Tracking system and marker device to be tracked by the tracking system

Abstract

To provide a tracking system for tracking a marker device attached to a medical device.SOLUTION: The marker device 501 comprises a sensing unit comprising a magnetic material 507,508 which is caused to mechanically oscillate by an external magnetic or electromagnetic excitation field, wherein the tracking system comprises a magnetic field generator for generating a predetermined magnetic or electromagnetic excitation field to induce the mechanical oscillation of the magnetic material, a transducer for converting the induced mechanical oscillation of the magnetic material 507,508 into one or more electrical response signals, and a position determination unit for determining the position of the marker device based on the one or more electrical response signals.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a tracking system for marker devices attached to medical devices, the respective marker devices, corresponding medical devices, a tracking method and a tracking computer program for tracking the marker devices. [Background technology]

[0002] Invasive, and particularly minimally invasive, medical procedures are a commonly used means of properly assessing and / or treating intravascular conditions in patients.

[0003] It is known, especially in minimally invasive medical procedures, to track medical devices used in such procedures by electromagnetic forces. However, such electromagnetic tracking has the drawback that, in order to determine not only the position but also the orientation of the medical device, the medical device needs to be equipped with several electromagnetic marker devices, each marker device adapted for, for example, 3 to 5 degrees of freedom (DoF).

[0004] Furthermore, commonly known electromagnetic marker devices are significantly larger than 1 mm. For example, the size of the electromagnetic marker device used by the tracking system disclosed in the paper "Validation of the Calypso Surface Beacon Transponder" by B. Maxwell et al., Journal of Applied Clinical Medical Physics, Vol. 17 (2016), pp. 223-234, is 8 mm.

[0005] A further problem is that electromagnetic marker devices often cannot be read from a relatively long distance, e.g., more than 30 cm. For example, the system disclosed in the above-mentioned paper by B. Maxwell et al. allows for reading marker devices from a distance of around 16 cm. Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, it is an object of the present invention to provide an improved tracking system, an improved marker device, a respective medical device, and a tracking method and a computer program for tracking the marker device. More specifically, it is an object of the present invention to provide a marker device that is small and can accurately indicate the position of a human, particularly a medical device used during a surgical operation on a patient undergoing a minimally invasive procedure. It is a further object of the present invention to provide a tracking device that can accurately track such a marker device. [Means for solving the problem]

[0007] According to a first aspect of the present invention, there is provided a surgical tracking system for tracking a marker device attached to a medical device. The marker device has a sensing unit having a magnetic body that produces a permanent magnetic moment, where the sensing unit is configured to convert an external magnetic or electromagnetic excitation field into mechanical vibration of the magnetic body. The tracking system includes a magnetic field generator that generates a predetermined magnetic or electromagnetic excitation field to induce mechanical vibration of the magnetic body of the sensing unit, a transducer that converts the magnetic or electromagnetic field generated by the induced mechanical vibration of the magnetic body into one or more electrical response signals, and a position determination unit that determines the position of the marker device based on the one or more electrical response signals.

[0008] Thus, a tracking system is provided that can be used to track the position and / or orientation of a medical device using respective marker devices attached to the medical device, which tracking system is particularly adapted to track the position and / or orientation of a medical device during a surgical procedure, and more particularly during a minimally invasive surgical procedure, as it allows for accurate position and / or orientation determination for the medical device using fairly small marker devices.

[0009] To this end, tracking systems use marker devices that are composed of a sensing unit made of a magnetic material having a permanent magnetic moment. When the sensing unit of the marker device is subjected to a predetermined external magnetic or electromagnetic excitation field, the magnetic material begins to vibrate in response to the excitation field. The mechanical vibration of the magnetic material generates a magnetic or electromagnetic (response) field, which is then converted by a respective transducer into one or more electrical response signals. These response signals are then used to derive the position of the marker device. More specifically, the mechanical vibration of the magnetic material can result in position-dependent magnetic field variations that can typically be expressed in terms of position response signals and can be used by a positioning device to determine the position of the marker device, and thereby the position of a medical device to which the marker device is attached.

[0010] In this context, the term medical device may specifically refer to a device used in a medical procedure. In some embodiments, the medical device may specifically correspond to a device used during a surgical procedure, in particular during a minimally invasive surgical procedure. In some embodiments, the medical device may refer to an interventional tool used in an interventional procedure performed on a human being, in particular a patient.

[0011] Generally, the proposed positioning technique using tracking systems and marker devices can be used for any medical device for which it is advantageous to locate / localize it. Thus, in some embodiments, the term medical device can be used for any additional medical device that can be used for localization. As an example, mention can be made of bandages and patches. In such cases, it may be important for safety reasons to track the position and / or orientation of such bandages and patches, for example, after a surgical procedure to determine if everything has been properly placed or removed (if necessary).

[0012] The term marker device is used in particular to refer to any device capable of indicating the position and / or orientation of an object to which it is attached. In particular, the term marker device refers to a device having a magnetically sensitive unit, i.e. a sensing unit comprising a magnetic material that reacts to a magnetic or electromagnetic excitation field by undergoing a respective mechanical vibration, in particular a rotational vibration. Such mechanical vibration is used by a tracking system to generate an electrical response signal that is used to derive the position (and orientation) of the marker device.

[0013] The term magnetic field generator specifically refers to a generator of a magnetic or electromagnetic excitation field. In some embodiments, the magnetic field generator has a magnetic field generating array including a plurality of generating units. In some embodiments, these generating units specifically correspond to respective coils arranged in a coil array. In some embodiments, each of these coils is controlled independently of the others. In some embodiments, such independent control is used to provide a non-uniform magnetic or electromagnetic excitation field, where ideally the magnetic field gradient is constant throughout the field of effect.

[0014] Generally, the concept of the present invention is based on the fact that the response of a magnetic body to a magnetic or electromagnetic excitation field (measured in terms of mechanical vibrations) can provide information about the position and / or orientation of a marker device having a sensing unit that includes the magnetic body, since the magnetic or electric excitation field affects the magnetic body differently depending on the relative position of the magnetic or electromagnetic excitation field.

[0015] Various possible approaches can be used to determine, or localize, the location of the marker device based on the response of the mechanical oscillator to a magnetic or electromagnetic excitation field, thereby localizing the medical device to which the marker device is attached. In this regard, two specific location approaches, also referred to as localization approaches, are used. One approach is to determine the location based on the coil sensitivity of each coil in the coil array. This approach involves calculating a spatial sensitivity profile B , based on the position and orientation of each coil in the coil array of the magnetic field generator in the tracking system. s、i In this case, the magnetic material of the sensing unit generates a specific mechanical vibration for each coil, specifically, B s、i (γ) versus the dynamic dipole moment for mechanical magnets

number

[0016] Another approach is based on gradient field encoding. This approach exploits the fact that the frequency of the marker device can be manipulated to provide independent position information. To this end, a non-uniform magnetic field, ideally with a constant magnetic field gradient over the entire area of ​​effect, is generated, for example, by applying low-frequency currents to selected coils in a coil array. Such a non-uniform magnetic field can be achieved, for example, by the independent control of the coils described above.

[0017] This additional magnetic field acts on the magnetic material of the sensing unit as a resonant magnetic field B rest , thereby changing the frequency of the vibration. Due to the non-uniformity of magnetic or electromagnetic fields, this frequency change depends on the position and orientation of the marker device.

[0018] These localization techniques are described in more detail below, and in some embodiments, one of these techniques may be sufficient, while a combination of both techniques may be useful to improve accuracy or to identify systematic errors (e.g., strong ferromagnetism in the workplace) that may lead to conflicting results between the two methods.

[0019] In some embodiments, the position determination unit determines at least five degrees of freedom of the marker device relative to a coordinate system provided by the tracking system based on the electrical response signal or more electrical response signals, the at least five degrees of freedom including the position and at least two orientation angles of the marker device relative to the tracking device.

[0020] In some embodiments, the tracking system defines or comprises a coordinate system with respect to which the marker device is located. To this end, a position determination unit determines at least five degrees of freedom (DoF) of the marker device based on one or more response signals. These five degrees of freedom allow determining not only the position but also the orientation (in terms of two orientation angles) of the marker device with respect to the coordinate system of the tracking system. This scheme therefore makes it possible to determine the position and orientation of the marker device, and therefore the position and orientation of the medical device to which the marker device is attached, using only one marker device.

[0021] According to some embodiments, a tracking system determines the position of a plurality of marker devices, each having a respective sensing unit, the magnetic material of each sensing unit being vibrable, particularly by rotation, such that different resonant frequencies produce different magnetic or electromagnetic fields that are converted into one or more electrical response signals specific to each marker device, whereby a position determination unit is provided for determining the position of one or more of the plurality of marker devices based on the respective one or more electrical response signals.

[0022] Preferably, the tracking device determines the position of several marker devices, the magnetic material of several marker devices being vibrable, preferably vibrable by rotation, such that different resonant frequencies result in different frequencies of their induced signals for each marker device, and the position determining unit is adapted to also determine the orientation of the marker devices based on the different frequencies of the emitted induced signals. By using marker devices with different resonant frequencies it is possible to distinguish between the various marker devices and to determine the respective positions, and preferably also the respective orientations, for each marker device.

[0023] Several marker devices may be attached to only one medical device, in which case the positioning unit may be adapted to determine the shape, position, and / or orientation of the medical device based on the determined positions of the several marker devices. The positioning unit may also be adapted to determine the shape, position, and / or orientation of the medical device based on the determined orientations relative to the several marker units. The marker devices may, in particular, determine the shape, position, and / or orientation of the medical device. While the marker devices are used, in particular, to determine the shape, position, and / or orientation of the medical device, it should be understood that the marker devices may also be used to determine the shape, position, and / or orientation of other elements to which multiple marker devices are attached, such as body tissues. In some embodiments, multiple marker devices are distributed among the medical devices used to treat the tissues and among the respective tissues in order to gather information about both elements and / or the relationship of both elements to each other.

[0024] In one embodiment, the position determination unit is adapted to determine the position of a particular marker device relative to the positions of other marker units. Also, the orientation of the marker device may be determined relative to the orientation of other marker devices. However, the position, and possibly also the orientation, may be determined relative to another reference. For this purpose, the tracking system also comprises a respective output unit for outputting the determined position and / or orientation of the marker device.

[0025] In some embodiments, the tracking system positioning unit is configured to compensate for the dependence of one or more electrical signals on temperature, and in some embodiments, the positioning unit is configured to apply a compensation algorithm to perform such compensation.

[0026] In a preferred embodiment, the tracking system, in particular the position determination unit, is configured to compensate for the dependence of one or more electrical signals on temperature, for which purpose the temperature-dependent behavior of the magnetic body, i.e. the temperature dependence of the resonant frequency of the magnetic body, is preferably determined either experimentally or by respective calculation.

[0027] In some embodiments, the tracking system therefore comprises a temperature sensor and / or an input means for inputting the temperature. To compensate for the temperature dependence, an algorithm is provided that takes the input temperature into account and correlates the input temperature with the known dependence of the resonant frequency of the magnetic material. This can result in a more accurate position determination / localization technique by removing the temperature effect from the electrical signal. That is, in some embodiments, temperature compensation is performed using a compensation algorithm, i.e., embodied in program code.

[0028] Alternatively or additionally, temperature compensation can be achieved by various means, such as a physical compensation element. That is, in some embodiments, the marker device itself, and more particularly the sensing unit, can compensate for the dependence of the resonant frequency of the mechanical vibrations of the magnetic body on temperature. To this end, the sensing unit comprises a compensation element adapted to modify the resonant frequency in a first frequency direction in response to temperature changes, which is opposite to a second frequency direction in which the resonant frequency of the sensing unit would be modified in response to temperature changes if the compensation element were not part of the sensing unit. In this way, the temperature-induced shift of the resonant frequency can be reduced or even eliminated. The first frequency direction thereby corresponds in particular to a direction of higher or lower frequency, and the opposite second frequency direction corresponds to a direction of lower or higher frequency, respectively.

[0029] The compensating element preferably has a magnetic material whose magnetization changes, thereby changing the resonant frequency according to temperature. In this case, the magnetic material is selected such that the direction of the correction of the resonant frequency is the first frequency direction, and is arranged in the sensing unit, in particular in the casing of the sensing unit. The compensating magnetic material is preferably arranged adjacent to the magnetic body and / or adjacent to the further magnetic body described below. This makes it possible to design a marker device in which undesirable temperature dependence can be significantly reduced or even eliminated, technically relatively easily and without taking up too much space in the casing.

[0030] In some embodiments, the position determining unit is configured to apply a compensation algorithm to compensate for one or more of the static background magnetic field and the dynamic background magnetic field.

