment, another magnetic element may also be suspended on an attachment portion, such as a filament, and is therefore free to perform rotational movements about the main axis of the sensing unit.

[0086] At equilibrium, magnetic objects 507 and 508 are aligned with their magnetizations in an antiparallel orientation. An external magnetic field pulse can be used to initiate resonant rotational oscillations. The attractive force determines the resonant frequency of the oscillation, which, for a spherical levitation magnet, is given by the following equation:

[0087]

[0088] Among them, M S Let ρ be the saturation magnetization of the magnetic material, ρ be its density, r be the diameter of the sphere, and B be the magnetic field generated by the fixed magnetic object. It can be approximated as a dipole field.

[0089]

[0090] Where m is the magnetic moment of the magnetic object.

[0091] Changes in the magnetic field generated by the oscillating magnetic object 508 can be detected via induced voltages in one or more detection coils of a transducer configured to convert a magnetic or electromagnetic field element generated by the mechanical oscillation of the magnetic object 508 of the sensing unit into an electrical response signal. A Fourier transform can be performed on the time trace of the detected signal to obtain a spectrum that allows determination of the resonant frequency.

[0092] Due to the low resonant frequency of a few kHz, the magnetic field is not shielded by the metal, and therefore all non-ferromagnetic metals can be used as structural or coating materials. Similarly, as long as the metal thickness does not significantly exceed the skin depth, marking devices can be placed in non-ferromagnetic metal objects without affecting their operation. At these frequencies, the skin depth is approximately 1 mm for very good conductors like copper, and approximately 10 mm for nitinol.

[0093] The sensing unit 501 therefore includes two magnetic objects 507 and 508, which, in equilibrium, are aligned with antiparallel magnetization. An external field pulse provided by a corresponding field generator can be used to levitate the magnetic object 508 (in... Figure 1In one embodiment, this corresponds to the rotational oscillation of a magnetic sphere around the main axis of the sensing unit, wherein another magnetic object 507 (also implemented as a magnetic sphere in this particular embodiment) is fixed. If, in another embodiment, another magnetic object 507 is also suspended in free space and can perform rotational oscillation, then both magnetic objects 507 and 508 can perform resonant reverse oscillation.

[0094] It is known to use magnetomechanical oscillators to determine the position and / or orientation of a tagging device relative to a tracking system. Furthermore, it is known to use LC oscillators to perform such position estimations. For example, a tagging device including a sensing unit is shown in the article "Validation of the Calypso Surface Beacon Transponder" by B. Maxwell et al., Journal of Applied Clinical Medical Physics, Vol. 17, pp. 223-234 (2016). However, the tagging device shown therein typically has a size of 8 mm. Providing a smaller tagging device would be beneficial. Unfortunately, as the size of the tagging device decreases, the accuracy of the measurement also decreases. Therefore, position determination measurements using the aforementioned tagging devices are not ideal, especially for small-sized tagging devices.

[0095] In other words, as the size decreases, the power level that can be generated at the oscillator and the dynamic dipole moment generated by the power reduction decrease. This can be seen from the following formula. The quality factor of the resonator cannot be higher than the quality factor of the coil. An approximate value for the quality factor of the coil can be written as:

[0096]

[0097] Where ω is the frequency, μ o ρ is the permeability of free space, τ is the resistivity, τ is the fraction including the radius of the conductor, and r is the radius of the coil. The coil is assumed to be cylindrical with a diameter matching its height. For a copper coil with a diameter of 1 mm, a quality factor of approximately 1 is achieved at 100 kHz. Therefore, typically, the maximum possible quality factor achievable in mechanical resonance is too low for efficient operation. Some materials, such as fused silica, exist that can provide a high quality factor in oscillations. These materials are typically very rigid and do not allow for efficiency for sufficiently high oscillation amplitudes (sufficiently high angles), i.e., generating sufficiently large field changes. This can lead to the need for a considerably high signal-to-noise ratio, which in turn necessitates a large amount of magnetic material, resulting in a large sensor.

[0098] The above formula overestimates the practically achievable Q value because it assumes all volumes are filled with conductive material and neglects proximity and skin effects, as well as losses in capacitors. Nevertheless, these values ​​result in working systems. Since, for example, the dynamic dipole moment of an LC oscillator is Q multiplied by the external magnetic field multiplied by the volume, the signal and r... 5 Proportional, while in the case of a mechanical oscillator (where energy is stored elastically), the signal is proportional to r. 3 Proportional, and in the case of the described embodiments, for example, referring to Figure 1 (Magnetomechanical oscillator, energy is stored in the magnetic field), signal and r 2 This is proportional because frequency is inversely proportional to linear size. Therefore, the recommendations presented here are well-suited for sensor miniaturization.

[0099] Therefore, through, for example Figure 1 The proposed design avoids the aforementioned problems. Since energy is primarily stored in the magnetic field, obtaining a high quality factor is relatively easy. High oscillation amplitudes are also readily achievable. Typically, the sensing unit can also employ an attachment portion that does not suffer severe wear, such as a thin filament. Therefore, the resonance can be easily altered by changing the magnetic field through mechanical movement of the magnets relative to each other. This change can then be used to determine the position of the sensing unit relative to the coordinate system provided by the tracking system, as discussed further below.

[0100] In embodiments with a fixed sphere, the fixed sphere may have a diameter of 620 μm, while the oscillating sphere 108 may have a diameter of 500 μm. The magnetic moment of the oscillating sphere 108 may be m ≈ 70 μm. 2 The fundamental frequency can be f0≈2kHz, and the quality factor can be approximately Q≈500. The SNR depends on a) the distance between the coil used to read the resonant frequency and b) the sensing device, as well as the coil parameters. For a handheld coil with a diameter of 10cm, 200 windings, and a resistance of 10Ohm, the theoretically achievable SNR at a distance of approximately 30cm and a sampling duration of 0.1s is approximately 4000. However, if almost no measures are implemented for background signal suppression, the typical SNR value for a display with a fixed sphere can be between 10 and 100. Therefore, the noise is primarily determined by fluctuations in the main power supply harmonics. For half the sphere diameter, i.e., an oscillating sphere, for example, 250μm, the magnetic moment can be m≈9μAm. 2 The fundamental frequency can be f0≈4kHz, the quality factor can remain unchanged, and the theoretical signal-to-noise ratio can be reduced to about 1000.

[0101] There are several ways to attach the attachment portion to the rotatable magnetic object 508.

[0102] For example, through-hole attachment can be used. In this case, a hole is drilled through the center of gravity and approximately perpendicular to the magnetization direction. Although the magnetic material is hard and brittle, several drilling methods exist, such as pulsed laser or electrical discharge machining (EDM). The wire travels through the hole and is glued in place. A vacuum suction process is preferred for this travel. Several types of adhesives can be used. Photocurable adhesives are economical. They should have low viscosity so that they fill the hole with the wire only by capillary force. Alternatively or additionally, the attachment portion can be fixed to the magnetic object 508 by mechanical means (e.g., by having a knot in the wire or by having some other thick portion in the wire (such as a droplet or thermally generated (melted) bead)). The latter is particularly easy to make with UHMWPE fibers. This attachment method reduces the magnetic dipole moment by only a small fraction and thus maintains a good signal. The shape of the magnetic object is not significantly altered, which would be important in the case of a sphere.

[0103] Clamp attachment can also be used. In this case, the magnetic object is divided into at least two parts. Preferably, a dividing plane is formed that is orthogonal to the magnetization and parallel to the wire attachment direction. The wire (i.e., the filament) is placed on this plane. Precise alignment is not necessary. The second magnetic part is placed on top. The magnetic parts are typically held together by magnetic force. Finally, an adhesive is applied to secure all the objects in place. The preferred type of adhesive is the same as that used in through-hole attachment. Furthermore, grooves can be ground into one or both of the magnetic objects to reduce the overall gap between them. This method produces results almost as good as the through-hole method, but does not require special manufacturing equipment. Typically, magnetic sub-objects are made not by dividing a single complete magnetic object, but by grinding two (identical) magnetic objects. The downside is that the process is more wasteful due to the use of two initial objects, and it can also be somewhat more labor-intensive.

[0104] The cheapest method is to attach the wire directly to the top of the magnetic object 508 using a suitable adhesive. The magnetic object 508 is held and aligned in some kind of tool. Both functions can be achieved using a suitable magnetic field. This tool can be funnel-shaped, with the wire traveling through it, and the magnetic object is attached to the funnel opening by magnetic force. The adhesive is applied to the funnel and cured. The assembly is then removed from the tool and the excess wire is cut off. This method can be very inexpensive and allows for ample expansion using the magnetic object. The disadvantages are the added material, reduced oscillation frequency, and the space required in the finished device.

[0105] In a further embodiment, an attachment and gluing structure can be used. The wire can be attached to the magnetic object 508 by first attaching the wire to a non-magnetic object and then gluing the non-magnetic object to the magnetic object. The non-magnetic object can be manufactured by injection molding or an equivalent inexpensive process. The shape of the non-magnetic object should allow for simple wire attachment; that is, it can have holes or clamping mechanisms, or even be as simple as a notch. The non-magnetic object is then glued to the magnetic object. Alternatively, it can be clamped or screwed to the magnetic object. This method is simple and inexpensive, but may require too much additional space for some applications.

[0106] In principle, all the methods discussed for attaching wire-magnetic objects are applied in the same way to wire-housing connections. However, since housing materials are generally easier to work with, slotting may be a good option. Clamping is also a good option. This may be cheaper, but may be more difficult to achieve a final seal.

[0107] According to Figure 1 In this embodiment, at least the wall 515 of the housing 502 of the marking device is a hard wall, for example, insensitive to external pressure. This avoids changes in the mechanical oscillations of the magnetic object 508 due to the influence of external pressure, because the distance between the magnetic object 508 and the restoring torque unit with respect to the magnetic object 507 remains constant, and therefore, the magnetic force interacting between the two magnetic objects does not change due to changes in the inter-sphere distance (caused by pressure). Therefore, the resonant frequency is also unaffected by any changes in the inter-sphere distance. This means that the mechanical oscillations of the magnetic object caused by the interaction of the external magnetic or electromagnetic excitation field with the restoring torque field of the magnetic object 507 depend primarily on the position and / or orientation of the marking device relative to the excitation field, and therefore allow for transfer to a coordinate system provided by the tracking device.

[0108] The aforementioned tagging device can be used to attach to any type of medical device that should be tracked during medical procedures. For this purpose, Figure 2 and Figure 3 The marking device 501 attached to the medical device 510 is schematically illustrated. Figure 2 and Figure 3 In a specific embodiment, medical device 510 corresponds to a guidewire. However, it should be understood that the medical device can also be any other kind of medical device, particularly any other kind of medical instrument, or even more specifically, any kind of medical instrument used to perform (minimally invasive) procedures where tracking can be beneficial. In some embodiments, the marking device can also be used to track different elements, such as tissue, bandages, etc. Examples of other devices and / or elements to be tracked are... Figures 4 to 6 This is provided in [the document], and discussed further below.

[0109] As discussed, in Figure 1 and Figure 2 In the specific embodiment shown, the marking device 501 is attached to a medical device 510, which corresponds to a guidewire. Portions 511 and 512 of the guidewire 510 can be used to give the housing 502 of the marking device 501 a fixed magnetic sphere 507 as a restoring torque unit and a rotatable magnetic sphere 508 as a magnetic object attached to the guidewire, the rotatable magnetic sphere 508 being attached to a rigid wall 515 via an attachment portion 506.

[0110] Figure 2 and Figure 3 The dimensions shown are merely exemplary. Dimensions may vary. However, the dimensions shown are well-suited for performing tracking during interventional procedures on human patients. Applying the law of proportionality to the observed exemplary SNR indicates that the indicated dimensions will provide sufficient SNR and accuracy for remote manipulation at a distance large enough to fully penetrate the patient. Therefore, the marking device 501 can be attached to the guidewire, thereby allowing guidewire tracking during interventional procedures.