[0031] In some embodiments, the positioning unit further applies a compensation algorithm to compensate for the static background magnetic field and / or the dynamic background magnetic field. The static background magnetic field is added to the magnetic field of the fixed magnetic body to produce a recovery magnetic field B seen by the vibrating magnetic body. restTherefore, the resulting resonant frequency changes, which is a source of error when using frequency changes of a vibrating magnetic material to determine position.

[0032] In some embodiments, this compensation is performed by a respective algorithm embodied in the tracking system, in particular applied by the positioning unit. For this purpose, the tracking system is equipped with one or more absolute magnetic field sensors adapted to measure the magnitude and orientation of the static background magnetic field. Based on the orientation of the marker device, frequency or magnetic field correction values ​​can be calculated to arrive at the correct position and / or orientation values.

[0033] To detect the static background magnetic field, any magnetic field sensor with sufficient sensitivity and range that can be incorporated into the tracking system can be used. As an example, a three-axis Hall sensor will be mentioned. Additionally or alternatively, a three-axis array of temperature-compensated micro robots with a well-defined zero magnetic field frequency can be used. From the change in the static background magnetic field with respect to each frequency, the magnitude and direction of the static background magnetic field can be determined. Ideally, their resonant frequencies are selected so that they do not interfere with the frequency of the sensing unit.

[0034] Instead of correcting for frequency shifts during evaluation, the coils of a multi-coil tracking system can be used to generate small offset magnetic fields to counterbalance the background magnetic field and / or the Earth's magnetic field. When an inhomogeneous magnetic field appears in the field of view due to the presence of ferromagnetic materials, a series of several three-axis magnetic field sensors can be employed to characterize the spatial magnetic field variations. Based on an interpolated background magnetic field map derived from these measurements, correction values ​​for sensing units with known positions and orientations can be calculated, and respective corrective offset magnetic fields can be applied, or a combination of the two correction methods can be used.

[0035] According to some embodiments, it is also possible to mitigate the effects of static and / or dynamic background magnetic fields on the marker device side. In this case, a sensing unit of the marker device can be designed that employs two suspended spheres with the same magnetic dipole moment and moment of inertia (or a suitable ratio between the two quantities). Since the counter vibrations occur at a single frequency, the first-order effects of static bias magnetic fields, such as the Earth's magnetic field, are nullified.

[0036] In some embodiments, the position determining unit is configured to apply a compensation algorithm to compensate for non-linearities resulting from different vibration amplitudes of the mechanical vibrations.

[0037] The position-determining unit is configured to compensate for nonlinearities in the system resulting from different vibration amplitudes of the mechanical vibrations of the magnetic material of the sensing unit. In some embodiments, this extends to further optional processing steps, in particular applying an inverse nonlinear filter to counteract the nonlinearities of the tracking system. This measures the nonlinearities of the tracking system and constructs a computational filter that reverses the effects of the nonlinearities. This is particularly useful when low-cost components, which tend to have more nonlinear behavior, are used.

[0038] Alternatively, a nonlinear filter is used as the first processing stage. If more than one signal is used, additional signal processing stages are required. If at least one receiving channel does not detect a response from the sensing unit of the marker device, thereby providing a constant background signal, this signal (and all other such signals) is correlated with the received signal, and the correlated components are subtracted from the signal-bearing channel. This subtraction can be performed in the time domain, the frequency domain, or a combination of both. If there are no channels without any sensor signals, a data processing strategy sometimes referred to as a "virtual gradiometer" is used. This decomposes many channels into virtual channels, which are linear combinations of physical channels, to minimize interference of response signals not caused by sensing. The coefficients for the linear combination are found by correlating the signals of the channels excluding the signal band of one or more sensing units.

[0039] In some embodiments, the magnetic field generating apparatus comprises a magnetic field generating array having a plurality of generating units arranged in a predetermined spatial arrangement, wherein one or more electrical response signals are indicative of characteristic mechanical vibrations of the magnetic body of the sensing unit induced by each of the plurality of generating units, and wherein the position determining unit is adapted to determine a position of the marker device based at least in part on the one or more electrical response signals indicative of the characteristic mechanical vibrations. In some embodiments, the position determining unit is adapted to determine, for each of the plurality of generating units, an amplitude of the characteristic mechanical vibration of the magnetic body from the one or more electrical response signals.

[0040] In some embodiments, the magnetic field generating device may include multiple generating units spatially arranged in a magnetic field generating array. In some embodiments, this spatial arrangement may be two-dimensional. However, three-dimensional spatial arrangements are also accepted as possibilities. In some embodiments, the magnetic field generating array corresponds to a coil array and the generating units correspond to one or more coils. In such cases, localization / localization is based at least in part on the coil sensitivities of the individual coils in the coil array. This approach is described in more detail below.

[0041] In some embodiments, the tracking system further comprises a control unit, wherein the magnetic field generating device comprises a magnetic field generating array having 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 others of the plurality of generating units, the control unit is adapted to control at least some of the generating units such that at least one spatial excitation field component of the magnetic or electromagnetic excitation field is modifiable by said control, and the position determining unit is adapted to determine a position of the marker device based at least in part on one or more signals indicative of the modification of the at least one spatial excitation field component. In some embodiments, the magnetic field generating device is adapted to sequentially generate a series of different further magnetic or electromagnetic encoding fields that vary in space and / or time, and the position determining unit is adapted to determine a position of the marker device based at least in part on one or more electrical responses transduced by the transducer based on the magnetic or electromagnetic field generated by the induced mechanical vibration of the magnetic body in response to each of the series of different further magnetic or electromagnetic encoding fields.

[0042] While localization can also be based on gradient field encoding, coil sensitivity localization is based on the amplitude distribution picked up by the coil array and can manipulate the marker's frequency to provide independent position information. To this end, a non-uniform magnetic field, ideally with a magnetic field gradient that is constant over the entire field of effect, is generated, for example, by applying low frequency currents to selected coils in the coil array.

[0043] This additional magnetic field acts on the magnetic material of the sensing unit as a resonant magnetic field B rest , thereby changing its frequency. Due to the non-uniformity of the magnetic field, this frequency change depends on the position and orientation of the marker device. By sequential application of several encoding magnetic fields (e.g., magnetic field gradients are applied in six different orientations), all three position parameters and two of the three orientation parameters of the marker can be determined. The remaining angles can be deferred from the higher-order response of the sensor to the external magnetic field, but at the expense of stronger magnetic field strength, to provide sufficient higher-order involvement. The basic encoding concept is related to gradient encoding in MRI, and therefore both frequency encoding and phase encoding can be performed.

[0044] In frequency encoding, a non-uniform magnetic field is applied during signal readout to produce the desired frequency shift. For the desired spatial resolution, the applied encoding field strength needs to be matched to the frequency sensitivity of the marker device and the frequency resolution provided by the tracking system.

[0045] In phase encoding, a non-uniform encoding field is applied before a single readout, i.e., a position-dependent frequency shift is applied only during a short window of opportunity to produce a position-dependent signal phase shift. If the phase resolution is not sufficient for accurate localization, the duration and / or amplitude of the phase encoding pulse can be varied in successive excitations to resolve ambiguities in the phase addition (greater than 2pi). Thus, full spatial information is obtained over the course of several readouts.

[0046] Phase encoding with one non-uniform magnetic field pattern (e.g., encoding one spatial axis) can be combined with frequency encoding with another non-uniform magnetic field pattern (e.g., encoding an orthogonal spatial axis) for effective localization. If the rough marker location is already known by a sensitivity-encoding method (which is faster due to its parallelism), it is sufficient to use several phase-encoding stages to provide the missing high resolution (high spatial frequencies) but not the full spatial information.

[0047] As described in the overview of this invention, a comparison of the localization results obtained by gradient versus sensitivity encoding can be used to identify systematic errors, for example, caused by background magnetic fields. It should also be noted that the linear response of a sensing unit employing, for example, two suspended magnetic spheres as the magnetic material to low-frequency external magnetic fields is suppressed, and in this case, the higher-frequency response can be used not only for localization but also for sanitary checks. However, the magnetic field sensitivity of these oscillators is so low that higher gradient magnetic fields are required for gradient magnetic field encoding.

[0048] In another aspect, a marker device for attachment to a medical device is provided. The marker device includes a casing and a sensing unit having a magnetic material that provides a permanent magnetic moment. The sensing unit is configured to convert an external magnetic excitation field or an external electromagnetic excitation field into mechanical vibration of the magnetic material, the induced mechanical vibration being independent of external pressure experienced by the sensing unit. In some embodiments, the casing is specifically a hard casing. In some embodiments, the marker device has an elongated shape with a maximum dimension of 5 mm or less and a minimum dimension of 1 mm or less. In some embodiments, a magnetic material is disposed within the casing such that the magnetic material is rotatable from an equilibrium orientation when the external magnetic excitation field or the external electromagnetic excitation field acts on the magnetic material. Thus, the sensing unit further includes a restoring torque unit that provides a restoring torque to return the magnetic material to an equilibrium orientation when the external magnetic excitation field or the external electromagnetic excitation field rotates the magnetic material from an equilibrium orientation that allows mechanical vibration of the magnetic material at a resonant frequency.

[0049] According to a further aspect, there is provided a marker device that allows for determining the position and / or orientation of a medical device to which the marker device is attached, the marker device having a casing and a sensing unit that allows for converting an external magnetic or electromagnetic excitation field generated by a magnetic field generating device into a mechanical, preferably rotational, vibration of a magnetic body provided in the sensing unit and having a permanent magnetic moment.

[0050] The sensing unit having the magnetic body may in particular comprise a casing or may be arranged within a casing. Specifically, the magnetic body is arranged within the casing. Thus, specifically, the magnetic body is arranged within the casing such that it can be rotated from its equilibrium direction by an external magnetic torque acting thereon. The external magnetic torque is the result of an external magnetic or electromagnetic excitation field acting on the magnetic body. That is, in some embodiments, the magnetic body is rotated from its equilibrium position by the external magnetic or electromagnetic excitation field.

[0051] The sensing unit further includes a restoring torque unit that applies a restoring torque to return the magnetic body to its equilibrium orientation when the magnetic body is rotated from its equilibrium orientation by an external magnetic or electromagnetic field. This results in rotational vibration of the magnetic body excited by the external magnetic torque from the external magnetic or electromagnetic field. The rotational vibration is thereby performed by the magnetic body at a respective resonant frequency determined by the spatial position and spatial orientation of the sensing unit, and thus the marker device, in the external magnetic or electromagnetic field. The resulting magnetic or electromagnetic field caused by the mechanical and rotational vibration of the magnetic body is thereby converted into one or more respective response signals. These response signals are therefore determined by the resonant frequency of the vibration.

[0052] In some embodiments, the rotational vibrations ultimately result in respective induced signals, because these induced signals depend on the position and orientation of the marker device in the external magnetic or electromagnetic field. These induced signals are generated in an excitation and induced signal unit of the tracking system. Specifically, the excitation and induced signal unit may comprise: i) a first coil adapted to generate a magnetic field that applies a magnetic torque to rotate the magnetic body of the tracking device from its equilibrium direction, thereby exciting rotational vibrations of the magnetic body; and ii) a second coil adapted to generate induced signals that depend on the spatial position and spatial orientation of the marker device. This allows the position and orientation of the marker device, i.e., six degrees of freedom of the marker device, to be determined using only one marker device, such that the position and orientation of a medical device equipped with the marker device can be determined.

[0053] Furthermore, this arrangement allows the tracking system to track based on marker devices that are relatively far away, for example, greater than 30 cm. The marker devices may also be relatively small, for example, smaller than 1 mm. To this end, in some embodiments, the casing of the marker device is cylindrical, and the outer diameter of the cylinder is smaller than 1 mm, more preferably smaller than 0.5 mm, and even more preferably smaller than 0.3 mm.

[0054] Preferably, the magnetic body is rotatable around a vertical rotation axis that runs longitudinally through the center of the magnetic body, and the magnetic body is rotationally symmetrical with respect to the vertical rotation axis. Specifically, the magnetic body may be a magnetic sphere or a magnetic cylinder. The restoring torque unit may also include a torsion spring mechanism that provides restoring torque. Additionally or alternatively, the restoring torque unit may also include a further magnetic body that provides restoring torque.

[0055] In one embodiment, the magnetic body is attached to one end of a mounting portion, such as a filament, and the other end of the mounting portion is attached to the casing. The mounting portion is adapted to prevent the magnetic body from striking the further magnetic body embodying the restoring torque unit due to their magnetic force and to rotate and oscillate the magnetic body. Preferably, the further magnetic bodies are fixedly attached to the casing. However, further magnetic bodies that are capable of rotating and oscillating relative to the casing may also be arranged within the casing. Specifically, the further magnetic body may be attached to one end of a mounting portion, such as a filament, and the other end of the mounting portion may be attached to the casing.