[0111] Using the labeling device also in other medical devices and / or other components can be useful, such as in... Figures 4 to 10 As shown.

[0112] For this purpose, Figure 4 At least one marking device 501 is shown, which is attached to an ultrasound probe 610 to track the position of the ultrasound probe 610 during ultrasound measurements of a patient 100.

[0113] Figure 5A The illustration shows the marking device 501, as described above, attached to a cardioid needle 710 for introduction into patient tissue. Figure 5A In one embodiment, a single marking device attached to the first end portion 711 of the core needle 710 is used to track the core needle as it is introduced into the patient's tissue. Alternatively, as... Figure 5B As shown, a plurality of marking devices 501, 501', 501" can be attached to the mandrel 710 along the length of the mandrel 710 from the first end portion 711 to the second end portion 712. The plurality of marking devices 501, 501', 501" can allow tracking of the position of the mandrel 710 relative to a coordinate system provided by a tracking system, and can also allow determination of the orientation and / or shape of the mandrel 710.

[0114] Figure 6 The illustration schematically depicts the use of a marking device 501 for tracking tissue. More specifically, in Figure 6In an exemplary embodiment, the marking device 510 is used to determine the location and / or orientation of the tumor 810. The location determination unit 900 of the tracking system is then used to determine the marking device and thus the position of the tumor 810 relative to a coordinate system provided by the tracking system (not shown). This allows for more accurate positioning of the tumor 810 for subsequent removal. For illustrative purposes, the dimensions of the marking device 501 used for tumor localization are shown below. Figure 6 The marking device 501 is further illustrated in relation to the human finger 101. As can be understood from this illustrative representation, the marking device 501 has a very small size while providing accurate positioning.

[0115] Figure 7 Another illustrative embodiment is schematically illustrated for using one or more marking devices 501, 501', 501"', 501"' for location determination and treatment control during medical procedures performed on a patient. In this case, information provided by the marking devices 501, 501', 501"', and 501"' attached to the patient's prostate 102 is combined with information from additional sensors such as pressure sensors, temperature sensors, radiation sensors, etc. This combination allows determination of the position and / or orientation of the prostate 102 relative to a coordinate system provided by a tracking system, and further measurement of parameters such as temperature, pressure, or radiation using the aforementioned sensors, wherein these measurements may be location-specific. This allows for obtaining a spatial mapping of these measurement parameters. Such a spatial mapping can allow for control and / or monitoring of the procedure.

[0116] Specifically, Figure 7 The patient's prostate 102 is shown. According to... Figure 7 In a particular embodiment, an ablation procedure should be performed on the prostate tissue. For this purpose, a plurality of marking devices 501, 501', 501" and 501"' are provided at several locations on the prostate 102. Furthermore, one or more temperature sensors (not shown) are provided at different locations on the prostate (preferably near the region of interest where the ablation treatment should be performed). This allows for spatial mapping of temperature progression during the ablation treatment and can help avoid temperature-induced damage and / or overtreatment of healthy tissue during the ablation procedure.

[0117] Figure 8An embodiment of a line for treating a cerebral aneurysm is illustrated schematically and exemplary. According to the described embodiment, the line 910 includes one or more marking devices 501, 501', 501'". Specifically, a first marking device 501 is located at a first end portion 911 of the line 910 on one side of the first end portion 911. Furthermore, another marking device 501' may be attached to a second end portion 912 of the line 910, and another marking device 501' may be installed within an intermediate section of the line 910, wherein the line 910 may include an inner cavity for arranging the marking devices 501'.

[0118] Figure 9 An embodiment of a liver shunt device 1100 including a line structure 1103 is illustrated schematically and exemplary. In this embodiment, the line structure 1103 has a first portion 1101 surrounded by a liner material and an exposed second portion 1102. In this embodiment, the first portion 1101 is lined using PTFE (polytetrafluoroethylene). Furthermore, in this embodiment, the first portion 1101 of the line structure has individual lines, while in the second portion 1102 of the line structure 1103, the lines are interwoven. The liver shunt device 1100 (also referred to simply as a liver shunt) includes a plurality of marking devices 501, 501', 501"', 501"'. For example, a first marking device 501 is arranged next to a corresponding line of the first portion 1101 of the line structure 1103 within a PTFE tube. A second marking device 501' is arranged "within the line" within the PTFE tube, i.e., the marking device 501' is arranged between the two ends of a corresponding line of the line structure 1103. Furthermore, third and fourth marking devices 501”, 501”' are arranged at the second part 1102 of the diversion device. Their positions and / or orientations can be determined by arranging the marking devices along the length of the diversion device. Additionally, the shape of the diversion device 1100 can be determined.

[0119] It should be noted that, in Figures 4 to 9 The arrangement of the marking devices is merely exemplary; more or fewer marking devices may be arranged at or within the corresponding medical device and / or element in the same or other locations. It is also possible that the corresponding device comprises only a single marking device. One or more marking devices attached to the corresponding medical device and / or element are marking devices according to at least one of the described embodiments.

[0120] In the following text, it is assumed that the length of the marking device is always approximately twice its diameter. All devices with a diameter of 0.3 mm or greater will enable real-time tracking with high accuracy at distances greater than 30 cm (more than 10 readings per second).

[0121] The marking device can be attached to the guidewire, for example, as shown in the reference above. Figure 2 and Figure 3This is explained and used for tracking such guidewires. Additionally, marking devices can be used to track catheters. Marking devices can also be placed on stents. To minimize interference from the marking device during stent implantation, the marking device should be as small as possible and should not exceed the stent wire diameter. Typical stent wire diameters are between 0.2 and 0.5 mm. Therefore, this will be a useful range for the marking device diameter. It is also possible to inject the marking device with a syringe, where the marking device can be inserted into smaller vessels in the lung or liver region without posing a risk to the patient. Typical injection diameters would be between 0.3 and 1.0 mm.

[0122] The described marking device is preferably configured to compensate for the temperature dependence of the resonant frequency. Reference will be made below. Figure 10 This describes one possible method for compensating for temperature-based shifts in resonant frequencies.

[0123] Also in Figure 10 In this embodiment, the marking device 3001 includes a housing 3002 and a magnetic object 3004 disposed within the housing 3002 such that it can rotate away from its equilibrium orientation when an external magnetic torque is applied to it. The marking device 3001 also includes a recovery torque unit 3003 adapted to provide a recovery torque to force the magnetic object 3004 back to its equilibrium orientation if an external magnetic or electromagnetic field has rotated it away from its equilibrium orientation, thereby allowing rotational oscillations of the magnetic object 3004 excited by the external magnetic or electromagnetic field, resulting in a corresponding magnetic torque. In this embodiment, the housing 3002 is cylindrical, and the magnetic object 3004 is rotatable about a virtual axis of rotation passing through it, wherein the magnetic object 3004 is rotationally symmetrical with respect to the virtual axis of rotation. Specifically, in this embodiment, the magnetic object 3004 is a magnetic sphere.

[0124] The restoring torque unit 3003 includes another magnetic object 3003 for providing restoring torque. Specifically, a magnetic object 3004 is attached to one end of an attachment portion (such as a filament) 3007, the other end of which is attached to a housing 3002. The attachment portion 3007 is adapted to prevent the magnetic object 3004 from touching the other magnetic object 3003 due to its magnetic attraction and to allow the magnetic object 3004 to rotate and oscillate. In this embodiment, the other magnetic object 3003 is fixedly attached to the housing 3002 using an adhesive 3009.

[0125] Magnetic object 3004 forms a first magnetic dipole, and another magnetic object 3003 forms a second magnetic dipole. The magnetic objects 3004 and 3003 are arranged such that, in a balanced orientation, the first and second dipoles point in opposite directions. The first and second magnetic objects 3004 are permanent magnets, wherein, in a balanced orientation, the north pole of magnetic object 3004 faces the south pole of magnetic object 3003, and vice versa.

[0126] The housing 3002 is cylindrical, wherein the cylindrical housing 3002 includes two end surfaces 3030 and 3031, and wherein the other magnetic object 3003 is fixedly attached to the first end surface 3030 and the end of the filament 3007 opposite to the end attached to the magnetic object 3004 is attached to the second end surface 3031 of the cylindrical housing 3002.

[0127] In this embodiment, the second end surface 3031 of the housing 3002 is formed by the hard wall 3008 of the housing 3002, wherein the magnetic object 3004 is attached to the hard wall 3008 via the attachment portion 3007, so that the influence of external pressure is not transmitted to the interior of the housing 3002.

[0128] The marking device 3001 also includes magnetic materials 3005 and 3006 arranged adjacent to another magnetic object 3003. These magnetic materials 3005 and 3006 influence the magnetic field generated by the other magnetic object 3003, wherein the influence of the magnetic materials 3005 and 3006 depends on temperature, thereby changing the strength of the magnetic field at the location of the magnetic object 3004, and thus changing the resonant frequency in response to temperature changes. The magnetic materials 3005 and 3006 are adapted such that their magnetization decreases with increasing temperature. Furthermore, the magnetic material 3006 is adapted such that its magnetization direction is opposite to that of the other magnetic object 3003, and the magnetic material 3005 is adapted such that its magnetization direction is the same as that of the other magnetic object 3003. Since the magnetic materials 3005 and 3006 are soft magnetic materials, they thus influence the resonant frequency in opposite frequency directions according to temperature; that is, one of these magnetic materials causes a change towards a higher frequency with increasing temperature, and the other causes a change towards a lower frequency with increasing temperature.

[0129] Therefore, the marking device 3001 is preferably configured such that its resonant frequency is independent of temperature. To compensate for any undesirable temperature-dependent frequency shift, magnetic materials 3005 and 3006 can be tailored to provide the same frequency shift in opposite frequency directions according to temperature changes. In particular, magnetic materials 3005 and 3006 can be selected and arranged to eliminate any temperature dependence of the resonant frequency of the marking device 3001. It is also possible that only one of the magnetic materials (i.e., a magnetic material that decreases its resonant frequency only with increasing temperature or a material that increases its resonant frequency only with increasing temperature) is used to reduce or even eliminate the temperature dependence of the resonant frequency of the marking device 3001. One or both of magnetic materials 3005 and 3006 can be considered as compensating elements for temperature-induced shifts in the resonant frequency.

[0130] Figure 11 A tracking system 1501 for tracking a tagging device as described above is schematically and exemplaryly shown, namely, a tracking system for wirelessly determining the position and / or orientation of a tagging device attached to a medical device based on one or more electrical response signals indicating the response frequency of oscillations of a magnetic object in a sensing unit. Figure 12 A prototype of the tracking system 1501 is shown as an example. The tracking system 1501 basically includes at least one field generator for a magnetic field and at least one magnetic field sensor, i.e., a transducer for converting a magnetic field or electromagnetic field generated by the oscillation of a magnetic object of the sensing unit into an electrical response signal.

[0131] The operating bandwidth is in the low kHz range and must be wide enough to cover the responses of several sensors operating in parallel at different frequencies, and possibly also to cover higher harmonics of the sensor resonant frequencies, for example, to improve the signal-to-noise ratio. The transmitted field amplitude is at most a few millitalas, while the field amplitude to be detected is between 1 / 10 nT and a few nT. Many different magnetic field generators (oscillating permanent magnets, cored / coreless coils, magnetostrictive field modulators, etc.) and many different magnetometers (Hall effect, various magnetoresistive sensors, magnetic resonant sensors, SQUIDS, etc.) can work. The technically simplest system is a coreless conductor loop for transmitting and receiving magnetic fields. For sensor applications, coils are usually good enough. The coil used to generate the magnetic field can also be used to receive the magnetic field. However, different coils can be used for these tasks, which offers some advantages.

[0132] exist Figure 11In this system, tracking system 1501 includes a transmitting coil 1503, which is connected to a microcontroller 1507 via a digital-to-analog converter 1506 (DAC) and an audio amplifier 1502 for generating an external magnetic or electromagnetic excitation field for a marking device 1520, which can be implemented as previously described. A receiving coil 1504 is also connected to the microcontroller 1507 via a low-noise amplifier 1505 and an analog-to-digital converter 1508 (ADC) for reading the resonant frequency. Microcontroller 1507 is connected to a display computer 1509. Microcontroller 1507 is configured for, for example, signal generation and reception, frequency evaluation, and control. Figure 12 A transmit / receive decoupler is also shown in the diagram.

[0133] Microcontroller 1507 generates transmit pulse (reference) Figure 13 The transmitted pulse (1350 in the upper trace) is amplified using an audio amplifier 1502 and then transmitted to a transmitting coil 1503, which may also be referred to as an excitation coil. In this embodiment, a separate receiving coil 1504 is used, which is decoupled from the transmitting coil 1503 using two additional decoupling coils 1510, for clarity. Figure 11 Two additional decoupling coils 1510 are not shown. The received signal is fed to a low-noise amplifier 1505 and then passed to the ADC 1508 of the microcontroller 1507, where a time trace typically 1 / 20th of a second is sampled at a rate of approximately 20 kS / s. In addition to the transmit pulse 1350, which can also be referred to as the excitation pulse, Figure 13 The induced voltage 1351 in the receiving coil 1504 due to the oscillation of the ball in the sensor and thus due to the sensor response is also shown. The spacing of the excitation pulses 1350 can be continuously adjusted by the microcontroller 1507.

[0134] In the embodiments described herein, the tracking system may specifically correspond to a multi-coil system. The use of several coils enables the determination of the position of the marking device by determining the location and orientation of the oscillating magnetic dipole in space. Different amplitudes of the received signal and known coil element sensitivities can be matched to a dipole model used to determine the position and orientation parameters. Figure 14 An example of a multi-coil system for implementation in a pillow or mattress is shown. When multiple receiving coils and channels are available, additional information can also be used to improve background signal suppression, as described further below.

[0135] exist Figure 14In this system, several coils 1652 form a multi-coil array integrated into the mattress 1651 of the bed in an imaging system 1650 (such as a C-arm system). The coils 1652 are preferably aluminum coils with less than 10% X-ray absorption. Therefore, no increase in patient dose is required when using the coils 1652.

[0136] The coil-based transmitting system of the tracking system will be described in more detail below. The coil-based transmitting system includes a transmitting amplifier and a transmitting coil. Optionally, a matching circuit and a "mute" circuit are also included. Since the shape of the transmitted signal is not critical in sensor applications, many amplifiers are suitable for the task (Class A, Class B, Class AB, Class D, etc., using transistors, vacuum tubes, thyristors, and more components). Since signal quality is not critical, an amplifier topology with the lowest loss can be chosen, which is a half-bridge or full-bridge amplifier using switches with low on-resistance. Preferred switches are MOSFETs or IGBTs. In the simplest case, the matching circuit is a simple capacitor in series with an inductor. Assuming the amplifier operates with a sufficient supply voltage, such a matching capacitor can be omitted, or the capacitance can be chosen to be so high that the resonant frequency of the coil with the capacitor is much lower than the operating frequency. The matching circuit is of interest for another reason. Medical devices should always be operated safely, and voltage reduction is important. By placing a capacitor in the middle of the coil, allowing current to flow through one coil segment, then through the matching capacitor, and subsequently to a second coil segment, the peak voltage difference can be reduced. This is even more true if the coil is divided into more segments, each connected to a suitable capacitor. This makes the coil and matching circuit a single unit. The field amplitude is conveniently controlled by pulse width modulation, i.e., the amplifier increases / decreases the current through the coil only within a portion of the cycle, or rapidly alternates between increasing / decreasing the current. Since precise signal shape is less relevant for sensing applications, it is best achieved by changing the state only twice within half a wave (or once at full power with the pulse length equal to half a wavelength). Ideally, the amplifier should not only have the ability to increase or decrease the current, but also maintain the current approximately constant or at the level specified by the matching circuit. This is achieved through the appropriate switching sequence of the transistors in the half-bridge or full-bridge. Typically, the amplifier's supply voltage should be quite low, and in the range below 50V. Furthermore, the matching circuit should be configured such that it does not exceed this 50V limit at any two points. In both cases, it is even better not to exceed 24V. This means that the number of windings should be kept low. However, the peak operating current should exceed 10A, and 100A is better.

[0137] Transmit / receive isolation will be described below. It is essential that when the transmitting system is not in transmit mode (i.e., not generating an excitation field), not too much noise from the transmitting system (i.e., from the field generator) is coupled into the receiving system, specifically into the transducer used to convert the magnetic or electromagnetic field generated by the mechanical oscillations of the magnetic object of the sensing unit into an electrical response signal. Furthermore, the transmitting amplifier should not short-circuit the received signal or even partially reduce it. Several possibilities exist for achieving this. If there are different transmitting and receiving coils, the two coils can be geometrically decoupled (see [reference]). Figure 15 ).

[0138] Figure 15 An implementation of a gradient measurement receiving coil design for suppressing transmitted and background signals in the receiving path is shown. A large coil 1452 has been chosen here, which allows the tracking device to be positioned approximately 30 cm above the upper coil. The gradient measurement design uses a geometric decoupling method: the transmitting coil loop 1451 is connected to generate a parallel field, while the receiving coil loop 1450 is connected to receive the field gradient and suppress the uniform field. This transmitting and receiving system provides inherent geometric decoupling through the use of a parallel transmitting loop and an antiparallel receiving loop, which can be referred to as a gradiometer configuration. This results in inherent geometric coupling. The system with the air coil is highly linear. Figure 15 Also shown is a DC blocking 1455, an audio amplifier 1454, and a low-pass emission filter 1453. Figure 15 The lower part of the diagram shows the outer winding of the receiving coil 1450 and the inner winding of the transmitting coil 1451.

[0139] In particular, Figure 15 The lower left image is a close-up of the middle section of the upper coil assembly. Peeking out from the bottom of the lower left image, only one turn of the transmitting coil 1451 is actually visible. The rest is obscured by the receiving coil, which is wound with much thinner wire. The DC blocking circuit 1455 is simply signal conditioning in front of the audio amplifier, as the signal for the audio amplifier can be generated by a simple PWM output. The low-pass filter 1553 is a filter between the output of the audio amplifier 1454 and the transmitting coil 1451. It serves two purposes: first, to prevent the introduction of high-frequency noise; and second, to combine the two output channels of the audio amplifier into one.

[0140] Geometric decoupling is not always possible, especially when using transmitter and receiver arrays. In such cases, a transformer can be introduced, with terminals connected to both the transmitting and receiving circuits. This transformer provides decoupling between the transmitting and receiving systems. This transformer solution can be used even when using combined transmit / receive coils. The transformer can be replaced by a capacitive (or even resistive) decoupling network with both combined and separate transmit / receive coils. The disadvantages of compensation methods are that they require considerable space, increase noise, and reduce the frequency operating range of the tracking system in the case of capacitive decoupling. A more robust and cheaper solution is to add circuitry that completely silences the transmit amplifier during the receive time. For this purpose, a cross diode can be added to the amplifier's output. Diodes with low capacitance at zero voltage (such as PIN diodes) are particularly useful. This provides high impedance when no current flows. To further enhance this, an electronic switch can be placed at the amplifier's output to short-circuit all residual noise signals during reception. The diode still provides the desired high impedance. It is also possible to build a special amplifier that is completely noiseless when not in operation and provides high impedance. In the case of half-bridge and full-bridge designs, this can be achieved by absolutely no switching operation in any component during reception, the use of low-output-capacitance transistors, providing approximately half the supply voltage at the output(s) in receive mode, no noise from the input connectors (optical insulators), and a highly filtered supply voltage (re-filtered or no power switching during receive operation).

[0141] The following discussion focuses on coil-based receiving systems for tracking systems. The receiving amplifier should be of a low-noise type. However, the requirement is not so high that it necessitates the use of uncommon receiving transistors. Standard low-noise bipolar or JFET silicon transistors are generally sufficient. The only special feature is that the amplifier needs to withstand the transmit pulse and begin operation shortly after it. Several ways exist to achieve this. In the case of decoupled transmit / receive systems (including combined transmit / receive coils with decoupling networks), the receiving amplifier does not require special features to achieve this. If decoupling is not present, the amplifier can be hardened to the transmit pulse. This can be done by adding a suitable capacitor to the amplifier's input and a crossover diode to the second terminal. This provides a suitable high impedance in the transmit case and shorts any excessively high voltages to a harmless level for the amplifier. Of course, the added capacitor needs to be rated for the maximum transmit voltage. The capacitance value needs to be so high that the signal at the amplifier does not drop too much in the receive case. For JFET-based amplifiers, this is generally not a critical issue. The crossover diode can be enhanced or replaced by a suitable electronic switch, such as an optocoupler with a MOSFET output. This has the advantage of further reducing the input voltage. If done properly, the receiving amplifier will not saturate and will start working immediately after the transmitted signal has sufficiently decreased.

[0142] The interface of digital systems will be discussed in more detail below, starting with the description of digital signal output and processing. While analog timer systems can generate output signals, digital systems such as DSPs or FPGAs are typically used. Different outputs can be used depending on the type of output amplifier. For analog amplifiers, some type of ADC can be used. Since output signal quality is not critical, a simple PWM-type analog output may suffice. Digital amplifiers are best interfaced using digital output lines. However, it is also possible to use analog outputs for them and implement a switching mode generator on the amplifier. In the case of optimally matched amplifiers, half-bridge or full-bridge amplifiers are best suited for generating switching modes directly on the digital system. Furthermore, switching modes used for receiving amplifier input protection and sending amplifier output denoising can also be generated directly by the digital system. A common characteristic of all output options is that they need to be fast enough to accurately maintain the phase of a single marker device within different excitations or between different marker devices. Therefore, the output needs to have the capability to switch updates on a raster finer than one-tenth of the full cycle time (ideally finer than one-hundredth of the full cycle time). For a sensing device, say, at 2kHz, this means updating on a finer grating than 220kHz, or even better, 200kHz. This doesn't mean it needs to be possible every time the state changes at a grating point. Therefore, for example, it's possible to have a serial interface for each amplifier that transmits the new switching state to the amplifier, and a protocol that executes the change at a specific time through the same serial interface. This is particularly useful for amplifier types that inherently require muting during the receiving phase. For this, a 1-bit serial interface requiring only an optocoupler on the amplifier can be implemented. This makes noise immunity from the digital transmitting side easy, as the stray capacitance in a single optocoupler can be very low.

[0143] The analog-to-digital interface will be discussed below. Analog-to-digital conversion is fairly standard. Since the signal is low-bandwidth, at least if only a single tagging device is used, the signal can be mixed to near DC and sampled. However, the signal from the tagging device has a fairly low frequency, typically below 10kHz. Today, there are many suitable ADC chips that sample this directly. In particular, since digital signal processing is worse than analog filtering, it is best to use a large amount of oversampling in the ADC. The frequency of the tagging device should be used at least 10 times, but 100 or 1000 times are also viable options. High oversampling makes the design of the ADC input filter simple and inexpensive, since only the sensor signal frequency needs to pass through, and no signal can pass through above the Nyquist frequency. However, filtering below the sensor frequency also helps to avoid the typically high background signal there. A high background signal can reduce the possible amplification before the ADC, thus increasing the ADC noise contribution. The ADC noise (number of effective bits) and samples should match the desired dynamic range and noise expectation. This means that the ADC should not be saturated when the maximum expected signal and all noise components are present. At the same time, the quantization noise of the ADC should be so low that it does not increase the overall noise. Noise here refers to all extraneous components in the recorded signal originating from real noise sources, such as coil resistance or receiver amplifier behavior. It also includes interfering components that cannot be eliminated by proper filtering and background signal subtraction. Typically, for modern ADC chips, this requirement might mean meeting it, for example, using a 2MS / s 18-bit ADC. While using a lower-specification ADC can be useful for cost savings, gain control can be increased to still achieve good overall performance.