[0056] In a preferred embodiment, the further magnetic body is rotatable about a vertical axis of rotation that runs through the center of the further magnetic body, and in this case, the further magnetic body is rotationally symmetrical about the vertical axis of rotation. The magnetic body may be a magnetic sphere or a magnetic cylinder. Furthermore, it is preferred that the vertical axis of the magnetic body and the further magnetic body are aligned with each other.

[0057] These techniques can produce rotational vibrations of the magnetic material that can make the recovery torque, and therefore the entire marker device, relatively small, the resonant frequency of the marker device can be set as desired, and the configuration of the marker device can still be relatively simple.

[0058] Here, in order to determine position, the resulting mechanical rotational vibration of the magnetic material must be independent of any external pressure to which the sensing unit is subjected. For example, if the marker device is used to track a medical device used in a minimally invasive procedure, the vibration of the magnetic material inside the marker device must not be affected by any pressure acting on the medical device from the outside, such as blood pressure or circulatory pressure.

[0059] For this reason, the marker device has a casing in which the sensing unit is located, because this casing has one or more rigid walls, specifically a hard casing, i.e., a casing with walls that do not change shape when external pressure acts on it. This means that the positioning of the magnetic body inside the casing is not significantly affected by external pressure due to the fact that the walls do not bend when subjected to external pressure. This, in turn, results in the spacing between the magnetic body and the restoring torque unit, which is also provided on the casing, remaining the same regardless of the pressure acting on the sensing unit from the outside. Therefore, the magnetic force acting between the magnetic body and the restoring torque unit does not change due to a spacing change caused by pressure-induced bending of any walls in this case, and is therefore independent of any external pressure acting on the sensing unit. Therefore, the resonant frequency of the rotational vibration caused by an external magnetic or electromagnetic field acting on the magnetic body is also not affected by any spacing change. This means that any resonant frequency change will typically be affected by the position and orientation of the sensing unit, and thus the marker device, in an external magnetic or electromagnetic field. As a result, the sensing unit is employed to locate / localize the marker device and thereby any medical device to which the marker device is attached.

[0060] Such a magnetic material located in a hard casing provides a miniature marker device with a relatively small size of less than 1 mm, making the marker device particularly suitable for use in tracking systems for tracking medical devices during minimally invasive surgical procedures.

[0061] According to yet another aspect, there is provided a medical device for use during surgery, the medical device having attached thereto a marker device as described above. The marker device is to be tracked by the tracking system as described above. In some embodiments, the medical device comprises a tip adapted to have the marker device attached thereto. In some embodiments, the medical device may include an interventional device or implant, particularly one or more of an electrical implant and / or an orthopedic implant. In some embodiments, the medical device may specifically include, for example, one or more of a surgical instrument, an imaging probe, an endoscope, a bronchoscope, or an ingestible tablet. Alternatively or additionally, the medical device may include one or more of a catheter, a wire, particularly a guidewire, a stent, one or more aneurysm coilings, one or more vena cava filters, a heart valve, a shunt, a needle, a wire, a stylet, or a radioactive seed. In some embodiments, the medical device is elongate. The medical device is adapted to have attached thereto a plurality of marker devices as described herein, the plurality of marker devices being arranged along a longitudinal axis of the medical device.

[0062] According to another aspect, there is provided a tracking method for tracking a marker device attached to a medical device as described hereinabove, using the tracking system as described hereinabove. The tracking system is particularly for use during surgical procedures. The tracking method comprises the steps of generating a magnetic or electromagnetic excitation field to induce mechanical vibrations of the magnetic material of the sensing unit, converting the magnetic or electromagnetic field generated by the induced mechanical vibrations of the magnetic material of the sensing unit into one or more electrical response signals, and determining a position of the marker device based on the one or more electrical response signals. In yet another aspect, there is provided a computer program including program code means for causing the tracking system as described hereinabove to perform the steps of the tracking method as described hereinabove, when the computer program is executed on a computer controlling the tracking system. [Brief explanation of the drawings]

[0063] In the following drawings: [Figure 1] 1A and 1B are diagrams illustrating schematically and exemplarily an embodiment of a marker device according to a first embodiment; [Figure 2] 1A and 1B are schematic and exemplary illustrations of a marker device attached to a medical device; [Figure 3] 3A-3C show different views of the marker device and the medical device according to FIG. 2. [Figure 4] 10A and 10B illustrate, schematically and exemplarily, further implementations of marker devices in a tracking system for tracking a medical device. [Figure 5A] 10A and 10B illustrate, schematically and exemplarily, further implementations of marker devices in a tracking system for tracking a medical device. [Figure 5B] 10A and 10B illustrate, schematically and exemplarily, further implementations of marker devices in a tracking system for tracking a medical device. [Figure 6] 1A and 1B are diagrams illustrating, in a schematic and exemplary manner, implementations of marker and tracking devices for determining the location of a tumor. [Figure 7] 1A-1C are schematic and exemplary diagrams illustrating implementations of multiple marker and tracking devices for determining the position, orientation, and / or shape of a region of interest in a patient's tissue. [Figure 8] 1A and 1B are diagrams showing, in a schematic and exemplary manner, an embodiment of a medical device corresponding to a wire for treating cerebral aneurysms, to which a marker device is attached. [Figure 9] 1A and 1B show schematic and exemplary embodiments of a medical device corresponding to an intrahepatic stent device to which a marker device is attached. [Figure 10] 1A-1C are schematic and exemplary diagrams illustrating embodiments of temperature-compensated marker devices. [Figure 11] FIG. 1 is a schematic and exemplary diagram illustrating a tracking system for tracking a marker device, according to an embodiment of the present invention; [Figure 12] FIG. 1 is a schematic and exemplary diagram illustrating a tracking system for tracking a marker device, according to an embodiment of the present invention; [Figure 13] 1A and 1B are schematic and exemplary diagrams illustrating excitation pulses and resulting induced voltages; [Figure 14] FIG. 1 is a schematic, exemplary illustration of a multi-coil array integrated into the mattress of a patient bed of an imaging system. [Figure 15] FIG. 1 shows a schematic and exemplary diagram of a receiving coil of a tracking system that detects magnetic or electromagnetic field variations induced by mechanical vibrations. [Figure 16] FIG. 1 illustrates a frequency spectrum used to determine a resonant frequency. [Figure 17] FIG. 2 is a schematic and exemplary diagram of an analog receive filter; [Figure 18] FIG. 1 is a diagram illustrating an exemplary Chebyshev II bandpass frequency response. [Figure 19] FIG. 10 shows the measured dependence of signal amplitude at different harmonics on sensor orientation for a single transmit / receive coil. [Figure 20] 10A and 10B show schematic and exemplary diagrams of further embodiments of marker devices; DETAILED DESCRIPTION OF THE INVENTION

[0064] 1 shows, in a schematic and exemplary manner, an embodiment of a marker device 501 attached to a medical device to be tracked by a tracking system used during a surgical procedure, in particular during a minimally invasive surgical procedure on a human, in particular a patient. The marker device 501 has a sensing unit made up of two magnetic spheres 507, 508.

[0065] The magnetic ball 508 is suspended from a mounting 506, such as a filament, and is therefore free to rotate about the primary axis of the sensing unit. In this embodiment, the further magnetic material 507 is fixed. However, in other embodiments, the further magnetic element may also be suspended from a mounting, such as a filament, and therefore may be free to rotate about the primary axis of the sensing unit.

[0066] At equilibrium, the magnetic bodies 507 and 508 are aligned with their magnetizations antiparallel. An external magnetic field pulse can be used to initiate a resonant rotational oscillation. The attractive forces determine the resonant frequency of this oscillation, which for a spherical suspended magnet is given by:

number

number

[0067] The magnetic field variations caused by this vibrating magnetic mass 508 can be detected via an induced voltage in one or several detection coils of a transducer configured to convert the magnetic or electromagnetic field caused by the mechanical vibration of the magnetic mass 508 of the sensing unit into an electrical response signal. A Fourier transformation of the time trace of the detection signal results in a spectrum that allows the determination of the resonant frequency.

[0068] Due to the low resonant frequency of a few kHz, all non-ferromagnetic metals can be used as structural or coating materials, since the magnetic field is not shielded by the metal. Similarly, the marker device can be encased in a non-ferromagnetic metal body without affecting the process, as long as the thickness of the metal does not significantly exceed the skin depth. At these frequencies, for a very good conductor such as copper, the skin depth is on the order of 1 mm, while for nitrile, the skin depth is about 10 mm.

[0069] Thus, sensing unit 501 includes two magnetic bodies 507, 508, which, at equilibrium, are aligned with antiparallel magnetizations. An external magnetic field pulse provided by a respective magnetic field generator can be used to initiate a rotational motion of suspended magnetic body 508, which in the embodiment of FIG. 1 corresponds to a magnetic sphere, about the main axis of the sensing unit, while the other magnetic body 507, also embodied as a magnetic sphere in this particular embodiment, is fixed. In another embodiment, if the other magnetic body 507 is also suspended in free space and allowed to undergo rotational motion, both magnetic bodies 507, 508 can undergo resonant counter-oscillation.

[0070] The use of magneto-mechanical oscillators to determine the position and / or orientation of a marker device relative to a tracking system is known. It is also known to use LC oscillators to perform such position determination. A marker device including a sensing unit is shown, for example, in the article "Validation of the Calypso Surface Beacon Transponder" by B. Maxwell et al., Journal of Applied Clinical Medical Physics, Vol. 17 (2016), pp. 223-234. However, the marker devices shown herein are typically 8 mm in size. It would be advantageous to provide smaller marker devices. Unfortunately, as marker devices become smaller, the accuracy of measurements also decreases. Therefore, position determination measurements using the marker devices listed above are not ideal, especially for small marker devices.

[0071] In other words, as the size is reduced, the power level that can be generated by the resonator and the dynamic dipole moment caused by that power decrease. This can be seen in the following equation: The Q factor of the resonator cannot be higher than the Q factor of the coil. An approximation to the Q factor of the coil can be written as:

number

[0072] The above equations overestimate the Q values ​​that can be achieved in practice. Nevertheless, these values ​​lead to a successful system: since the dynamic dipole moment of the LC oscillator is Qx external magnetic field x volume, the signal is γ 5 In the case of a mechanical oscillator (where the energy is stored elastically), the signal scales according to γ 3 and for example in the embodiment described with reference to FIG. 1 (magneto-mechanical oscillator, energy stored in a magnetic field), the signal scales according to γ 2 Therefore, the proposal presented here is very well suited for sensor miniaturization.

[0073] Therefore, the problems mentioned above are avoided, for example, by the design proposed in Figure 1. Since the energy is stored mainly in the magnetic field, a high Q factor can be achieved relatively easily. High vibration amplitudes are also easily possible. The sensing unit typically also employs a mounting such as a thin filament that is not subject to severe wear. This allows the resonance to be easily changed 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 a coordinate system given by a tracking system, as will be further described below.

[0074] In an embodiment with a fixed sphere, the diameter of the fixed sphere may be 620 μm, while the diameter of the vibrating sphere 108 may be 500 μm. The magnetic moment of the vibrating sphere 108 is m≈70 μAm 2 The fundamental frequency may be f0 ≈ 2 KHz, and the Q factor may be roughly Q ≈ 500. The SNR depends on a) the coil used to read out the resonant frequency, b) the sensing device, and the coil parameters. For a handheld coil with a diameter of 10 cm, 200 turns, and 10 Ohm resistance, the theoretically achievable SNR at a spacing of about 30 cm and a sampling duration of 0.1 s is roughly 4000. However, typical SNR values ​​for a fixed sphere demonstrator may be in the 10-100 range if little or no measures are taken to suppress background signals. Therefore, the noise is mainly determined by fluctuations in the lead-in power harmonics. At half the sphere diameter, i.e., 250 μm for the vibrating sphere, the magnetic moment is m ≈ 9 μA m 2 , the fundamental frequency may be f0≈4 kHz, the Q factor may remain unchanged, and the theoretical SNR may drop to about 1000.

[0075] There are several ways how the attachment portion can be attached to the rotatable magnetic body 508.