[0144] Data processing will be discussed below. The raw ADC data must be processed before data evaluation. Since significant oversampling is expected, the first processing step can be a decimation step. This has the major advantage of reducing the data size and therefore the computational power required for other steps. Optionally, the decimation step may include additional filters, i.e., bandpass filters near the expected signal frequency. This simplifies other processing steps and reduces the dynamic range of the signal, which in turn saves computational power (with fewer bits of variables). Another optional data processing step is to apply an inverse nonlinear filter to reduce the nonlinearity of the receiving system. This means measuring the nonlinearity of the entire receiving system and constructing a computational filter to reverse the effects of the nonlinearity. This is particularly useful when using low-cost components, as they tend to have more nonlinear behavior.

[0145] This nonlinear filter can alternatively be used as the first processing step. If more than one received signal is used, there are other signal processing steps. If at least one receiving channel does not detect a signal from the sensing unit and thus provides a measurement of the background signal, this (and all other such signals) is correlated with the received signal and the correlated component is subtracted from the signal-carrying channel. This subtraction can be done in the time domain, the frequency domain, or a mixture of both. If there is no channel without a signal from the sensing unit, a data processing strategy sometimes called a “virtual gradient meter” can be used. This decomposes multiple channels into virtual channels, which are linear combinations of physical channels, to minimize interference from signals not generated by the sensor. The factor of the linear combination can be found by correlating the signals of channels other than the signal bands of the sensing unit(s).

[0146] Furthermore, the data evaluation will be explained below. Frequency is the main parameter extracted from the signal acquired from the sensing unit. Due to the high quality factor of the resonator (time constant up to seconds), subsequent excitation pulses typically end before the oscillation has fully decayed (see reference). Figure 13 This necessitates having the correct phase and timing to amplify existing oscillations. This requires real-time frequency extraction between subsequent excitations. Frequency extraction can be achieved using comparison algorithms that minimize the phase difference between the measured signal and a pre-calculated time trace spanning a range of frequencies, or via Fourier analysis (which is the preferred method). High-resolution frequency information can be obtained by zero-filling in the time domain or frequency interpolation followed by locating the resonant peaks in the spectrum using peak-finding or curve-fitting procedures. To further improve the accuracy and reliability of frequency determination, higher-order harmonics of the detected resonant signal can be incorporated into the evaluation, for example, using weighted frequency estimation based on several harmonics or by checking the consistency of frequency determination among several harmonics (see reference). Figure 16 (Spectrum in the upper right corner).

[0147] exist Figure 16 In the referenced example, the second harmonic signal is an order of magnitude smaller than the fundamental frequency signal. Therefore, better filtering is required. Various filter stages can be used to optimize the resonant frequency and its higher harmonic signals, such as analog excitation filters (e.g., DC blocking and low-pass), analog receiver filters (e.g., bandpass filters), and digital receiver filters (e.g., IIR response filters for real-time processing) (sixth-order Chebyshev II type). Figure 16 In the system, the center position of the f0 resonant peak is determined based on the maximum peak in the filtered spectrum. The timing of the next in-phase excitation pulse is calculated based on f0. The system's repetition frequency ranges from 5 to 30 Hz, providing real-time tracking of the frequency response.

[0148] exist Figure 16The image shows the signal spectra with and without digital bandpass filtering (1051 vs. 1050). The dashed points are within the range selected for evaluation. Different point symbols represent different filter types that do not actually show differences and can therefore be ignored. Figure 17 In the middle, the bandpass is attached to a commercial low-noise audio range amplifier, of which the type is FEMTOMSSS Technik GmbH's DLPVA-100-BUN-S. Figure 18 In the diagram, the actual 40dB suppression spectrum of the digital filter is compared to the range of the selected band. No significant difference is shown between the two implementations. The filter shown is applied to... Figure 16 The data shown leads to the difference between 1050 and 1051.

[0149] Based on the determined frequency and the known timestamp of the received signal, the correct timing of the next excitation pulse can be calculated. The number and width of the excitation pulses are suitable for generating oscillations with sufficiently high amplitude to produce a sufficient signal in the receiving coil.

[0150] The following describes in detail the process used to determine the location of a tagged device via a tracking system and thus for locating the medical device to which the tagged device is attached. For such localization, frequency effects are irrelevant (sensitivity encoding, discussed further below) or negligible (gradient field encoding, also discussed further below). For localization using a gradient field method that also acts on the sensor frequency, these compensations are not necessary because it is only necessary to assess frequency variations over sub-second time intervals. These variations are less dependent on the oscillation amplitude.

[0151] The signal from the magneto oscillator is induced by the voltage u in coil i. i The detection of (t) is a result of the change in the magnetic field due to the oscillating motion of the magnetic moment m(t) of the suspended magnetic sphere at position r0:

[0152]

[0153] Among them B S,i (r) represents the coil sensitivity of detection coil i at position r, which is essentially constant over time. In the final step, the magnetic moment has been replaced by the following formula.

[0154]

[0155] in, M is a unit vector describing the spatial orientation of magnetization. sat It is the saturation magnetization of the material used (typically between 1.30 and 1.45 T / μ0 for NdFeB), and V 球体 It is the volume of the magnetic object.

[0156] As can be seen from (4), a large dynamic magnetic moment is desired to induce a high voltage in the receiving coil. Since the size of the marking device and therefore the volume of the magnetic sphere, which can be used as both a magnetic object and a restoring torque unit, must be small in most applications, a large oscillation amplitude can be used to increase the signal, resulting in a large... However, in the recovery field B provided by the fixed sphere rest The recovery torque does not change with the angle between the magnetization m of the oscillating sphere and the angle. (That is, the amplitude of the oscillation) increases linearly:

[0157]

[0158] Considering the torque due to friction with a damping coefficient C and having mass m s and radius r s The torque required for the angular acceleration of a sphere Equations for motion can be established:

[0159]

[0160] Small angle approximation And replace m=M sat V 球体 get

[0161]

[0162] The system's high-quality factor allows for further approximation of C≈0 and makes it possible to calculate the angular resonant frequency as "

[0163]

[0164] Since micro-oscillators are typically driven to amplitudes much greater than 10°, this approximation is invalid in general. For large angles, the recovery torque is smaller, and therefore the frequency decreases, resulting in amplitude-dependent frequencies. in, The change in recovery torque during oscillation also introduces nonlinearity into the sensor response, which manifests as higher-order harmonics with a fundamental frequency in the spectrum.

[0165] In addition to the nonlinear restoring torque, the force between the two magnetic spheres also depends on the mutual orientation of their magnetization:

[0166]

[0167] For a given sensor design, the force always points along the connecting vector of the two magnetic spheres; however, its amplitude becomes zero at the oscillation amplitude of 90°, and even at higher angles it changes from attraction to repulsion.

[0168] If the excitation field generated by the transmitting coil has a constant amplitude, then the oscillation amplitude... The amplitude will decrease as the distance between the coil and the sensor increases (the excitation field decreases), and therefore the frequency will decrease. The amplitude also depends on... Figure 10 The relative orientation between the coil and the sensor is shown.

[0169] For tracking systems, it is necessary to determine the orientation and 3D position of the marking device as described above. Two independent position determination methods can be used for localization. In some cases, one method may be sufficient, while in others, combining the two methods may help increase accuracy or identify systematic errors that lead to contradictory results between the two methods.

[0170] The first method can be based on coil sensitivity location determination / positioning. This method utilizes the fact that each coil i in the coil array has a different spatial sensitivity distribution B based on its position and orientation. S,i (r) is a fact.

[0171] According to formula (3), a single magnetic oscillator (i.e., a single magnetic object) has a characteristic mechanical oscillation, which then generates a response with a characteristic amplitude for each coil, the characteristic amplitude being determined by the dynamic dipole moment of the magnetic object. Compared to B S,i The orientation of (r) is determined accordingly. For the reconstruction of the sensor position and orientation, a set of forward functions as given by formula (4) needs to be determined.

[0172] Finally, we expect a mapping between the six position and orientation coordinates of the marking device in the coordinate system provided by the tracking system and the voltage amplitudes of the fundamental or higher harmonics of all receiving channels. The following formula describes how to get rid of the time dependency in Equation (4) so ​​that only the amplitude needs to be considered. We do this by including all independent variables (i.e., the position vector r = (x, y, z)). T and orientation vector start:

[0173]

[0174] The required coil sensitivity curve can be calculated based on known coil geometry, measured at defined locations and then interpolated, or determined in a hybrid of both (i.e., by fitting a model to the experimental results with appropriate fitting parameters). For magnetized oscillations, a clear description of the oscillation frequency ω and amplitude α0 in the marked frames will be...

[0175]

[0176] The main term indicates the local marker frame and has already used the expansion of trigonometric functions for the low oscillation amplitude α0. Therefore, the time variation will be:

[0177]

[0178] The first term characterizes the fundamental frequency response, and the second term characterizes the second harmonic frequency response. A torsion matrix is ​​used. The magnetization in a general orientation in space can be calculated, i.e. Therefore, starting from (11), the voltage amplitudes of the fundamental frequency and the second harmonic frequency can be determined as follows:

[0179] and

[0180]

[0181] Therefore, the total voltage of coil i will be

[0182]

[0183] From the set of forward functions (14) and (15) and the measured response amplitude, the location and orientation of the marker device can be calculated by solving the system of equations using a nonlinear solver for standard mathematical methods. The accuracy of the solution will improve with the number of receiving coils and with the orthogonality (i.e., the magnitude of the difference) between their respective coil sensitivities. By solving the system of equations in the least squares sense, mismatches between six unknowns and more (or fewer) numbers of receiving channels can be taken into account.

[0184] Position determination / localization can also be performed based on gradient field encoding. While coil sensitivity localization is based on the amplitude distribution picked up by the coil array, the frequency of one or more marking devices can be manipulated to provide independent position information. For this purpose, the tracking system can be provided with a control unit capable of independently controlling each coil in the coil array to generate a non-uniform magnetic or electromagnetic excitation field with an ideal constant field gradient within the workspace. This can be achieved, for example, by applying a low-frequency current to selected coils in the coil array. This additional field alters the recovery field B acting on the oscillating magnetic object. rest And thus its frequency changes (Equation 9).

[0185] Due to the non-uniform nature of the excitation field, frequency variations will depend on the marker's location and orientation. By sequentially executing controls to apply several encoded fields (e.g., field gradients applied at six different orientations), all three locations of the marker and two of the three orientation parameters can be determined. The remaining angles can be determined based on the higher-order response delay of the marking device's sensing unit to an external magnetic or electromagnetic excitation field, however, at the cost of higher field strengths required to generate sufficient higher-order contributions. The basic encoding idea is related to gradient encoding in MRI; therefore, both frequency encoding and phase encoding can be performed.

[0186] For frequency coding, a non-uniform field is applied during signal readout to generate the desired frequency shift. For the desired spatial resolution, the applied coding field strength must be suitable for the frequency sensitivity of the tagging device and the frequency resolution delivered by the system. Assuming a magnetic sphere with a diameter of 0.5 mm is used as the magnetic object, the frequency sensitivity of the NdFeB tagging device is... For a spatial resolution of Δr = 1 mm and an assumed frequency resolution of Δf = 10 mHz, approximately...

[0187]

[0188] A field gradient will be required. This gradient intensity is approximately 100 times lower than that of a typical MRI system. Therefore, a dedicated water-cooled gradient coil is not necessary, but the coils of the transmit-receive array can be used for field generation.

[0189] For phase coding, a non-uniform coding field is applied before signal readout; that is, the position-related frequency shift is applied only within a short window during which the phase shift of the position-related signal accumulates. When the phase resolution is insufficient for precise positioning, the duration and / or amplitude of the phase coding pulses can be varied in sequential excitation, making it possible to discern ambiguities in phase increments (greater than 2π). Thus, complete spatial information is obtained over several readouts. Phase coding with one non-uniform field pattern (e.g., encoding a spatial axis) can be combined with frequency coding with another non-uniform field pattern (e.g., encoding orthogonal spatial axes) for efficient positioning. If a rough location of the marked device is already known according to a sensitivity coding method (which is faster due to its parallel nature), then a few phase coding steps providing only the missing high-resolution (high spatial frequency) components, rather than complete spatial information, are sufficient.

[0190] As described below, comparing the localization results obtained by gradient encoding and sensitivity encoding can be used to identify systematic errors, such as those caused by the background field. Furthermore, it should be noted that the linear response to low-frequency external fields of sensors employing two suspended spheres can be suppressed; in this case, higher-order frequency responses can be used for localization or sanity checks. However, the field sensitivity of these oscillators is much lower, necessitating a higher gradient field for gradient field encoding.

[0191] The following sections will describe parameter determination and location determination for tightly coupled sensors.

[0192] Determining the location (meaning three positions and three orientation parameters) and measuring additional parameters (such as pressure or temperature) using only a few coils is particularly difficult. However, due to space constraints, using only a few coils is cost-effective and preferred in some applications. Therefore, it is desirable to modify the detection process and hardware to operate using only a few coils. One way to do this is by using several marking devices and / or sensors in a coupled manner. Here, coupling means combining several sensors / marking devices, each operating at different known frequencies, in a fixed relative orientation within an assembly. Typically, the sensors are attached to a rigid frame, but technically it is only necessary to know the relative positions of the sensors / marking devices at the evaluation point in time.

[0193] With sufficient sensors, position can be determined using only two coils. This is perhaps the easiest to observe when compared to conventional electromagnetic navigation systems. These typically include several (usually more than six) transmitting coils and one receiving coil, which is positioned and its orientation is evaluated. However, due to the rotational symmetry of the coils, rotation of the coils about their axis (the axis of the dynamic dipole moment) cannot be detected.

[0194] In this comparison, the set of rigidly coupled sensors can be considered as a transmitting array, and a single transmit-receive coil can be considered as a tag. Therefore, the sensor / tag array can be positioned somewhere on a loop around the dynamic dipole axis of the transmitting coil. Note that if the coil is not circular, the loop is not a perfect circle in space, but this does not change the independent variable. Therefore, the position cannot be determined using a single coil, but using two coils (with non-parallel dynamic dipole moments), the symmetry is broken, and the position and orientation of the sensor / tag array can be determined.

[0195] The evaluation of different sensor signals is best accomplished using a complete modeling approach, as described further below. In short, a model is generated for each sensor / tag device in the array, i.e., in the form of differential equations. This model predicts the response of the sensing unit to a given excitation field. Together with the transmitting / receiving system model (including amplifiers, filters, and coils), the overall response of the array can be predicted. Knowing past excitation pulses (typically only a few pulses with decay times are needed), the expected received signal for the tag device position and parameter values ​​can be calculated.

[0196] Pre-existing knowledge can also be incorporated into the process. Only the maximum displacement velocity of the sensor relative to the coil is allowed. Here, the only difference from the previously described method is that the process is not performed for a single sensor, but for a set of coupled sensors in an array, or for several arrays simultaneously. For sensor arrays, there is also a set of available pre-existing knowledge, namely the relative positions and orientations of the sensors / tags in the array. Employing a full-parameter method, or at least a zero-amplitude frequency extrapolation method, is particularly useful because it is difficult to make all many sensors operate simultaneously with the desired amplitude. However, the full-model method is computationally intensive to some extent. To reduce the required computational power, it may be beneficial to first use individually interpreted sensor / tag evaluation methods and use their results as starting values ​​for the final full-model-based position and value reconstruction.

[0197] The following sections will explain some aspects of calibration, with the first being calibration in the presence of conductive and soft ferromagnetic materials.

[0198] The presence of conductive, and particularly soft ferromagnetic, materials can interfere with positioning by distorting the field generated by the oscillating magnet of the marker or sensor and / or by distorting the field generated by one or more transmitting coils. To a lesser extent, it can also alter sensor readings, particularly since compensation for amplitude effects may reduce accuracy. Therefore, a field calibration process is desirable. Furthermore, measures to identify potential field interference are also preferred. Therefore, methods for detecting interference are discussed first.

[0199] Typically, tracking systems like those described herein use a transmit / receive coil array. The coils can be separate transmit-only and receive-only coils, or the same coil can be used for both functions. In any case, in this configuration, one coil can transmit and all other coils directly receive the transmitted signal. The received signal is compared to a stored reference value. If the actual received signal deviates too much from the stored value, some action is triggered, such as a warning for inaccuracy, triggering a self-calibration process, or suggesting a calibration process involving user interaction, or a combination of these. Several coils can also be used for transmission simultaneously. The transmit pulse should contain multiple frequencies. This can be achieved by generating pulses or by using frequency scanning or some intermediate method, as is well known in the literature. Frequency analysis is important because eddy currents traveling on conductive structures are highly frequency-dependent. Therefore, a significant change can be the ratio of the received signal at two different frequencies exceeding a certain limit. It can also be significant if at least one spectral component changes its defined value. However, a uniform change across the entire spectrum can be attributed, for example, to a gain variation in the receive amplifier. Therefore, if, for example, the receive amplifier is constructed in a way that allows for gain variation, this effect can be used to set a new gain value in software to compensate for that gain variation. If a gain change is expected in the transmitting amplifier rather than in the receiving path, the independent variable holds in a similar manner. Here, as a correction, the transmitting amplitude is changed in the calculation model (causing a change in the oscillation amplitude of the sensor, etc.). Theoretically, the impedance of a single coil can also be measured and its change used as an indication of changes in the eddy current environment. However, the ability to measure impedance does not naturally come with electronic equipment and requires specialized equipment. Not only can the coupling of coils be used to detect changes in the eddy current environment, but the known characteristics of sensor / marker devices within their operating range can also be used.

[0200] In particular, sensors can be integrated into the transmitting / receiving coil array itself. Even a single sensor / tag is useful. For example, if a single tag is integrated into the system at a fixed position relative to one or more coils, the change in the tag's response is an indication of a changing eddy current environment. Even more advantageous is the integration of a sensor / tag device that is sensitive to low-frequency magnetic fields but insensitive or only minimally sensitive to other physical properties that may change rapidly. This tag device is not only an indication of a static magnetic field but also an indication of the presence of ferromagnetic material. To detect ferromagnetic material, the coils will be fed not only with current at the frequency of the sensor / tag device's oscillation but also with current at much lower frequencies. The current feeding can be done coil-by-coil or using several coils. If the measured sensor response (i.e., due to frequency variations of the applied low-frequency magnetic field) differs from the stored expectation, it is possible that the ferromagnetic material distorts the field. If enough coils are present in the system, it is not even necessary to place the field-dependent sensor / tag at a known location. With sufficient coils, the position of the tagging device can be determined independently by using the sensitivity of the coils at the oscillation frequency of the sensor / tagging device and by using the sensitivity of the sensor / tagging device to a near-DC magnetic field (gradient field encoding).

[0201] If the positions obtained by both methods diverge, the eddy current (or ferromagnetic) environment has changed. However, it is even better if many such markers are incorporated into the system, rather than just one. It is also better to have them at known locations than unknown locations. However, knowing some properties of the location, rather than having no location information at all, is also useful. A practical way to achieve partial knowledge is to place the sensors / markers on a rigid structure that ensures a known and time-stable position and orientation relative to each other. This calibration "frame" with the sensors / markers can be permanently or periodically placed within the operating volume of the tracking system. If the tracking system detects a deviation from the expected relative position and orientation, the system is affected by eddy currents or ferromagnetic interference.

[0202] Furthermore, if the sensor / tag device is also sensitive to near-DC magnetic fields and the coil array has sufficient coils, the relative position of the sensor / tag can be determined independently at very low frequencies where only ferromagnetic materials interfere with the field, and at the sensor / tag resonant frequency where both ferromagnetism and eddy currents cause field distortion. Therefore, if, for example, the ferromagnetic material contributes to the interference, information about the properties of the interfering object can be generated.

[0203] Furthermore, the optimal method for detecting interference is a complete mathematical model of the transmitting / receiving amplifier, coils, and (one or more) marking devices / (one or more) sensors. This model also includes known positions and orientations, both absolute and relative. In the first step, all positions / orientations and physical parameters are optimized in a way that minimizes error. This step includes, for example, prior knowledge about the relative positions of the fixed-position markings attached to the coil array and within the potential frame. Incidentally, the "frame" does not need to be something introduced solely for calibration; the marking device itself, consisting of many oscillators, can serve as the frame. In the second step, the total weighted error between the expected signal and the delivered signal is calculated. If the error exceeds a certain threshold, it is determined that some material is interfering with the field. Based on the nature of the error (i.e., whether it occurs on an AC-sensitive component or a DC-sensitive component), the nature of the interfering material can be inferred.

[0204] The following section explains the dependence of signal amplitude in different harmonics on sensor orientation relative to a single coil. Specifically, Figure 19 The diagram illustrates the measurement dependence of signal amplitude in different harmonics on sensor orientation relative to a single transmit-receive coil. If the excitation field is parallel to the magnetic dipole orientation, no excitation occurs, and the signal is zero. The highest oscillation amplitude is achieved for orthogonal alignment of the field and dipole. Note that the spatial pattern of even-order harmonics is orthogonal to that of odd-order harmonics. This can be seen from the zero point of the second-order harmonic amplitude at the orientation corresponding to the maximum value in the fundamental signal (first harmonic) and the third harmonic. The amplitude ratio plot (center curve plot) highlights this difference in orientation dependence: the second harmonic changes from zero to a maximum (or singularity) relative to the first harmonic, while the third harmonic is flat relative to the first harmonic. The knowledge that the dynamic response at even-order harmonics is orthogonal to the dynamic response orientation of odd-order harmonics can be used to determine the third orientation angle of the sensing unit.

[0205] The location and / or orientation of the marking device can be obtained through sensitivity encoding or gradient encoding, and thus the location and / or orientation of the medical device with the marking device attached thereto. In some embodiments, a combination of both may be used.

[0206] The following will describe how the oscillation amplitude can be determined by using the amplitude of the harmonics of the fundamental frequency.

[0207] One method for determining the oscillation amplitude is to evaluate the harmonics of the induced signal in the coil. As a nonlinear oscillator, a magnetomechanical oscillator generates harmonics at the resonant frequency in its dynamic dipole moment. These harmonics are picked up in one or more receiving coils. Preferably, care is taken not to suppress these multiples of the fundamental frequency during the sampling and filtering steps. The spectrum of the harmonics depends on the details of the sensor. There can be sensors that primarily generate odd-order harmonics (at 3ω0, 5ω0, ...) and sensors that generate both even-order and odd-order harmonics (at 2ω0, 3ω0, 4ω0, ...). However, hybrid types can be constructed. The dynamic dipole moment of odd-order harmonics tends to align with the dynamic dipole moment of the fundamental frequency, while even-order harmonics tend to align perpendicular to the dynamic dipole moment of the fundamental frequency and perpendicular to the axis of rotation. Therefore, odd-order harmonics are conceptually the easiest to use because, for example, the ratio of the dynamic dipole moment of the third harmonic to the dipole moment of the fundamental frequency is reflected as a corresponding ratio of the recording voltage in a single coil, for example, and is evaluated as the peak amplitude of the spectrum. However, since the amplification in the receiving system can be frequency-dependent, it is preferable to apply a correction to maintain the true ratio of the third-harmonic dynamic dipole moment to the fundamental frequency dynamic dipole moment. This ratio can be measured within a predetermined integration period. For each sensor, a calibration of this ratio against the oscillation amplitude or direct frequency shift can be provided, and thus the correction is applied. In the case of even-order harmonics, the situation is slightly more complex because the direction of the dynamic dipole moment is not aligned with the fundamental frequency dynamic dipole moment. Therefore, it is often necessary to use more than one coil here, or to determine the orientation of the coil relative to the sensor by other means.