[0076] For example, through-hole mounting can be used. In this case, a hole is drilled through the center of gravity, approximately perpendicular to the magnetization. Magnetic materials are hard and brittle, but there are several methods for drilling holes, such as pulsed lasers or electrical discharge machining (EDM). The thread is threaded through the hole and glued in place. Threading is best accomplished using a vacuum suction process. Several types of adhesives can be used. An economical option is a light-curing adhesive, which has a low viscosity and easily draws the thread through the hole via capillary forces alone. Additionally or alternatively, the attachment portion can be fixed to the magnetic material 508 by mechanical means, such as by tying a knot in the thread or by creating some other thick spot on the thread, such as an adhesive drop or a thermally induced (molten) bead. The latter is particularly easy with UHMWPE fibers. This attachment method only slightly reduces the magnetic dipole moment, thus preserving a good signal. The shape of the magnetic material cannot be significantly altered, which is important in the case of spheres.

[0077] Clamp mounting can also be used. In this case, the magnetic body is divided into at least two components. The dividing plane is preferably perpendicular to the magnetization and parallel to the thread attachment direction. The thread, or filament, is placed in this plane. Precise alignment is not necessary. A second magnetic part is placed on top. The magnetic parts are usually held together by magnetic force. Finally, an adhesive is applied to secure everything in place. The type of adhesive is preferably the same as in the through-hole mounting process. Additionally, grooves can be made in one or both of the magnetic bodies to narrow the overall gap between them. This method achieves nearly the same results as the through-hole method, but does not require any special equipment for its manufacture. Typically, magnetic sub-body parts are made by attaching two (identical) magnetic bodies rather than by dividing one complete magnetic body. The downside is that this process can be wasteful and somewhat labor-intensive, since two magnetic bodies are used initially.

[0078] The least expensive method is to attach the end of the thread directly to the magnetic material 508 using a suitable adhesive. This magnetic material 508 is held and aligned by some kind of tool. A suitable magnetic field can accomplish both functions. This tool can be in the shape of a funnel through which the thread passes and the magnetic material is magnetically attached to the funnel opening. An adhesive is applied to the funnel and allowed to harden. The assembly is then pulled out of the tool and the unwanted portion of the thread is cut off. This method is very inexpensive and makes full use of the magnetic material. The drawbacks are that it adds significant material, lowering the resonant frequency and requiring space in the finished device.

[0079] In further embodiments, a further adhesive process may be used in conjunction with the structure to which it is attached. The thread can be attached to the magnetic material 508 by first attaching the thread to the non-magnetic material and then gluing the non-magnetic material to the magnetic material. The non-magnetic material can be manufactured by bond molding or an equivalent inexpensive process. The shape of the non-magnetic material must be such that it allows for easy thread attachment, i.e., it has holes or a fastening mechanism, perhaps as simple as an indentation, whereby the non-magnetic material is glued to the magnetic material. Alternatively, the non-magnetic material may be clamped or screwed onto the magnetic material. This method is simple and inexpensive, but may require significant space depending on the application.

[0080] In principle, all of the methods mentioned for thread-to-magnetic attachment apply equally to thread-to-casing attachment, except that the through-hole method is considered the better choice because the casing material usually fares better with it. Clamping is also a good choice; this will be cheaper, but sealing the finish can be more difficult.

[0081] In the embodiment according to Fig. 1, at least the wall 515 of the casing 502 of the marker device is a rigid wall that is insensitive to external pressure. This keeps the distance between the magnetic body 508 and the restoring torque unit constant from the point of view of the magnetic body 507, which in turn keeps the mechanical vibration of the magnetic body 508 due to the influence of external pressure unchanged, since the magnetic interaction between these two magnetic bodies does not change due to the (pressure-induced) inter-sphere spacing change. Therefore, the resonant frequency is also insensitive to any inter-sphere spacing. This means that the mechanical vibration of the magnetic body 508 induced by the interaction of an external magnetic or electromagnetic excitation field with the restoring torque field of the magnetic body 507 depends mostly on the position and / or orientation of the marker device relative to this excitation field, thus allowing for transformation into the coordinate system given by the tracking system.

[0082] The marker devices described above may be adapted to be attached to any type of medical device to be tracked during a medical procedure. To this end, FIGS. 2 and 3 schematically show a marker device 501 attached to a medical device 510. In the specific embodiment of FIGS. 2 and 3, the medical device 510 corresponds to a guidewire. However, it should be understood that the medical device may be any other type of medical device, specifically any type of medical instrument, and even more specifically any type of medical instrument used in performing (minimally invasive) surgery, where tracking is advantageous. In some embodiments, the marker devices are also used to track various elements, such as tissue, bandages, etc. Examples of additional devices and / or elements to be tracked are shown in FIGS. 4-6 and are further described below.

[0083] As mentioned, in the specific embodiment shown in Figures 2 and 3, the marker device 501 is attached to a medical device 510 corresponding to a guidewire. Portions 511 and 512 of the guidewire can be used to form a casing 502 for the marker device 501, with a fixed magnetic ball 507 as a restoring torque unit and a rotating magnetic ball 508 attached to a solid wall 515 as a magnetic body via attachment portion 506.

[0084] The dimensions shown in Figures 2 and 3 are illustrative only. These dimensions may vary. However, the dimensions shown are highly suitable for tracking during interventional procedures on human patients. Applying scaling laws to the demonstrator SNR, it can be seen that the dimensions shown provide sufficient SNR and accuracy for remote surgery far enough away to fully penetrate the patient. Therefore, a marker device 501 can be attached to a guidewire to enable tracking of the guidewire during an interventional procedure.

[0085] It may also be beneficial to use marker devices for other medical devices and / or other elements, as shown in Figures 4-10.

[0086] To this end, FIG. 4 shows at least one marker device 501 attached to the ultrasound probe 610 for tracking the position of the ultrasound probe 610 during ultrasound measurements on the patient 100 .

[0087] FIG. 5A shows the marker device 501 described above attached to a stylet 710 that is used to introduce the stylet into the patient's tissue. In the embodiment of FIG. 5A, only one marker device is used to track the stylet following introduction into the patient's tissue, and this marker device is attached to the first end 711 of the stylet 710. Alternatively, as shown in FIG. 5B, multiple marker devices 501, 501', 501" may be attached to the stylet 710 along the length of the stylet 710 from the first end 711 to the second end 712. This multiple marker devices 501, 501', 501" allows the position of the stylet 710 to be tracked relative to a coordinate system provided by the tracking system, but also allows the orientation and / or shape of the stylet 710 to be determined.

[0088] FIG. 6 schematically illustrates the use of marker device 501 to track tissue. More specifically, in the exemplary embodiment of FIG. 6, marker device 510 is used to determine the position and / or orientation of tumor 810. A positioning unit 900 of the tracking system is then used to determine the position of the marker device, and thus the position of tumor 810, relative to a coordinate system provided by the tracking system (not shown). This allows for more accurate localization of tumor 810 for subsequent resection. To aid in understanding the dimensions of marker device 501 used for tumor localization, FIG. 6 also illustrates marker device 501 in relation to a human finger 101. As can be seen from this helpful representation, marker device 510 is minimal in size while still providing accurate localization.

[0089] FIG. 7 schematically illustrates another easy-to-understand embodiment in which one or more marker devices 501, 501′, 501″, 501′″ are employed for position determination and therapy control during a medical procedure performed on a patient. In such a case, information provided by marker devices 501, 501′, 501″, 501′″ attached to the patient's prostate 102 is combined with information from further sensors, such as pressure sensors, temperature sensors, radiation sensors, etc. This combination makes it possible to determine the position and / or orientation of the prostate 102 relative to a coordinate system provided by the tracking system, and further measure parameters such as temperature, pressure, or radiation using the sensors mentioned above, since these measurements are correlated to specific locations. This allows for a spatial mapping of these measured parameters to be obtained. Such spatial mapping makes it possible to control and / or monitor the course of the treatment procedure.

[0090] In particular, FIG. 7 shows the prostate gland 102 of a patient. In a particular embodiment according to FIG. 7, an ablation procedure is to be performed on the prostate tissue. To this end, a number of markers 501, 501', 501", 501" are provided at several locations on the prostate gland 102. One or more sensors (not shown) are also provided at various locations on the prostate gland, preferably near the area of ​​interest where the ablation procedure will be performed. This allows for spatial mapping of the temperature evolution during the ablation procedure, which helps to avoid temperature-induced damage and / or over-treatment of healthy tissue during the ablation procedure.

[0091] FIG. 8 shows a schematic and exemplary embodiment of a wire for treating cerebral aneurysms. The wire 910 is provided with one or more marker devices 501, 501', 501" according to the above embodiments. Specifically, a first marker device 501 is located at a first end 911 of the wire 910 on one side of the first end 911. A further marker device 501' may also be attached within an intermediate section of the wire 910 to which a further marker device 501" may be attached at a second end 912 of the wire 910, and the wire 910 may have an internal cavity in which the marker device 501" is located.

[0092] FIG. 9 schematically and exemplarily illustrates an embodiment of an intrahepatic shunt device 1100 having a wire structure 1103. In this embodiment, the wire structure 1103 has a first part 1101 surrounded by a linear material and a bare second part 1102. In this embodiment, the first part 1101 is lined using PTFE (polytetrafluoroethylene). Also in this embodiment, the first part 1101 of the wire structure has separate wires, while the second part 1102 of the wire structure 1103 has interwoven wires. The intrahepatic shunt device 1100, which may simply be referred to as an intrahepatic shunt, includes several marker devices 501, 501′, 501″, and 501′″. For example, a first marker device 501 is disposed inside a PTFE tube next to each wire of the first part 1101 of the wire structure 1103. A second marker device 501' is disposed "in-wire" within the PTFE tube, i.e., the marker device 501' is disposed between the two ends of each wire of the wire structure 1103. A second marker device 501' is disposed "in-wire" within the PTFE tube, i.e., the marker device 501' is disposed between the two ends of each wire of the wire structure 1103. A third marker device 501" and a fourth marker device 501''' are also disposed on the second portion 1102 of the shunt device. By placing marker devices along the length of the shunt device, its position and / or orientation can be determined. The shape of the shunt device 1100 can also be determined.

[0093] 4-9, the arrangement of the marker devices is exemplary only, i.e., there may be more or fewer devices arranged at the same or other positions on or within each medical device and / or element. It is also possible that each device comprises only one marker device. The one or several marker devices attached to each medical device and / or element are marker devices according to at least one of the described embodiments.

[0094] In what follows, we assume that the length of a marker device is always approximately twice its diameter. Any device with a diameter of 0.3 mm or greater will allow real-time tracking (more than 10 readings per second) at intervals greater than 30 cm with high accuracy.

[0095] Marker devices can be attached to guidewires, such as those described above with reference to Figures 2 and 3, and used to track such guidewires. Marker devices can also be used to track catheters. Marker devices can be placed on stents. To minimize disturbances caused by the marker device during stent placement, the marker device should be as small as possible and should not exceed the stent wire diameter. Typical stent wire diameters are 0.2-0.5 mm. This is therefore a useful range for marker device diameters. Marker devices can also be injected with a syringe, in which case they can be implanted in smaller blood vessels in the pulmonary or intrahepatic areas without risk to the patient. Typical diameters when implanted are 0.3-1.0 mm.

[0096] The marker device is preferably configured to compensate for the dependence of the resonant frequency on temperature. One possibility for compensating for the temperature-based shift in resonant frequency will be described below with reference to FIG.

[0097] 10 also shows a marker device 3001 including a casing 3002 and a magnetic body 3004 disposed within the casing 3002, the magnetic body 3004 being rotatable from its equilibrium orientation when an external magnetic torque acts on the magnetic body 3004. The marker device 3001 further includes a restoring torque unit 3003 adapted to apply a restoring torque to return the magnetic body 3004 to its equilibrium orientation when the magnetic body is rotated from its equilibrium orientation by an external magnetic field or an external electromagnetic field, so as to enable rotational oscillation of the magnetic body 3004 excited by the external magnetic field or the external electromagnetic field providing the magnetic torque. In this embodiment, the casing 3002 is cylindrical, the magnetic body 3004 is rotatable about a virtual rotation axis running longitudinally through the center of the magnetic body 3004, and the magnetic body 3004 is rotationally symmetrical with respect to the virtual rotation axis. Specifically, in this embodiment, the magnetic body 3004 is a magnetic sphere.

[0098] The restorative torque unit 3003 has a further magnetic body 3003 that provides the restorative torque. Specifically, the magnetic body 3004 is attached to one end of an attachment portion 3007, such as a filament, and the other end of the attachment portion 3007 is attached to the casing 3002. The attachment portion 3007 is adapted to prevent the magnetic body 3004 from touching the further magnetic body 3003 due to their magnetic force and to cause the magnetic body 3004 to rotationally vibrate. In this embodiment, the further magnetic body 3003 is fixedly attached to the casing 3002 using an adhesive 3009.