[0208] While both sensor position and orientation can be reconstructed with a large coil set (e.g., >= 6), a few coils (e.g., 3-5) should at least allow for the reconstruction of the sensor's orientation relative to the coils using a method similar to the positioning determination method described further in detail below. The true ratio of the dynamic dipole moments of the even-order harmonics can then be determined using the coil sensitivity. The intermediate steps of orientation determination can be omitted, and a direct mapping of the ratio of the fundamental frequency amplitude to the harmonic amplitude in the coils can be established using linear algebraic methods. It should be understood that the methods described here in the frequency domain can be mapped to methods in other bases (e.g., the time domain). In the time domain, frequency analysis is mapped to oscillation shape analysis. These mapping methods are well known in the mathematical literature.

[0209] The determination of the oscillation amplitude based on the time-domain envelope function will be described below.

[0210] Another way to determine the oscillation amplitude is to utilize the nonlinear decay behavior of the signal. The damping of a sensor is typically nonlinear. Nonlinear decay means that with double the stored energy, the average power dissipation of the sensor is not doubled, but increases by a factor slightly greater than 2. This is likely due to the stretching of the filament caused by the force modulation described above. Equation (9) shows that at low oscillation amplitudes, the attraction between magnetic objects is largely constant, but at higher amplitudes, they are no longer constant. This force variation in the first approximation depends on the square of the oscillation amplitude, corresponding to an approximation of the cosine function via a parabola. This squared correlation is the cause of the nonlinearity in the dissipation. The varying force between the magnetic objects causes the periodic stretching of one or more filaments, which leads to the dissipation contribution. Other effects may also cause nonlinear behavior. In summary, these effects result in the envelope shape of the decay curve over a given time depending on the initial amplitude. Therefore, if the sensing unit of the tagging device has a constant initial oscillation amplitude and the distance and / or orientation of the sensing unit of the tagging device changes relative to one or more receiving coils, a scaled version of the initial decay envelope is found. However, if the excitation amplitude of the sensing unit changes, not only does the total amplitude of the decay curve change, but its shape also changes. This means that amplitude effects and distance / orientation effects can be eliminated, and therefore the initial oscillation amplitude can be reconstructed using, for example, a lookup table of pre-recorded decay curves.

[0211] This again leads to the possibility of determining the zero-amplitude frequency or the controlled constant-amplitude excitation as described above. This method requires only a single coil to operate. However, it is somewhat sensitive to movement of the sensing unit during recording, as this also alters the shape of the envelope. Therefore, it is beneficial to incorporate models of possible sensing unit movements into the evaluation. For example, if it is known that the sensing unit of the marking device will not perform rapid acceleration, it is useful to use the assumption of continuous motion to correct the attenuation curve envelope.

[0212] The following text will explain how to determine the oscillation amplitude based on the signal amplitude response to changes in the excitation field.

[0213] Another method for determining the oscillation amplitude is to analyze the sensor signal's response to different intensities of magnetic or electromagnetic excitation fields. In this case, current pulses are systematically varied, and the response of one or more sensors to different excitation pulses is evaluated. The transmitted pulse current, duration, and phase, or combinations thereof, can be varied. For example, suppose there are two excitation pulses. If the distance is high and the local field amplitude is low, the two pulses are designed to generate twice the amplitude that a single pulse would produce. However, if the distance is low and the local field at the sensor is high, the amplitude will be less than twice the amplitude. This results in a characteristic reduction of the received voltage relative to the expected factor of two. Therefore, the ratio of one or more of the received signal (Fourier) amplitudes of the sensor for a given excitation mode is a measure of the excitation amplitude and can again be used to extrapolate to a zero-amplitude frequency and / or for a constant excitation amplitude. In addition, other quantities, such as frequency and decay time, can also be evaluated. The ratios of these quantities are also characteristic of the oscillation amplitude and can be used to extrapolate to a zero-amplitude frequency.

[0214] The following section describes how to determine the correct parameters based on the full model with all contributing factors.

[0215] All the methods described above are merely evaluation methods, and some of them require changes to the emitted field pulses. No hardware changes to the system are required to perform these evaluations. Therefore, implementing them all is logical. This can be done by simply running the evaluations in parallel and combining the results in a manner that minimizes noise (i.e., a weighted average based on relative noise). While this is relatively straightforward and easy to implement, better results can be expected by using a mathematical approach using true integration, which will be outlined below. Conversely, mathematically complex methods are considerably more difficult to implement and may require far too many computational resources to run on cost-effective computer hardware. The foundation of a proper mathematical approach is a mathematical model of the sensing unit. This model predicts the sensing unit's response to a magnetic or electromagnetic excitation field and the state of the current sensing unit. The sensing unit state could be the current deflection angle and rotational speed of a levitated sphere corresponding to a magnetic object.

[0216] In some embodiments, models of the transmitting and receiving coils, including filter and amplifier characteristics, must also be generated. This can be represented by differential equations, but Fourier parametric representations are also not uncommon here, provided that the transmitting and receiving systems are sufficiently linear in nature.

[0217] Finally, a model for the coil's transmit and receive sensitivities is required. This can be simply a set of spatial points with additional sensitivities and an interpolation algorithm between those points. It can also be based on a simulation of the coil, grounded in Biot-Savart's law. This model can then predict the sensor's voltage response at any given location and orientation using a given history of excitation pulses and external parameters. Therefore, the procedure involves varying the sensor's location and orientation, as well as the physical parameters affecting the sensor, in a way that best matches the recorded signals and simulation results in the simulation. Many well-known optimization methods, such as gradient descent or random walks, can be used. The match can be defined as the root mean square of the sum of the differences between measured sample points and simulated sample points. If this quantity is minimized, the match is optimal. Additional constraints can be introduced to alter the best fit, for example, by modeling the expected relative location and orientation, or by constraining the maximum expected sensor acceleration and / or the measured quantity (which, for example, gives constraints on the maximum rate of change of these quantities). Additional sensor inputs, such as an accelerometer on a handheld coil system, can also be used for at least one independent input for distance and orientation variations. Since full-model-based evaluation processes are computationally intensive, they can be combined with one or more of the previous methods to provide a good starting point for further optimization.

[0218] The processor can also be configured to compensate for the effects of gravity, as will be explained below.

[0219] The processor, and more specifically the location determination unit, is also configured to compensate for the Earth's magnetic field and other static field effects.

[0220] A static background field is added to the field of a stationary magnetic object, and thus modulates the recovered field B seen through an oscillating magnet. rest This changes the resonant frequency according to formula (8), and is therefore the source of error sensed via the frequency change of the oscillator. For a 0.5 mm diameter magnetic sphere made of NdFeB with a saturation magnetization of 1.3 T / μ0, the field generated by the fixed sphere at the center of the oscillating sphere is 16.1 mT and 6.8 mT for center-to-center distances of 0.75 mm and 1.0 mm, respectively. The Earth's magnetic field is between 25 and 65 μT. For the aforementioned distances of 0.75 mm and 1.0 mm, the frequency difference between the parallel and antiparallel alignment of the static field component with the maximum Earth's magnetic field of 65 μT will produce frequency differences of approximately 5 Hz and 9 Hz, respectively. Different mitigation strategies for this are described below.

[0221] The mitigation on the marking device side is achieved by using two suspended spheres with the same magnetic dipole moment and moment of inertia (or a suitable ratio of the two quantities) as magnetic objects, instead of the single sphere previously described as a magnetic object. Since the reverse oscillations occur at a single frequency, the first-order effects of a static bias field (such as the Earth's magnetic field) are eliminated.

[0222] Another mitigation strategy is to use an absolute field sensor in the system to measure the amplitude and orientation of the static background field. Based on the sensor orientation determined using the methods discussed earlier, frequency or field corrections can be calculated to achieve improved position determination. To sense the static background field, any magnetic field sensor with sufficient sensitivity and footprint that can be integrated into the tracking system can be used. A cost-effective option could be a 3-axis Hall sensor. An alternative would be a 3-axis array of a temperature-compensated microrobot with a well-defined zero-field frequency.

[0223] The amplitude and orientation of the background field can be determined by the changes in its corresponding frequencies. Ideally, its resonant frequencies are chosen such that they do not interfere with the sensing unit of interest. Instead of correcting for frequency shifts in the evaluation, coils from a multi-coil tracking system can be used to generate small offset fields to balance the Earth's magnetic field and other background fields. If inhomogeneities exist in the field of view due to the presence of ferromagnetic materials, several sets of 3-axis magnetic field sensors can be used to characterize the spatial field variations.

[0224] The marking device should have a high quality factor and requires a large frequency sweep to be sensitive to the measured quantity within the range required for the specific application. The high quality factor is particularly important at high oscillation amplitudes where the highest signal is generated. Both properties can be degraded because the two magnetic objects have a strong attraction, and the force increases strongly with the contraction of distance (the fourth power of the distance, see Equation (9)). The strong force results in a relatively strong tension in at least one filament holding at least one magnetic object. This tension itself does not lead to a dissipation path. However, especially at large oscillation amplitudes, the force between the magnetic objects decreases, and therefore the tension on the attachment decreases periodically. This results in periodic elongation and shortening of the attachment, which can often lead to heat generation. Therefore, power is extracted from the oscillator. The force also depends strongly on the distance between the magnetic objects, and becomes very large if the objects are close to each other.

[0225] To address this problem, a method for reducing force and its variation is described. It consists only of a portion of magnetic material magnetized in the opposite direction next to another magnetic object, such as... Figure 20 As shown.

[0226] exist Figure 20In this embodiment, the sensing unit 4001 includes a magnetic object 4008, which is a permanent magnet suspended from a rigid wall 4010 of the housing 4002 via an attachment portion 4006 (such as a filament, preferably a high-strength wire). The rigid wall 4010 is preferably made of a metal or polymer that is insensitive to external pressure. Furthermore, the remainder of the housing 4002 may also be made of metal or polymer. The housing 4002 may be filled with gas, or it may provide a vacuum space. Another magnetic object 4007 is fixed to the inner end surface of the housing 4002 via an adhesive 4011. The two magnetic objects 4007, 4008 are generally magnetized in opposite directions. However, fixing the magnetic object 4007 also includes a portion 4012 having a reverse magnetization orientation.

[0227] Therefore, if two magnetic spheres are involved, in this example, at least one sphere acquires a cap magnetized in opposite directions. The cap is located next to the other magnetic sphere. If one sphere is stationary and the other is oscillating, it is preferable to have the cap on the stationary sphere. In this way, the dynamic dipole moment of the sensor does not decrease. Only the oscillation frequency is slightly lower. However, the roles of the spheres can also be reversed. The reverse magnetization portion is so small that the net force between the magnetic objects remains attractive at all operating distances. If the reverse magnetization portion is small enough, the attraction condition can be satisfied just when the magnetic objects come into contact.

[0228] There are several ways to create a cap with reverse magnetization. One method is to add some magnetic material to the top of at least one magnetic object. The magnetic material can be soft or hard magnetic. It can be a solid, continuous magnetic object or a magnetic coating, or something in between. The magnetic material tends to align itself in a way that creates reverse magnetization. Furthermore, it tends to adhere to the magnetic object. However, the added material should be glued to the magnetic object, especially if the two main magnetic objects might occasionally touch each other. To maintain the originally desired shape, some material can be removed from the magnetic object to be altered, for example, by grinding.

[0229] Alternative methods exist for forming reverse-magnetized regions. It can be created simply by reverse-magnetizing the desired region of a magnetic object. This can be achieved by applying a strong pulse of current through a conductor near the magnetic object. However, this is not very practical due to overheating. It can be more easily achieved by heating only the affected portion of the magnetic object to near or above the Curie temperature. This will result in a reversal of the magnetization. This effect can be enhanced by applying a pulse or constant magnetic field in the opposite direction. The field can also contain a strong gradient by using some hard or soft magnetic material near the region to be affected. Since the heating must be quite localized, the temperature increase needs to be very rapid so that the total energy deposited into the magnetic object is low and does not bring it close to the Curie temperature overall. A suitable heating source could be a laser. Resistance or induction heating methods can also work.