[0099] The magnetic body 3004 forms a first magnetic dipole, and the further magnetic body 3003 forms a second magnetic dipole, and the magnetic body 3004 and the further magnetic body 3003 are arranged such that in the equilibrium direction the first dipole and the second dipole point in opposite directions. The first magnetic body 3004 and the second magnetic body 3003 are permanent magnets, and in the equilibrium direction the north pole of the magnetic body 3004 points towards the south pole of the further magnetic body 3003 and vice versa.

[0100] The casing 3002 is cylindrical, and the cylindrical casing 3002 has two end faces 3030, 3031, and the further magnetic body 3003 is fixedly attached to the first end face 3030, and the end of the filament 3007 opposite the end attached to the magnetic body 3004 is attached to the second end face 3031 of the cylindrical casing 3002.

[0101] In this embodiment, the second end face 3031 of the casing 3002 is formed by a rigid wall 3008 of the casing 3002, and in this case the magnetic body 3004 is attached to the rigid wall 3008 via an attachment portion 3007 such that the influence of external pressure is not transmitted to the inside of the casing 3002.

[0102] The marker device 3001 further comprises magnetic materials 3005, 3006 arranged adjacent to the further magnetic body 3003. The magnetic materials 3005, 3006 influence the magnetic field generated by the further magnetic body 3003, and this influence of the magnetic materials 3005, 3006 is temperature dependent, so that a change in temperature changes the strength of the magnetic field at the location of the magnetic body 3004, and thus the resonant frequency. The magnetic materials 3005, 3006 are adapted to weaken their magnetization as the temperature increases. Furthermore, the magnetic body 3006 is adapted to have its magnetization direction opposite to that of the further magnetic body 3003, and the magnetic material 3005 is adapted to have its magnetization direction the same as that of the further magnetic body 3003. Therefore, the soft magnetic materials 3005, 3006 affect the resonant frequency as a function of temperature in opposite frequency directions, i.e., one of these magnetic materials will change towards higher frequencies as the temperature increases, and the other of these magnetic materials will change towards lower frequencies as the temperature increases.

[0103] Preferably, the marker device 3001 is configured in this manner so that its resonant frequency is not temperature-dependent. To compensate for any undesirable temperature-dependent frequency shifts, the magnetic materials 3005, 3006 can be engineered to cause the same frequency shift in the opposite frequency direction in response to temperature changes. Specifically, the magnetic material 3005 is selected and positioned such that any temperature dependence of the resonant frequency of the marker device 3001 is eliminated. Also, only one of the magnetic materials, i.e., a magnetic material that decreases the resonant frequency as the temperature increases, or a material that increases the resonant frequency as the temperature increases, can be used to reduce or even eliminate the temperature dependence of the resonant frequency of the marker device 3001. One or both of the magnetic materials 3005, 3006 may be considered elements that compensate for the temperature-induced shift in the resonant frequency.

[0104] Fig. 11 shows a schematic and exemplary tracking system 1501 for tracking the marker device described above, i.e., a tracking system for wirelessly determining the position and / or orientation of a marker device attached to a medical device based on one or more electrical response signals indicative of the response frequency of the vibration of the magnetic material in the sensing unit. Fig. 12 shows an exemplary prototype of the tracking system 1501. The tracking system 1501 basically comprises at least one magnetic field generator and at least one magnetic field sensor, i.e., a transducer for converting the magnetic or electromagnetic field caused by the induced vibration of the magnetic material in the sensing unit into an electrical response signal.

[0105] The operating frequency band must be in the low kHz range, wide enough to cover the responses of several sensors operating in parallel at different frequencies, and possibly even higher harmonics of the sensor resonant frequency, for example, to increase the signal-to-noise ratio (SNR). The transmit field amplitude can be up to several milliteslas, while the sensed field amplitude is in the range of a tenth of a nano-T to several nano-T. Many different magnetic field generators (e.g., vibrating permanent magnets, cored / coreless coils, magnetostrictive field modulators) can work well, as can many different magnetometers (e.g., Hall-effect sensors, various types of magneto-resistive sensors, magneto-resonant sensors, SQUIDS sensors). The technically simplest system is an air-core conductor loop for transmitting and receiving a magnetic field. A coil is typically sufficient for sensor applications. The coil that generates a magnetic field can also be used to receive the magnetic field. However, various coils offering several advantages can be employed for these tasks.

[0106] In Figure 11, the tracking system 1501 comprises a transmit coil 1503 connected to a microcontroller 1507 via a digital-to-analog converter 1506 (DAC) and an audio amplifier 1502 for generating an external excitation field or an external electromagnetic excitation field for a marker device 1520, which may be embodied as described above. The receive 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 out the resonant frequency. The microcontroller 1507 is configured for, for example, signal generation and reception, frequency evaluation and control. A transmit / receive decoupler is also shown in Figure 12.

[0107] The microcontroller 1507 generates transmit pulses (see top trace 1350 in FIG. 13 ) that are amplified using an audio amplifier 1502 and then sent to a transmit coil 1503, sometimes referred to as an excitation coil. This implementation employs a separate receive coil 1504 that is decoupled from the transmit coil 1503 using two additional decoupling coils 1510, not shown in FIG. 11 for clarity. The receive signal is fed into a low-noise amplifier 1505 and passed to the microcontroller's 1507 ADC 1508, where a time trace, typically 1 / 20 of a second, is sampled at a rate of approximately 20 kS / s. In addition to the transmit pulses 1350, sometimes referred to as excitation pulses, FIG. 13 also shows an induced voltage 1351 in the receive coil 1504 due to spherical vibrations in the sensor and therefore the sensor response. The spacing of the excitation pulses 1350 can be continuously adjusted by the microcontroller 1507.

[0108] In the embodiments described herein, the tracking system specifically corresponds to a multi-coil system. Using several coils allows for location determination relative to the marker device by determining the position and orientation of an oscillating magnetic dipole in space. The various amplitudes of the received signals along with known coil element sensitivities can be fitted to a dipole model to determine position and orientation parameters. An example of a multi-coil system for implementation in a pillow or mattress is shown in FIG. 14. With many receive coils and channels available, additional information can also be used to enhance background signal suppression as described further below.

[0109] 14, several coils 1652 form a multi-coil array that is integrated into a patient bed mattress 1651 of an imaging system 1650, such as a C-arm system. Coils 1652 are preferably aluminum coils with x-ray absorption of less than 10%. Therefore, no increase in patient dose is required if coils 1652 are used.

[0110] The following provides a more detailed, exemplary description of a coil-based transmission system for a tracking system. A coil-based transmission system includes a transmission amplifier and a transmission coil. In some cases, it also includes a matching circuit and a "mute" circuit. Because the transmit signal shape is not critical for sensor applications, many amplifiers are suitable for this task (e.g., Class A, Class B, Class AB, Class D, etc., which employ a variety of components, such as transistors, vacuum tubes, and thyristors). Because signal quality is not critical, the lowest-loss amplifier topology can be selected: a half-bridge or full-bridge amplifier employing 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. Provided the amplifier operates at a sufficient supply voltage, such a matching capacitor can be omitted or selected with a capacitance high enough that the resonant frequency of the coil due to the capacitor is significantly lower than the operating frequency. Matching circuits are also interesting for another reason: medical devices must always operate safely, making voltage reduction a challenge. The peak voltage difference can be narrowed by placing a capacitor in the center of the coil so that the current flows through one coil section, then through the matching capacitor, and then through the second coil section. This becomes even more true when the coil is divided into more sections, each connected to a corresponding capacitor. This makes the coil and matching circuit a combined unit. The magnetic field amplitude is continuously controlled by pulse-width modulation, i.e., the amplifier increases / decreases the current through the coil for only a small fraction of the period, or alternates between increasing and decreasing the current rapidly. Since the exact signal shape is not very important for sensor applications, this is best achieved by changing state only twice within a half wavelength (or once at full power, where the pulse length is the same as the half wavelength). Ideally, the amplifier not only has the possibility to increase or decrease the current, but also to keep it nearly constant or at the level dictated by the matching circuit. This is achieved by the correct switching sequence of the transistors in the half-bridge or full-bridge. The amplifier's supply voltage should usually be quite low, in the range below 50 V.Furthermore, the matching circuit should be set so that this 50V limit is not exceeded at any two points, and even better, not to exceed 24V in either case. This means that the number of turns should be kept low. However, the peak operating current should be above 10A, and even better, above 100A.

[0111] In the following, we will talk about transmit / receive separation. While the transmit system is not in transmit mode, i.e. no excitation field is generated, it is essential that noise from the transmit system, i.e. noise from the magnetic field generator, does not penetrate too much into the receive system, i.e. the transducer that converts the magnetic or electromagnetic field generated by the induced mechanical vibrations of the magnetic material of the detection unit into an electrical response signal. Furthermore, the transmit amplifier should not shorten or even partially reduce the receive signal. There are several possibilities to achieve this. If we have different transmit and receive coils, we can geometrically separate the two coils (see Figure 15).

[0112] FIG. 15 shows an example of a geometric receive coil design for transmit and background signal suppression in the receive path. Here, a large coil 1452 is chosen, allowing tracking of marker devices up to a spacing of approximately 30 cm above the upper coil. The geometric design uses a geometric decoupling method, in which the transmit coil loop 1451 is connected to generate a parallel magnetic field, while the receive coil loop 1450 is connected to receive a magnetic field gradient and suppress a uniform magnetic field. This transmit / receive system uses parallel transmit loops and anti-parallel receive loops, resulting in inherent geometric decoupling, which may be referred to as a gradiometer configuration. This results in inherent geometric coupling. This system with air-core coils is highly linear. FIG. 15 also shows a DC block 1455 with an audio amplifier 1454 and a low-pass transmit filter 1453. The bottom of FIG. 15 shows an example of the outer turn of the receive coil 1450 and the inner turn of the transmit coil 1451.

[0113] Specifically, in Figure 15, the bottom left image is a close-up of the center portion of the upper coil assembly. In the bottom left image, only one turn of the transmit coil 1451 is actually visible peeking out from the bottom. The rest is hidden by the receive coil, which is wrapped with thinner wire. Since the signal to the audio amplifier is generated by a simple PWM output, the DC blocking circuit 1455 is simply a signal conditioner in front of the audio amplifier. Low pass filter 1553 is a filter that sits between the output of audio amplifier 1454 and transmit coil 1451. This filter has two purposes: first, to avoid introducing high frequency noise, and second, it is there to combine the audio amplifier output channels into one.

[0114] Geometric decoupling is not always possible, especially when a series transmitter and receiver are used. In this case, a transformer can be introduced into the transmitter circuit and at the terminal connected to the receiver circuit. This transformer provides decoupling between the transmitter and receiver systems. This transformer solution can also be used when combined transmitter / receiver coils are used. Instead of a transformer, a capacitive (or even resistive) decoupling network can be used with both combined transmitter / receiver coils and separate transmitter / receiver coils. The drawbacks of this compensation method are that it requires significant space, adds noise, and, in the case of capacitive decoupling, reduces the frequency range of the tracking system. A more robust and inexpensive solution is to add a circuit that completely mutes the transmitter amplifier during receiver times. For this, cross diodes can be added to the amplifier output. Diodes with zero voltage and low capacitance, such as PIN diodes, are particularly useful. This provides a high impedance when no current is flowing. To further enhance this, an electronic switch can be placed at the amplifier output that shorts out any residual noise signals during receiver times. The diode still provides the desired high impedance. It is also possible to build special amplifiers that completely eliminate noise and provide a high impedance when not in use. In half-bridge and full-bridge designs, this can be achieved by never switching any components when receiving, using low output capacitance transistors, having about half the supply voltage at the output in receive mode, not letting any noise come out of the input connector (optical isolation), and having a highly filtered supply voltage (either filtering multiple times or not switching the power supply at all during the receive process).

[0115] In the following, we will discuss the coil-based receiving system of a tracking system. The receiving amplifier must be of the low-noise type. However, the requirements are not so high that rare receiving transistors must be used. Standard low-noise dipole or JFET silicon transistors are usually sufficient. The only special feature is that the amplifier must survive the transmit pulse and start operating immediately after it. There are several ways to achieve this goal. In the case of a decoupled transmit / receive system (including a combined transmit / receive coil with a decoupling network), the receiving amplifier does not need any special features to achieve this goal. In the absence of any decoupling, the amplifier can be made robust against the transmit pulse. This can be done by adding an appropriate capacitor to the amplifier's input and a cross diode to the second terminal. This provides an adequate high impedance when transmitting, sparing all high voltages to a level that is harmless to the amplifier. Naturally, the added capacitor must be rated for the maximum transmit voltage. The capacitance value must be high enough so that the signal at the amplifier is not significantly reduced when receiving. With JFET-based amplifiers, this is usually not a significant issue. The cross diode may be augmented 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 receive amplifier will not go into saturation and will function correctly as soon as the transmit signal has been sufficiently reduced.