[0230] Furthermore, some methods for determining the presence of field interference can also serve as a good starting point for methods used to compensate for the effects of field interference. This exemplary method can be most easily illustrated when it is assumed that a conductive material, rather than a ferromagnetic material, is present to induce eddy currents. When the above model is applied, the correct position can be obtained from the evaluation of the near-DC correlated signal (gradient field encoding), but incorrect position and local field amplitude can be obtained at the sensor frequency and its harmonics (coil sensitivity encoding). Therefore, the higher frequency field can be distorted in a way that matches expectations. After applying the distortion, all positions and sensing unit readouts will be improved. It is beneficial not only to rely on position evaluation based on the near-DC magnetic field, but also because AC sensitivity encoding is much faster.

[0231] The most critical part of this compensation method is determining the correct model for the AC field distortion. A simple solution is, for example, to parameterize the field shift function using a simple 3D polynomial. This means using the field values ​​at the locations transformed by the 3D polynomial, rather than the actual field values. This is computationally efficient, but may lack physical insight, and it's not obvious how measurements of coil coupling can be incorporated into this framework. Therefore, it's better to use a model that is closer to physical reality. For example, it's preferable to use a field model of the conductive plates near the coil system to induce the desired field distortion.

[0232] Therefore, the position, angle, thickness, and size of some virtual plates are essentially variable until the model's expectations and measured data match. How to model such conductive plates is well-known in electromagnetic simulation literature. This type of modeling has the added advantage of incorporating the shape of objects that might appear in a particular environment. Thus, if a special device (e.g., an X-ray C-arm) is brought close to the field of view, that device is known and can be modeled beforehand, so that only the precise orientation and position must be optimized by the system software. Another advantage is that the assumed location of interfering objects can be displayed by the system, or the data can be transmitted to a second system to perform the display task. In this way, the user can be specifically pointed at the object interfering with the measurement, and the user may want to move or remove them. During this process, the coupling data of the coils essentially acts as an array of metal detectors. The incorporation of ferromagnetic materials is conceptually the same as that of conductive materials that generate eddy currents. However, ferromagnetic material simulation is computationally slightly more intensive, and because of the potential lack of a clear reference position defined by a dedicated marking device, it may not yield an exact location. But again, it is best to model a set of ferromagnetic materials (such as sheets and rods) and place and deform them around the coil array in the simulation. Here, providing a database of possible ferromagnetic objects would be highly beneficial for the model. Furthermore, the process of mutual coupling measurements can be enhanced by measuring harmonic generation in the coil environment. The presence of harmonics is a strong indicator of soft ferromagnetic materials, and the measured signals provide valuable input regarding the size and location of the object.

[0233] The generation of excitation pulses will be described below.

[0234] The tracking system and optionally the field generator preferably include software that generates the timing and shape of the excitation pulses. The excitation pulse generator is preferably aware of the hardware's capabilities. Different types of amplifiers and filters may be present. One type of amplifier is capable of generating current waveforms that closely follow very arbitrary paths. These are referred to herein as "analog amplifiers".

[0235] Another type can only increase the current at a predetermined rate, decrease the current at a similar rate, and keep it approximately constant. Essentially, these amplifiers apply a voltage with a positive or negative sign at the coil or act as a short circuit. These are referred to here as "digital amplifiers." Digital amplifiers can have different switching speeds, i.e., the number of state changes allowed per unit time. If the switching speed is much higher than the oscillation speed, the digital amplifier again functions like an analog amplifier. Therefore, this type of amplifier can be conceptually considered as an analog amplifier.

[0236] If the switching speed is only approximately the same as the oscillation frequency of the marking device, then the handling must be slightly different. However, this is a more difficult case, so all discussion will focus on this. This type of amplifier has several advantages over analog amplifiers. The main advantage is that the amplifier's efficiency is typically very high, and 98% efficiency is easily achieved. Another advantage is the ease of interface connection with computing systems. A matching circuit can exist between the amplifier and the coil. The simplest matching circuit is simply a capacitor connected in series with the coil. Using a matching circuit, the maximum current through the coil at a given amplifier supply voltage is increased. However, this matching circuit has the disadvantage of blocking low-frequency currents.

[0237] Some sequences may require low-frequency current. The solution to this problem can be twofold. First, a matching circuit that is transparent at both high and low frequencies can be provided. An example of such a circuit is a coil or coil-capacitor circuit connected in series with a first matching capacitor. Another approach is to have a switch that bypasses the matching circuit and closes when near-DC current is needed. A capacitor can also be integrated in the bypass path if the resonant frequency is low enough. In the same way, multiple switches and capacitors can be used to provide different matching frequencies throughout the series. Moreover, note that even when the circuit is tuned near DC, some current at the marked device frequency is still available. It should be noted that DC current may not necessarily be available during readout. There are two main elements to providing this capability. First, DC current interference with the readout must not be allowed. There is always a problem if the transmitting and receiving coils are combined. A DC source can provide a short-circuit path to the signal. This must be avoided, and a proper matching circuit prevents it.

[0238] The matching circuit must introduce a sufficiently high impedance between the coil and the DC source. This can be achieved by adding an additional coil in series with an inductor on the order of the transmitting / receiving coil inductance. If this is not necessary, the inductor can have a parallel switch to short-circuit it. Many other solutions exist. The second condition is that the DC source does not introduce too much noise, i.e., the current source noise does not interfere with the accurate measurement of the marking device. This can be achieved with a suitable analog filter in the case of DC transmission.

[0239] The filter can be bypassed by a suitable switch (e.g., a MOSFET optocoupler) during AC pulse transmission. It is also feasible to completely avoid switching actions in the DC source during signal reception and use only the slowly decaying current in the coil. It is also feasible to perform only a few switching actions during reception and discard the received data if it is corrupted. The DC field source can also be a completely separate coil, or the field generator can be a (moving) permanent magnet. This avoids most problems. Another problem with the presence of DC current during signal reception is that the coil may provide different environments for the sensor. This means, for example, that some coils can be shortened for AC current, and the AC field no longer penetrates the coil, thus altering the field value in nearby coils. This effect must be considered when calculating position and / or orientation. Two main field elements interact with the marking device. One is the near-DC amplitude of the current, i.e., the average current value over a period of approximately 0.1 seconds (approximately 0.01 seconds to approximately 1 second). The other is the Fourier amplitude at the sensor / marker's resonant frequency (as a complex value, since phase is important). Therefore, the first task is to map these two values ​​to the generation of the sequence.

[0240] The following sections will describe the mapping of Fourier amplitude and current to specific time-domain pulse patterns as needed.

[0241] It is also useful to generate a software subsystem that performs this precise type of mapping (i.e., to obtain the desired near-DC current and the desired Fourier amplitude (and frequency) as input and generate a time-domain pulse sequence). It is also desirable for the software to return information on whether the desired value can be achieved within the limitations imposed by the hardware application, such as the maximum current or maximum heating or regulatory limits, such as patient heating or peripheral nerve stimulation. Instead of simple yes / no information, information about the severity of undesirable side effects can be provided. This information can be provided by each individual transmit channel (each transmit coil). Another return value could be the actual best-fit output DC current and (one or more) Fourier amplitudes. The input can be not only a combination of frequency and Fourier amplitude, but also various Fourier amplitudes at different frequencies. The maximum length of the pulse sequence can also be a parameter for the input of this function. Internally, it works as follows: In the case of an analog amplifier, a first result can be generated simply by performing an inverse Fourier transform on the desired Fourier amplitude (and DC value) within the desired transmit time. If the process results in a waveform that cannot be achieved due to some limitations, this is reported back, and a scaled version can be generated. Possible filter characteristics are considered through appropriate convolution. If several switching filter states exist, all switching filter states can be tested, and the switching filter state with the lowest requirements for the amplifier can be selected. Note that several heuristics are available, making it unnecessary to evaluate all filter states in most cases. For example, if a better filter is available, a filter with a frequency far from the resonant frequency can be omitted. For digital amplifiers, the inverse Fourier transform (including filter effects) provides a good starting point for optimization. In this first approximation step, the resulting peaks in the time spectrum are approximated by two (or at most a few) ramps and the flat region in between. Thus, for example, a half-cycle of a sine wave that starts and ends at zero is first approximated by a flat (zero) portion, then a ramp, then a flat portion, then a ramp, and finally a flat (zero) region. The timing of the different portions is arranged in such a way that they reach approximately the same region. Following this first approximation is a second step, where the starting positions of the ramps and flat portions are shifted to achieve a best fit with the desired Fourier values. The best fit can be the least squares sum of the differences (complex numbers) between the desired and realized Fourier components. All commonly used optimization algorithms, such as gradient descent, can be used.

[0242] The following text will describe the mapping from the expected Fourier values ​​at the marked device to the current in the coil.

[0243] The next higher level of abstraction in the pulse generation process is the software item that takes specific field Fourier values ​​and directions at a particular location as input and converts them into a requirement for the current in the coil. The evaluation algorithm typically provides a measure of the sensor / tag's location and orientation. This location is not, and does not need to be, a location in 3D space. However, a 3D location is the ideal case. For example, if only one coil exists, it might only be possible to determine the field value at the sensor in the sensitive direction. However, this also translates to some virtual location and orientation in 3D space. Therefore, these cases do not require special processing in the software. The conversion to the coil current requirement is then a result of the optimization process. There exists a model that calculates the Fourier field components at a specific spatial location from the current in the coil. This is the basis of optimization, where the Fourier components of the coil current are optimized in a way that generates the desired field components. Often, there is no explicit way for the coil current to form the desired field. There may also be cases where the desired current is incompatible with the limitations of the hardware system. Lower-level software returns values ​​describing the negative impacts, and the software uses this information to optimize the current. The goal of optimization is to achieve a good trade-off between the Fourier components of the field realized at the tag device and the negative impacts. This means that deviations and side effects from the desired field are combined into a numerical value, and for that value, a standard optimization algorithm is used to find its maximum or minimum. The combination of numerical values ​​can be a weighted sum of squares. Of course, for this entity, a large number of working mathematical combinations can be found. Finally, this part of the program returns to the calling program (at a higher level) the field realized at the stated location and the quality value used for its optimization.

[0244] The generation of the desired field Fourier values ​​for the marker / sensor will be described below.

[0245] At this level of abstraction, the software system actually handles the measurements that need to be taken. Therefore, the input to the program is the current requirement of how accurately and quickly things should be measured. These requirements depend on the specific application using the sensor / tag and are therefore not part of this document. The requirements can vary considerably. For example, if only a single sensor is involved, the requirement would be, for instance, to measure a single quantity as accurately as possible every 0.1 seconds. If the application is a tracking solution with multiple coupled tags, the desired result could be a position update of the entire tag assembly every 0.1 seconds, regardless of which tag / sensor contributes to the signal (based on coil sensitivity), and an independent position check using a gradient method every 1 second. The program can also access the current state of the sensor / tag (position / oscillation parameters, etc.) and the simulation model described elsewhere in this document. Based on this, the optimal excitation field Fourier values, including the orientation for each tag device, can be calculated. These parameters can be passed to a lower software level previously described (with the desired execution somewhere in the future) to ultimately generate a current. In the case of a single sensor, this will work immediately, and the plan can be written to a hardware output buffer. However, for tracking, for example, tag device components, there may not be a pulse shape that perfectly excites all individual tag devices. In particular, the phase will not fit all individual tag devices. Therefore, the software may have to try to focus the optimal excitation only on a subset of the existing tag devices and try to find a solution that gives a working pulse sequence. This is the general working principle of this software optimization. It attempts to change the desired excitation of various sensors and focus on some sensors to still obtain the desired result. The conceptually simplest approach is to iterate through all possible subsets of tag devices and check which subset of excitation gives the best information about the desired parameters. Since there are many possible subsets, the program needs to add some heuristics to reduce complexity. For example, if a given tag device is excited, it can first observe what other tag devices are also excited and these tag devices can always be grouped together. If a suitable solution is found, it can be written to the output buffer. Including a near-DC magnetic field may require additional logic, depending on the hardware implementation. If the hardware can apply a DC magnetic field while recording the signal, the software does not have to do anything very special except apply one or a few gradients during readout. However, if the DC gradient and readout are incompatible, an additional optimization step is needed to generate the correct DC field or gradient sometime between excitation pulses. The logic behind the optimization remains the same. Parameters are varied until the simulation predicts sufficiently good measurements for the application.