[0116] The interface with digital systems is described in more detail below, starting with digital signal outputs and their processing. While analog timer systems can generate output signals, digital systems such as DSPs or FPGAs are typically used. Different output amplifier types are used. Analog amplifiers use some type of ADC. Because output signal quality is not critical, a simple PWV-type analog output is sufficient. Digital amplifiers are best interfaced using digital output lines. However, it is also possible to implement a switching pattern generator in the amplifier using an analog output. For best-matched amplifiers, half-bridge or full-bridge, it is best to generate the switching pattern directly in the digital system. Furthermore, switching patterns for receive amplifier input protection and transmit amplifier output noise cancellation are also generated directly by the digital system. A common feature of all output options is that they must be fast enough to tightly maintain phase across various excitations of a single marker device or between various marker devices. Therefore, this output must be capable of switching updates on a raster finer than 1 / 10 of the full cycle time, and even finer than 1 / 100 of the full cycle time. For example, for a 2 kHz detection device, this means updating on a raster finer than 220 kHz, or even better, 200 kHz. This does not mean that a switching state change must be possible at every raster point. Therefore, for example, a serial interface can be provided for each amplifier, which communicates the new switching state to the amplifier and the protocol, so that this change is performed at a fixed time over the same serial interface. This is particularly useful for amplifiers that are essentially silent during the receive phase. For this purpose, a 1-bit serial interface can be implemented, which requires only one optocoupler per amplifier. This makes it easier to achieve noise immunity from the digital transmitter, since the stray capacitance of a single optocoupler can be very low.

[0117] Below, we will discuss the analog-to-digital interface. Analog-to-digital conversion is fairly standard. Because the signal has low bandwidth, it is possible to mix it down to near DC and sample it if only one marker device is used. However, the frequency of the signal from the marker device is somewhat low, usually below 10 kHz. Today, there are many suitable ADC chips that sample it directly. Especially since digital signal processing is trivial compared to analog filtering, it is best to use significant oversampling for the ADC. At least 10 times the marker device frequency should be used, with 100 or even 1000 times being valid options. Significant oversampling makes the ADC input filter design easier and cheaper, since it passes the sensor signal frequency while blocking signals above the Nyquist frequency. However, filtering below the sensor frequency also helps to ensure that the usual high background signal is not present. High background signals reduce possible amplification before the ADC and increase the ADC noise contribution. The ADC noise (effective number of bits) and samples must match the required dynamic range and noise expectations. This means that the ADC must not saturate while still providing the maximum expected signal and all noise components. At the same time, the ADC's quantization noise must be low enough to avoid increasing overall noise. Here, noise refers to all unwanted components in the recorded signal due to actual noise sources, such as coil resistance and receiver amplifier behavior. This also includes interference components that cannot be eliminated by appropriate filtering and background signal subtraction. Typically, with modern ADC chips, this requirement can be met, for example, with a 2 MS / s 18-bit ADC. To save cost, it can be beneficial to employ an ADC with lower specifications and gain control, while still achieving good overall performance.

[0118] In the following, we will discuss data processing. Before data evaluation, the raw ADC data needs to be processed. Since significant oversampling is desirable, the first processing stage is a decimation stage. This has the main advantage of reducing the data size, thereby reducing the computational power required for further stages. In some cases, the decimation stage may include another filter, namely a bandpass filter around the expected signal frequency. This can simplify further processing stages and narrow the dynamic range of the signal, which in turn saves computational power (variables with fewer bits). A further optional data processing stage is inverse nonlinear filtering to counteract nonlinearities in the receiving system. This means that the nonlinearity of the entire receiving system is measured and a computational filter is constructed to counteract the effects of the nonlinearity. This is particularly helpful when low-cost components are used, as such components tend to exhibit more nonlinear behavior.

[0119] This nonlinear filter can alternatively be used as the first processing stage. If more than one signal is used, there are further signal processing stages. If at least one receiving channel does not detect a signal from the sensing unit, thereby resulting in a constant background signal, this (and all other such signals) is correlated with the received signal, and the correlated component is subtracted from the channel bearing the signal. This subtraction can be performed in the time domain, the frequency domain, or a mixture of both. If there are no channels without a signal from the sensing unit, a data processing strategy sometimes called a "virtual gradiometer" can be used. This resolves multiple channels into a virtual channel, which is a linear combination of the physical channels, so as to minimize interference from signals not emitted by the sensor. The coefficients for the linear combination can be determined by correlating the signals of the channels excluding the signal band of the sensing unit.

[0120] Further data evaluation will be described below. Frequency is the main parameter extracted from the acquired signal from the sensing unit. Due to the high Q of the resonator (time constants of up to seconds), subsequent excitation pulses usually expire before the vibrations have completely decayed (see Figure 13). Therefore, they must have the correct phase and timing to amplify the ongoing vibrations. This requires real-time extraction of the frequency between subsequent excitations. This frequency can be extracted using a comparison algorithm that minimizes the phase difference between the measured signal and a pre-calculated time trace spanning the frequency range, or by Fourier analysis, which is the preferred method. High-resolution frequency information can be obtained by locating the resonance peak in the subsequent spectrum, which may involve time-domain zero padding, frequency-domain interpolation, or peak detection or curve-fitting procedures. To further improve the accuracy and robustness of the frequency determination, higher harmonics of the detected resonance signal can be incorporated into this evaluation, for example, using weighted frequency estimation based on several harmonics or by determining the consistency of the frequency determination between several harmonics (see the upper right spectrum in Figure 16).

[0121] In the example shown in Figure 16, 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 signal at the resonant frequency and its higher harmonics, including analog excitation filters such as DC blocks or low-pass filters, analog receive filters such as band-pass filters, and digital receive filters (sixth-order Chebyshev II) such as ITR resonant filters for real-time processing. In Figure 16, the center position of the f0 resonant peak is determined from the largest peak in the filtered spectrum. From f0, the timing of the next in-phase excitation pulse is calculated. The repetition rate of the system is between 5 and 30 Hz, resulting in a real-time trace of the frequency response.

[0122] In Figure 16, the signal spectrum is displayed with and without digital bandpass filtering (1051 vs. 1050). The dotted line indicates the range selected for evaluation. The various dots correspond to different filter types, which in practice make no difference and can therefore be ignored. In Figure 17, the bandpass is attached to a commercially available low-noise audio-range amplifier, model DLPVA-100-BUN-S from FEMTO Messtechnik GmbH. In Figure 18, the actual 40 dB suppression spectrum of the digital filter is traced across the selected band. There is no noticeable difference between the two implementations. The filter shown is applied to the data shown in Figure 16, resulting in the difference between 1050 and 1051.

[0123] From the determined frequency and known timestamp of the received signal, the correct timing can be calculated for the next block of excitation pulses, the number and width of which are adapted to cause oscillations of sufficiently high amplitude to create a sufficient signal in the receiving coil.

[0124] The following describes in detail the process by which a tracking system is used to determine the position of a marker device, thereby locating a medical device to which the marker device is attached. In such localization, frequency effects are irrelevant (sensitivity encoding is discussed further below) or negligible (gradient field encoding is discussed further below). In localization using gradient field methods that also affect the sensor frequency, these compensations are not necessary because only frequency changes over time periods of less than one second need to be determined. These changes are not significantly dependent on vibration amplitude.

[0125] The signal of the magneto-mechanical oscillator is the voltage u induced in the coil i as a result of the magnetic field fluctuations due to the oscillatory motion of the magnetic moment m(t) of the suspended magnetic sphere at the position γ0. i It is detected at (t).

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[0126] From (4), it follows that a large dynamic magnetic moment is desirable to induce a high voltage in the receiving coil. Since the size of the marker device, and hence the bulk of the magnetic spheres used as the magnetic body and the restoring torque unit, respectively, must be small for most applications, a large

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[0127] mass m s , radius r s The damping coefficient C required for the angular acceleration of a ball and the friction in the case of torque

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[0128] Small angle approximation sinφ≒φ, and substitution m=M sat V shere leads to the following:

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[0129] The high Q of this system allows for a further approximation C≈0, allowing the calculation of the angular response frequency as follows:

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[0130] This approximation is not typically valid, since micro-oscillators are usually driven to amplitudes much larger than 10°. At larger angles, the restoring torque becomes smaller, causing a decrease in frequency, and the amplitude-dependent frequency ω(φ max )=ω0k(φ max ) (where k(φ max )>1). The variation of the restoring torque during vibration also leads to nonlinearities in the sensor response, which are magnified by the presence of higher harmonics of the fundamental frequency in the spectrum.

[0131] In addition to the nonlinear restoring torque, the force between the two magnetic spheres depends on the orientation of their magnetization relative to each other.

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[0132] For a given sensor design, this force is always directed along the connection vector of the two magnetic spheres, but its magnitude goes to zero at an oscillation amplitude of 90° and goes from attractive to repulsive at larger angles.

[0133] If the excitation field generated by the transmitting coil has a constant amplitude, the oscillation amplitude φ max The amplitude also depends on the relative orientation of the coil and the sensor, as shown in Figure 10.

[0134] A tracking system requires that the orientation and 3D position of the marker devices described hereinbefore be determined. Two independent localization techniques can be used for localization. In some cases, one technique is sufficient, while in other situations a combination of both techniques is useful to improve accuracy or to identify systematic errors that result in conflicting results between the two methods.

[0135] The first approach is coil sensitivity based localization. In this approach, for each coil i in the coil array, a spatial sensitivity profile B S、i Take advantage of the fact that (γ) is different.

[0136] According to equation (3), one magnetic oscillator, i.e., one magnetic body, has a specific mechanical vibration, and this vibration is B S、i (γ), the dynamic dipole moment of the magnetic material

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[0137] For this reason, it is desirable to map the six position and orientation coordinates of the marker device in the coordinate system given by the tracking system to the voltage amplitudes at the fundamental frequency or higher harmonics on all receive channels. In the following equation, we describe how to get rid of the time dependency in equation (4) so ​​that we only need to consider the amplitudes. We start by including all the arguments, i.e., the position vector γ = (x, y, z) γ, direction vector φ=(φ, θ, Ψ) T :

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[0138] The required coil sensitivity profile may be calculated from known coil geometry, measured at defined positions and then interpolated, or may be determined as a mixture of both, i.e., in a model that can fit experimental results with well-fitting parameters. For magnetization oscillations, the explicit description in terms of frequency ω and amplitude α, in the frame of the marker, is:

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[0139] From the forward function set (14), (15) and the measured response amplitude, the marker device position and marker device orientation can be calculated by solving the simultaneous equations using standard mathematical nonlinear methods. The accuracy of this solution increases with the number of receive coils and the orthogonality (i.e., the magnitude of the difference) between those receive coil sensitivities. The mismatch between the six unknowns and more (or fewer) receive channels can be taken into account by solving the simultaneous equations in a least-squares sense.

[0140] Localization / positioning is also based on gradient field encoding. While coil sensitivity localization is based on the amplitude distribution picked up by the coil array, the frequencies of one or more marker devices can be manipulated to obtain independent position information. For this purpose, the tracking system is equipped with a control unit that can independently control each coil in the coil array to generate a non-uniform magnetic or electromagnetic excitation field, ideally with a constant magnetic field gradient over the entire area of ​​effect. This is achieved, for example, by applying low-frequency currents to selected coils in the coil array. This additional magnetic field generates a restoring field B acting on the vibrating magnetic material. rest changes, and therefore its frequency also changes (Equation 9)

[0141] Due to the non-uniformity of this excitation field, the frequency change depends on the orientation and position of the marker. By sequentially controlling the application of several encoding magnetic fields, for example, by applying magnetic field gradients at six different orientations, all three position parameters and two of the three orientation parameters of the marker can be determined. The remaining angles may be deferred from the higher-order response of the sensing unit of the marker device to the external magnetic or electromagnetic excitation field, but a higher magnetic field strength is required to provide sufficient high-order involvement. The basic encoding concept is related to gradient encoding in MRI, and therefore both frequency encoding and phase encoding can be performed.

[0142] In frequency encoding, a non-uniform magnetic field is applied during signal readout to produce a desired frequency shift. For a desired spatial resolution, the applied encoding field strength must be matched to the frequency sensitivity of the marker device and the frequency resolution provided by the system. The frequency sensitivity of a NdFeB marker device with a magnetic sphere of 0.5 mm diameter as the magnetic material is

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[0143] In phase encoding, a non-uniform encoding field is applied before a single readout, i.e., only during a brief window of opportunity to create a position-dependent signal phase shift. If the phase resolution is insufficient for precise localization, the duration and / or amplitude of the phase-encoding pulse can be varied in successive excitations to resolve ambiguities in phase generation. Thus, complete spatial information is obtained over the course of several readouts. Phase encoding with one non-uniform magnetic field pattern (e.g., encoding one spatial axis) can be combined with frequency encoding with another non-uniform magnetic field pattern (e.g., encoding an orthogonal spatial axis) for effective localization. If the rough marker device location is already known from a sensitivity encoding technique (which is fast due to its parallelism), it is sufficient to use only a few phase encoding stages that provide the missing high-resolution (high spatial frequency) components rather than the complete spatial information.

[0144] As described below, a comparison of the localization results obtained by gradient versus sensitivity encoding can be used to identify systematic errors, for example, caused by background magnetic fields. It should also be noted that the linear response of sensors employing two suspended spheres to low-frequency external magnetic fields is suppressed, and in this case, the higher-frequency response can be used for localization or sanity checks. However, the magnetic field sensitivity of these oscillators is so low that higher gradient magnetic fields are required for gradient magnetic field encoding.

[0145] In the following, parameter determination and position determination will be discussed for tightly coupled sensors.

[0146] Determining position (meaning three positional parameters and three orientation parameters) and measuring further parameters (such as pressure and temperature) can be challenging, especially when using only a few coils. However, using only a few coils is cost-effective and, in some applications, preferable due to space limitations. Therefore, it is desirable to modify the detection procedure and hardware to work with a few coils. One way to do this is to use several devices and / or sensors in a coupled form. By coupled, we mean here an assembly of several sensor / marker devices, each operating at a well-known frequency, combined in a fixed relative orientation. Typically, the sensors are mounted on a rigid frame, but technically, only the relative positions of the sensor / marker devices need to be known at the time of identification.

[0147] With enough sensors, it is possible to determine position with just two coils. This is best seen in comparison with conventional electromagnetic navigation systems. These typically consist of several transmitting coils, usually seven or more, and one receiving coil whose position and orientation are assessed. However, rotation of the coil around its axis (the axis of the dynamic dipole moment) cannot be detected due to the coil's rotational symmetry.

[0148] In this comparison, a series of tightly coupled sensors can be seen as a transmit array, and one transmit coil as a marker. It is then possible to locate the sensor / marker device array somewhere in a ring centered on the dynamic dipole axis of the transmit coil. Note that if the coil were not round, the ring would not be a perfect circle in space, but this does not change the logic. Therefore, while one coil cannot determine position, two coils (with non-parallel dynamic dipole moments) break the symmetry and allow the location and orientation of the sensor / marker array to be determined.

[0149] Determining the various sensor signals is best done with a full model approach, described further below. Briefly, a model of each array sensor / marker device is created, i.e., in differential equation form. This model predicts the sensing unit response to a given excitation field. In conjunction with the transmit / receive system model (including amplifiers, filters, and coils), the full response of the array can be predicted. Knowing past excitation pulses (typically only a few decaying pulses are required), the expected received signal for a marker device position and parameter values ​​can be computed.

[0150] It is also possible to incorporate prior knowledge into this procedure (i.e., only allowing the maximum displacement velocity of the sensor relative to the coil). Here, the only difference from the previously described methods is that this process is performed not for a single sensor, but for a set of connected sensors in an array, or for several arrays simultaneously. With a sensor array, there is a set of prior knowledge available, i.e., the relative positions and relative orientations of the sensors / markers in the array are known. This is particularly helpful, since it is difficult to simultaneously operate many sensors at the desired amplitudes, so employing a fully parametric method or at least a zero-amplitude frequency extrapolation method is helpful. However, full-model approaches are somewhat computationally intensive. To reduce the required computational power, it is advantageous to first use the previously described single sensor / marker estimation methods separately and use their results as starting points for the final full-model-based position and value reconstruction.

[0151] Below, several calibration aspects are described, first addressing calibration in the presence of conductive soft ferromagnetic materials.

[0152] Conductive and particularly soft ferromagnetic materials can interfere with localization by distorting the magnetic field generated by the oscillating magnet of the marker or sensor and / or by distorting the magnetic field generated by the transmitting coil. In particular, compensation for amplitude effects can be less accurate, and sensor readings may also be altered to a lesser extent. Therefore, a calibration procedure for the magnetic field is desirable. Furthermore, it is also desirable to provide a measure to determine whether a magnetic field disturbance is currently occurring. Therefore, we first discuss how to detect disturbance problems.

[0153] Typically, the tracking system described herein uses a transmit / receive coil array. The coils may be separate transmit-only and receive-only coils, or the same coil may be used for both functions. In either 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 significantly from the stored value, some action is triggered, such as an inaccuracy warning, a self-calibration process, or a calibration process with user interaction, or a combination of these. It is also possible for several coils to transmit simultaneously. The transmitted pulse must contain multiple frequencies. This can be achieved by pulsing, frequency sweeping, or using several vectors, as is well understood herein. Frequency analysis is important because eddy currents flowing through conductive structures are primarily frequency-dependent. Therefore, a noticeable change is when the ratio of the received target signals at two different frequencies exceeds a certain limit. It is also significant if at least one spectral component changes by a specified value. However, a uniform change across the spectrum is due to a gain change, for example, in the receive amplitude. Therefore, for example, if the receive amplitude is configured such that a gain change is possible, this effect can be used to set a new gain value in software that compensates for this gain change. This logic also applies if a gain change is expected to occur in the transmit amplitude but not in the receive path. Note that, as a correction, the transmit amplitude is changed in the computational model (leading to a change in the vibration amplitude of the sensor, etc.). It is theoretically possible to measure the impedance of a single coil and use this change as an indicator of changes in the eddy current environment. However, the ability to measure impedance necessarily lacks electronics and requires specialized equipment. Known properties of the sensor / marker in its operating range, as well as the coil coupling, can be used to detect environmental changes in the eddy current environment.

[0154] Specifically, sensors can be integrated into the transmit / receive coil array itself. Even a single sensor / marker can be useful. For example, if only one marker is integrated into the system, changes in the marker's response at a fixed position relative to the coil are indicative of changes in the eddy current environment. It is even more desirable to incorporate a sensor / marker device that is sensitive to low-frequency magnetic fields but less sensitive to other properties that may change quickly. This marker device is an indicator not only of the static magnetic field but also of the presence of ferromagnetic materials. To detect ferromagnetic materials, the coils are filled not only with current at the frequency of the sensor / marker device vibration, but also with current at a much lower frequency. Current filling can be done for each coil or using several coils. If the measured sensor response (i.e., the frequency change due to the applied low-frequency magnetic field) does not match the stored prediction, it is possible that the ferromagnetic material distorts the magnetic field. If there are enough coils in the system, the location of the field-dependent sensor / marker does not even need to be known. With enough coils, the marker device position can be determined using the sensitivity of the coils at the sensor / marker device frequency and, independently, the sensitivity of the sensor / marker device to near DC magnetic fields (gradient magnetic field encoding).

[0155] If the positions obtained by the two methods differ, the eddy current (or ferromagnetic) environment has changed. However, it is better if many such marker devices are incorporated into the system rather than just one. It is also better to have them in known positions rather than in unknown positions. However, it is also useful to know only some properties of the position rather than having no position information at all. A practical way to achieve partial knowledge is to place the sensor / marker devices on a rigid structure that ensures a known, time-stable position and orientation relative to each other. Such a calibration "frame" of sensor / marker devices is permanently or occasionally inserted into the tracking system's computational budget. If the tracking system finds relative positions and orientations that deviate from expectations, the system is being disturbed by eddy currents or ferromagnetic materials.

[0156] Furthermore, if the sensor / marker device is also sensitive to near DC magnetic fields and there are enough coils in the coil array, the relative position of the sensor / marker can be determined independently at very low frequencies, where only ferromagnetic materials perturb the magnetic field, and at the sensor / marker resonant frequency, where both ferromagnetic and eddy currents cause field distortions. Thus, information about the nature of the perturbation can be elicited, for example, if ferromagnetic materials are involved in this perturbation.

[0157] Furthermore, the best approach to detecting disturbances is a full mathematical model of the transmit / receive amplitudes, coils, and marker devices / sensors. This model includes known positions and orientations, both relative and absolute. In the first stage, all position / orientation and physical parameters are optimized to minimize errors. This stage includes, for example, a fixed-position marker attached to the coil array and prior knowledge of its relative position in a potential difference frame. As an aside, the "frame" need not be introduced solely for calibration; a marker device consisting of many oscillators can naturally serve as a frame. In the second stage, the total weighted error between the expected signal and the delivered signal is calculated by computer. If the error exceeds a certain threshold, it is concluded that a substance is disturbing the magnetic field. From the nature of the error (i.e., whether the error occurs in the AC or DC sensitivity component), the nature of the disturbing entity can be inferred.

[0158] The dependence of signal amplitude at various harmonics on sensor orientation relative to a single coil will now be discussed. Specifically, Figure 19 provides a good example of the measured dependence of signal amplitude at various harmonics on sensor orientation relative to a single transmit / receive coil. When the excitation field is aligned parallel to the magnetic dipole orientation, no excitation occurs and the signal is zero. The largest oscillation amplitude is achieved at orthogonal alignment of the field with the dipole. Note that the spatial pattern of the even harmonics is orthogonally aligned with the odd harmonics. This is evident by the second harmonic amplitude (first harmonic) and the zero of the third harmonic at the orientation corresponding to the maximum in the base signal. The amplitude ratio plot (center graph) highlights this difference in orientation dependence, with the ratio of the second harmonic to the first harmonic going from zero to a maximum value (or singularity), while the ratio of the third harmonic to the first harmonic is 1:1. The knowledge that the dynamic response at the even harmonics is oriented orthogonally to the dynamic response at the odd harmonics can be used to determine a third orientation angle of the sensing unit.

[0159] Both sensitivity encoding and gradient encoding can provide the position and / or orientation of the marker device and the medical device to which the marker device is attached, and in some embodiments a combination of both is employed.

[0160] Below we will discuss how the vibration amplitude can be determined using the amplitudes of the harmonics of the fundamental frequency.

[0161] One way to determine the vibration amplitude is to evaluate the harmonics of the induced signal in the coil. Due to nonlinear vibration, magneto-mechanical oscillators generate harmonics of the resonant frequency due to their dynamic dipole moments. These harmonics are picked up by the receiving coil. Care should be taken during the sampling and filtering stage to avoid suppressing these multiple fundamental frequencies. The harmonic spectrum depends on the sensor's details. Some sensors produce predominantly odd harmonics (3ω0, 5ω0, etc.), while others produce even and odd harmonics (2ω0, 3ω0, 4ω0, etc.). However, mixed types can also be constructed. The dynamic dipole moments of the odd harmonics tend to align with the dynamic dipole moment of the fundamental frequency, while the even harmonics tend to align perpendicular to the dynamic dipole moment of the fundamental frequency and perpendicular to the axis of rotation. Therefore, odd harmonics are conceptually the easiest to use. Odd harmonics are conceptually easiest to use because the ratio of the dynamic dipole moment of, say, the third harmonic to the fundamental frequency dipole moment is reflected as a corresponding ratio in the recording voltage in one coil, and is therefore considered, for example, a spectral peak amplitude. However, because the amplitude in the receiving system is frequency-dependent, a correction is applied to find the true ratio of the third harmonic dipole moment to the fundamental frequency dipole moment. This ratio is measured over a predetermined integration period. A correction is applied because a calibration of this ratio to the vibration amplitude or direct frequency shift can be provided for each sensor. For even harmonics, the situation is somewhat more complicated because the direction of the dynamic dipole moment does not align with the fundamental frequency dynamic dipole moment. Therefore, here, it is usually necessary to employ more than one coil, and the coil orientation relative to the sensor must be determined by other means.

[0162] For large coil sets (e.g., ≥ 6), both the sensor position and orientation may be reconfigured, and even for a small number of coils (e.g., 3-5), it should be possible to reconfigure the sensor orientation relative to the coils using at least a method similar to the positioning method described in detail below. The true ratio of dynamic dipole moments per harmonic can then be determined using the coil sensitivities. The intermediate step of orientation determination can be eliminated, and a direct map of the ratio of fundamental frequency amplitude to harmonic amplitude in the coil can be established using linear algebraic methods. It is understood that the methods described here in the frequency domain can be mapped to methods in other bases, such as the time domain. In the time domain, frequency analysis is mapped to vibrational analysis. These mapping methods are well known in the mathematical literature.

[0163] In the following, we will discuss the determination of vibration amplitude based on the time-domain envelope function.