[0246] The generation of the startup sequence will be described below.

[0247] Algorithms typically assume that considerable knowledge already exists about the tagging devices that can be used to optimize the sequence. This is usually not entirely available at the start of the sequence. For example, depending on the application, it may be known how many tagging devices should be present in the application and what frequency range is permissible. However, the precise frequencies and locations will be unknown. Therefore, a specific start-up sequence is needed to attempt to find all possible tagging devices at all possible locations. The simplest possible start-up sequence is as follows: The working volume is divided into a spatial 3D or abstract grid. If there are not enough coils for full 3D encoding, an abstract grid is used. Each spatial point is divided into different orientations. The procedure iterates through each location and each angle at that location, applying the highest transmission power for a given frequency and preset transmission time. The system then records the potential signal from the sensor / tag. Typically, a single transmission pulse excites not only one tagging device but also many others simultaneously. However, the procedure ensures that even the tagging device with the weakest possible signal will be detected. An optional next step is to individually excite each sensor with different amplitudes. This allows the extraction of nonlinear properties. Another optional step is to excite each tagging device in the presence of a DC field, or to measure the signal phase (again in all directions) after the DC field, to determine the tagging device's sensitivity to the DC magnetic field. These basic procedures can be greatly accelerated by using some knowledge about the system. For example, it's possible that if distant volumes have already been searched for the sensor / tag, many or all of the closer volumes will receive the highest possible amplitude at least at some angles. Therefore, only a few remaining parameters need to be applied to the closer volumes. The same logic can be used to evaluate the nonlinear characteristics of the sensor / tag or its response to a DC magnetic field.

[0248] The strategies used for high temporal resolution measurements are explained below.

[0249] For many applications, high temporal resolution is desired. Therefore, for both position and parameter determination, a strategy utilizing magnetomechanical oscillators to achieve high temporal resolution is desirable. The simplest method for high temporal resolution is simply to reduce the repetition time. The repetition time is the time interval between subsequent excitation pulses. After each excitation pulse, the frequency and amplitude are determined, from which physical values ​​and position can be calculated, as described elsewhere. However, the quality factor of a marking device tends to be relatively high, and the oscillation amplitude has not yet decreased significantly by the time of the next excitation pulse. To always obtain the desired excitation for the marking device, the phase of the next excitation must be considered. Typically, we want "in-phase excitation," i.e., excitation in a way that the marking device receives energy from the very beginning of the excitation pulse. How to optimize timing is described elsewhere. In-phase excitation minimizes the transmitted energy, and therefore the excitation pulse length can be kept to a minimum. This increases the overall signal-to-noise ratio.

[0250] High repetition rates have several drawbacks. First, the system typically cannot receive values ​​during and shortly after the excitation pulse, and therefore the signal-to-noise ratio may not be optimal. Second, each transmitted pulse destroys some knowledge about the phase of the sensor oscillation. Phase information can only survive to a certain extent when the excitation pulse and sensor orientation are kept tightly controlled and precisely known, which is technically challenging. Phase information over longer time intervals may be useful because information about the average frequency (and therefore the average physical quantity) is encoded within it. When evaluating double-length intervals, the measurement of the average physical quantity is much more accurate than evaluating only the first and second halves independently and averaging the two results. Therefore, it may be worthwhile to extract more than one measurement from a single signal pulse, rather than having as many excitation pulses as measurements. This can be easily accomplished by dividing the signal into several sub-parts and evaluating each sub-part individually.

[0251] This simplistic approach doesn't account for the fact that measurements improve with longer datasets. To incorporate this, the set can be stratified into subsets, and each subset within each stratum can be evaluated, scaling the mean to match the longer dataset. Thus, for example, the dataset (an undisturbed decaying signal) could be evaluated holistically first. It could then be split into two, and the two split datasets evaluated separately. Then, the same numerical value could be added to each result so that its mean matches the mean of the entire set. This process could be repeated to eventually have 4, 8, etc., subsets. This approach can be mathematically refined into a full-model-based evaluation. To do this, a model of the evolution of the physical parameters (and possibly the spatial movement of the sensor) is generated. This model could be a polynomial of some degree or some other suitable mathematical function. This function should describe the physical properties of the measured quantity in a way that requires only a small number of parameters. Thus, for example, when the parameter is blood pressure, a Fourier series model is better, as it describes the pressure waveform of a heartbeat better than a polynomial. The parameters are then varied to match the measured dataset as well as possible. If discrete measurement points are ultimately needed, they can be easily computed using the model's output for certain time points.

[0252] Those skilled in the art, through studying the accompanying drawings, the disclosure, and the claims, can understand and implement other variations of the disclosed embodiments when practicing the claimed invention.

[0253] In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality.

[0254] A single unit or device can perform the functions of several items recited in the claims. Although some measures are recited in different dependent claims, this does not mean that combinations of these measures cannot be used advantageously.

[0255] Determination tasks, such as determining the resonant frequency based on inductive signals, determining the position and / or orientation signals based on the resonant frequency, and determining the calibration curve, which are performed by one or more units or devices, can also be performed by any other number of units or devices. Control of the tracking system can be implemented as program code devices and / or dedicated hardware.

[0256] Computer programs can be stored / distributed on suitable media, such as optical storage media or solid-state media supplied together with or as part of other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunications systems.

[0257] No reference numerals in the claims should be construed as limiting the scope.

[0258] This invention relates to a tracking system and a tagging device, the tracking system being used to track the tagging device, and the tagging device being adapted to be attached to a medical device. The tracking system is provided for use during surgery, wherein the tagging device includes a sensing unit comprising a magnetic object providing a permanent magnetic moment, wherein the sensing unit is configured to convert an external magnetic or electromagnetic excitation field into mechanical oscillations of the magnetic object; and wherein the tracking system includes: a field generator for generating a predetermined magnetic or electromagnetic excitation field for inducing mechanical oscillations of the magnetic object of the sensing unit; a transducer for converting the magnetic or electromagnetic field generated by the induced mechanical oscillations of the magnetic object into one or more electrical response signals; and a position determination unit for determining the position of the tagging device based on the one or more electrical response signals.

Claims

1. A marking device configured to attach to a medical device, the marking device comprising: The shell, and The sensing unit includes a magnetic object that provides a permanent magnetic moment, the magnetic object being arranged such that it can rotate away from its equilibrium orientation when an external magnetic field or electromagnetic field is applied to it. The sensing unit further includes a recovery torque unit, which is used to convert an external magnetic excitation field or electromagnetic excitation field into mechanical oscillations of the magnetic object. The restoring torque unit is adapted to provide a restoring torque to force the magnetic object back to the equilibrium orientation if an external magnetic torque has already caused the magnetic object to rotate away from the equilibrium orientation, thereby allowing the rotational oscillation of the magnetic object.

2. The marking device according to claim 1, wherein, The magnetic object is capable of rotating about a virtual rotation axis, which passes through the magnetic object at its center.

3. The marking device according to any one of the preceding claims, wherein, The recovery torque unit includes a torsion spring mechanism configured to provide the recovery torque, and / or wherein the magnetic object is configured to perform rotational motion about the main axis of the sensing unit.

4. The marking device according to any one of the preceding claims, wherein, The magnetic object is rotatably attached to the housing via a first attachment portion, and the restoring torque unit is fixedly or rotatably attached to the housing via a second attachment portion, wherein the magnetic object and the torque unit are arranged within the housing.

5. The marking device according to claim 4, wherein, The magnetic object is configured to be suspended from the first attachment portion.

6. The marking device according to any one of the preceding claims, wherein, The sensing unit is configured to detect one or more locations and / or positions of the marking device, wherein the magnetic object is a magnetic ball.

7. The marking device according to any one of claims 2-6, wherein, The restoring torque unit includes another magnetic object adapted to provide the restoring torque.

8. The marking device according to claim 7, wherein, The other magnetic object is configured to rotate about the virtual rotation axis, which passes through the other magnetic object at its center, wherein the other magnetic object is rotationally symmetrical about the virtual rotation axis.

9. The tagging device according to any one of the preceding claims is configured to perform one or more of the following: tracking the location of the medical device and / or being configured to track tissue.

10. The marking device according to any one of claims 4-9, wherein, The attachment portion is configured to be attached to the magnetic object by any one or a combination of the following methods: fixed attachment by using glue, clamp attachment, knot attachment, thermally generated beads, ultra-high molecular weight polyethylene (UHMWPE) fiber attachment, and / or notch attachment.

11. The marking device according to any one of the preceding claims, wherein, The medical device is any of the following: guidewire, ultrasound probe, probe, wire, liver shunt device, liver shunt device including wire structure, stent, catheter, electrical implant, orthopedic implant, surgical instrument, imaging probe, endoscope, bronchoscope, swallowable pill, one or more aneurysm coils, one or more vena cava filters, heart valve, needle, tube, radioactive seed.

12. The marking device according to any one of the preceding claims, wherein the marking device has an elongated shape, the maximum dimension being less than or equal to 5 mm and / or the minimum dimension being equal to or greater than 1 mm.

13. An assembly of a marking device, comprising a plurality of marking devices according to any one of the preceding claims, wherein, At least some of the multiple marking devices are configured to apply different resonant frequencies.

14. A tracking system for tracking components of a tagging device according to any one of claims 1-12 or according to claim 13, wherein, The tracking system includes: A field generator is configured to generate a predetermined magnetic or electromagnetic excitation field to induce mechanical oscillations in the magnetic object of the sensing unit. A transducer configured to convert a magnetic or electromagnetic field generated by mechanical oscillations caused by the magnetic object into one or more electrical response signals. A location determination unit is configured to determine the location of the marking device based on the one or more electrical response signals.

15. The tracking system according to claim 14, wherein, The position determination unit is adapted to: Based on the one or more electrical response signals, at least five degrees of freedom for the marking device relative to a coordinate system provided by the tracking system are determined, the at least five degrees of freedom including the position of the marking device relative to the tracking system and at least two orientation angles.

16. The tracking system according to any one of claims 14-15, wherein, The field generator includes a magnetic field generating array comprising a plurality of generating units arranged in a predetermined spatial arrangement, wherein one or more electrical response signals indicate characteristic mechanical oscillations of the magnetic object of the sensing unit caused by at least some of the plurality of generating units, and wherein the position determining unit is adapted to determine the position of the marking device based at least in part on one or more electrical response signals indicating the characteristic mechanical oscillations.

17. The tracking system according to claim 16, wherein, The position determination unit is adapted to determine the amplitude of the characteristic mechanical oscillation of the magnetic object of one or more of the plurality of generating units based on the one or more electrical response signals.

18. The tracking system according to any one of claims 13-17, wherein the tracking system is configured to perform tracking from a distance greater than 30 centimeters, the tracking being based on the tagging device.

19. The tracking system according to any one of claims 13-18, wherein, Establish or utilize a repetition frequency, which is between 5 and 30 Hz.

20. A tracking method for tracking a tagging device according to any one of claims 1 to 11 or a component of a tagging device according to claim 12 using a tracking system according to any one of claims 13 to 19, wherein, The method includes: A magnetic or electromagnetic excitation field is generated to induce mechanical oscillations in the magnetic object of the sensing unit. The magnetic field or electromagnetic field generated by the mechanical oscillation caused by the magnetic object of the sensing unit is converted into one or more electrical response signals. The location of the marking device is determined based on the one or more electrical response signals.

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