[0164] Another way to determine the vibration amplitude is to exploit the nonlinear decay behavior of the signal. The damping of the sensor is usually nonlinear. Nonlinear damping means that at double stored energy, the average dissipated power of the sensor does not double, but increases by a factor somewhat higher than two. The reason for this may be the elongation of the filament due to the force modulation described above. Equation (9) shows that at low vibration amplitudes, the attractive force between the magnetic bodies is approximately constant, but at higher amplitudes it is no longer constant. This force variation is governed to a first approximation by the square of the vibration amplitude, which corresponds to the approximation of a cosine function by a parabola. This squared dependence is the reason for the nonlinearity in the dissipation. The periodically changing force between the magnetic pair elongates the filament, which leads to dissipation contributions. Other effects may also result in nonlinear behavior. Collectively, these effects result in a situation where the envelope shape of the decay curve over a given time period is governed by the initial amplitude. Therefore, if the initial vibration amplitude of the sensing unit of the marker device is constant and the spacing and / or orientation of the sensing unit of the marker device is changed relative to the receiving coil, a scaled-down initial decay envelope is seen. However, if the excitation amplitude of the sensing unit is changed, not only does the overall amplitude of the decay curve change, but its shape also changes. This can be disentangled from the amplitude and spacing / orientation effects, so that the initial vibration amplitude can be reconfigured, for example, using a look-up table of pre-recorded decay curves.

[0165] This also leads to the possibility of measuring the zero amplitude frequency or controlled constant amplitude excitation as described above. However, this also changes the shape of the envelope and is somewhat sensitive to the movement of the sensing unit during recording. For example, if it is known that the sensing unit of the marker device does not undergo rapid accelerations, it is useful to combine the assumption of continuous movement with the decay curve envelope.

[0166] Determination of the vibration amplitude based on a single amplitude response to a variation in the excitation field will now be described.

[0167] Another way to determine the vibration amplitude is to analyze the sensor signal response to various intensities of the excitation field. In this case, the current pulse is systematically varied, and the sensor's response to the various excitation pulses is evaluated. The current, duration, and phase of the transmit pulse are varied, or a combination thereof. For example, assume there are two excitation pulses. If they are spaced widely apart and the local magnetic field amplitude is low, the two pulses are specified to produce an amplitude twice that of a single pulse. However, if they are spaced closely together and the local magnetic field at the sensor is large, the amplitude falls below this double. This results in a characteristic decrease in the received voltage relative to the expected double. Therefore, the ratio of the sensor's received signal (Fourier) amplitude for a given excitation pattern is a measure of the excitation amplitude and can also be used to extrapolate to zero-amplitude frequency and / or to obtain a constant excitation amplitude. In addition, other quantities, such as frequency and decay time, can also be evaluated. The ratio of these quantities is also characteristic of the vibration amplitude and can be used to extrapolate to zero-amplitude frequency.

[0168] In the following, we will discuss the determination of the correct parameters based on a full model of all factors.

[0169] All of the above methods are merely evaluation methods, and some require modifying the transmitted magnetic field pulse. No modifications to the hardware system are required to perform this evaluation. Therefore, it makes sense to implement them all. This is done by simply evaluating the results in parallel and in conjunction with the results, minimizing noise, i.e., performing a weighted average according to the relative noise. While this is relatively easy and straightforward to implement, better results can be expected using a truly integrated mathematical approach, as summarized below. Conversely, mathematically advanced approaches can be significantly more difficult to implement and require significantly more computational resources to run on cost-effective computer hardware. The basis of a proper mathematical approach is a mathematical model for the sensing unit. This model predicts the sensing unit's response to the excitation magnetic field and its corresponding state. The sensing unit state is the current deflection angle and rotation speed of the suspended ball, which corresponds to the magnetic material.

[0170] In some embodiments, a model of the transmit and receive coils needs to be generated, including filter and amplifier characteristics. As long as the transmit and receive systems are sufficiently linear in nature, this can be formulated into differential equations, although Fourier parameterizations are not uncommon here.

[0171] Finally, a model for the coil transmit and receive sensitivity must be provided. This can simply be a set of spatial points with sensitivity and an interpolation algorithm between them. It can also be based on a simulation of the coil based on the Biot-Savart law. This model can then predict the sensor's voltage response at any location and orientation for a given history of excitation pulses and external parameters. Therefore, the procedure involves varying the sensor's position and orientation, affecting the physical parameters in the simulation, and varying the sensor to achieve the best possible fit between the recorded signal and the simulation. Many well-known optimization methods, such as gradient descent or random walk, can be used. This fit can be defined as the root-mean-square sum of the differences between measured and simulated sample points. The fit is best if this quantity is lowest. This best fit can be influenced by introducing additional constraints, such as a model for the expected relative position and relative orientation, or by constraints on the maximum expected sensor acceleration and / or a model for the measured quantities, for example, which impose constraints on the maximum rate of change of these quantities. Additional sensor inputs, such as an accelerometer, are also used in handheld coil systems, for at least one input that is independent of spacing and orientation changes. Since the full model-based evaluation process is computationally intensive, this process can be combined with one or several of the previous models to provide a good starting point for further optimization.

[0172] As will be explained below, the processor may also be configured to compensate for the effects of gravitational forces.

[0173] The processor, and more particularly the position determination unit, may also be configured to compensate for geomagnetic field effects and other static magnetic field effects.

[0174] The static background magnetic field is added to the magnetic field of the fixed magnetic body, resulting in a recovery magnetic field B, which is seen by the vibrating magnet. restThis modulates the resonant frequency according to Equation (8), thereby changing the oscillator frequency and resulting in a source of error in detection. For a 0.5 mm diameter magnetic sphere made of NdFeB with a saturation magnetization of 1.3 T / μ, the magnetic field generated by the fixed sphere at the center of the vibrating sphere is 16.1 mT at a center-to-center spacing of 0.75 mm and 6.8 mT at a center-to-center spacing of 1.0 mm. The geomagnetic field ranges from 25 to 65 μT. The frequency difference between parallel and antiparallel alignment of the static magnetic field component of the maximum geomagnetic field of 65 μT results in a frequency difference of approximately 5 Hz for the 0.75 mm spacing and approximately 9 Hz for the 1.0 mm spacing. Various mitigation strategies are presented below.

[0175] The mitigation on the marker device side is the use of a design that employs two suspended spheres with identical magnetic dipole moment and moment of inertia (or an appropriate ratio of the two quantities) as the magnetic material, rather than the single sphere previously described. Because counter-oscillation occurs at a single frequency, the first order effect of a static bias field, such as the Earth's magnetic field, is nullified.

[0176] Another mitigation strategy is to use an absolute magnetic field sensor in the detection system to measure the magnitude and orientation of the static background magnetic field. Based on the sensor orientation determined using the methods described above, frequency or magnetic field corrections can be calculated to achieve improved positioning. For detecting the static background magnetic field, any magnetic field sensor with sufficient sensitivity and range that can be incorporated into the tracking system can be used. One cost-effective option may be a three-axis Hall sensor. An alternative is a temperature-compensated micro-robotic three-axis array with a well-defined zero-field frequency.

[0177] From the changes in their respective frequencies, the magnitude and direction of the background magnetic field can be determined. Ideally, these resonant frequencies are chosen so that they do not interfere with the frequencies of the sensing unit of interest. During evaluation, rather than correcting for frequency shifts, the coils of a multi-coil tracking system can also be used to generate small offset fields that balance the Earth's magnetic field and other background fields. When there is an inhomogeneous magnetic field in the field of view due to the presence of ferromagnetic materials, several sets of three-axis magnetic field sensors can be employed to characterize the spatial magnetic field variations.

[0178] The marker device must have a high Q factor and a large frequency sweep that is sensitive to the quantity being measured over the range required for a particular application. A high Q factor is especially important at high vibration amplitudes, where the highest signals are emitted. There is a strong attractive force between two magnetic bodies, and as this force increases dramatically with decreasing separation (up to the fourth power of this separation, see Equation (9)), both properties can be compromised. This strong force results in a relatively strong pull on at least one filament holding at least one magnetic body. This pull does not itself provide a dissipation path. However, especially at large vibration amplitudes, the weakening of the force between the magnetic bodies periodically weakens the pull on the attachment portion. This can result in a periodic expansion and contraction of the attachment portion, which typically generates heat. Therefore, power is extracted from the oscillator. This force also strongly depends on the separation between the magnetic bodies and becomes very large when the magnetic bodies are close to each other.

[0179] To overcome this problem, we will describe a method of reducing the forces and force variations, which simply consists of magnetizing one part of the magnetic material in the opposite direction next to another, as shown in Figure 20.

[0180] In FIG. 20, sensing unit 4001 includes magnetic body 4008, which is a permanent magnet, suspended from rigid wall 4010 of casing 4002 via attachment 4006, such as a filament, preferably a high-strength wire. Rigid wall 4010 is preferably made of a metal or polymer that is insensitive to external pressure. Other parts of casing 4002 may also be made of metal or polymer. Casing 4002 may be filled with gas or may have a vacuum space. Another magnetic body 4007 is fixed to the inner end surface of casing 4002 with adhesive 4011. The two magnetic bodies 4007 and 4008 are typically magnetized in opposite directions. However, fixed magnetic body 4007 also has a portion 4012 with opposite magnetization orientation.

[0181] Therefore, when two magnetic spheres are involved, in this example, we obtain a cap in which at least one magnetic body is magnetized in an opposite direction. The cap is located next to the other magnetic sphere. If one magnetic sphere is stationary and the other is vibrating, it is best to have the cap attached to the stationary sphere. In this way, the dynamic dipole moment of the sensor is weakened. The frequency is only slightly reduced. However, it is also possible to reverse the roles of the spheres. Since the oppositely magnetized areas are very small, the net force between the magnetic bodies is still attractive at all usable distances. If the oppositely magnetized areas are small enough, the attractive condition can be met right up to the point where the magnetic bodies come into contact.

[0182] There are several ways to create a reverse magnetized cap. One involves simply adding a magnetic material to the top surface of at least one magnetic body. The magnetic material can be magnetically soft or magnetically hard. It can be a solid, continuous magnetic material, magnetic paint, or something in between. The magnetic material naturally tends to align to form opposing magnetizations, and also tends to stick to the magnetic body. Nevertheless, this additional material needs to be adhered to the magnetic body, especially if the two primary magnetic bodies may occasionally come into contact. To preserve the original desired shape, some material can be removed from the magnetic body being changed, for example by grinding.

[0183] There are alternative ways to create a zone of reverse magnetization. This can be achieved by a strong pulse of current through a conductor near the magnetic material. However, this is not very practical due to excessive heating. This can be more easily achieved by simply heating the affected portion of the magnetic material to near or above its Curie temperature. This results in a reversal of the magnetization. This effect can be enhanced by applying a pulsed or constant magnetic field in the opposite direction. The magnetic field can also incorporate a strong gradient by using somewhat harder or softer magnetic materials near the affected zone. Because the heating must be quite localized, the total energy stored in the magnetic material must be low, and the temperature rise must be very rapid so as not to approach the Curie temperature overall. A suitable heating source can be a laser. Resistive or inductive heating methods would also work well.

[0184] Some methods for determining the onset of magnetic field disturbances are also a good starting point for methods for compensating for their effects. These exemplary methods are most easily explained under the assumption that there are conductive materials that induce eddy currents, but no ferromagnetic materials. Applying the above model, determining the dependent signal near DC (gradient field encoding) can yield the correct position, but at the sensor frequency and its harmonics, potentially yielding incorrect position and local magnetic field amplitudes (coil sensitivity encoding). Therefore, higher frequency magnetic fields can be distorted to match this prediction. After the distortion is applied, all position and sensing unit readouts improve. Because AC sensitivity encoding is fairly fast, it is beneficial not to rely solely on position estimation based on the magnetic field near DC.

[0185] The most important part of this compensation method is to define a correct model for the AC magnetic field distortion. A simple solution is to parameterize the magnetic field shift function, for example, using a simple 3D polynomial. This means that the magnetic field values ​​at the actual position are not used, but rather the magnetic field values ​​at the position transformed by the 3D polynomial. This is computationally efficient, but lacks physical insight; for example, it is not clear how measurements of coil coupling are incorporated into this concept. Therefore, it is better to use a model that is closer to physical reality. For example, it is better to use a magnetic field model conductive plate near the coil system that induces the desired magnetic field disturbance.

[0186] Therefore, the basic position, angle, thickness, and size of several virtual plates are varied until the model predictions and measured data match. How to model such conductive plates is well known in the electromagnetic simulation literature. This type of modeling has the added advantage of easily incorporating objects whose shapes are likely to occur in a particular environment. Therefore, when a spatial device, such as an X-ray C-arm, approaches the field of view, the device can be modeled before it is known, and only its exact orientation and position can be optimized by the system software. A further advantage is that the position of a potential disturbing object can be displayed by the system, or the data can be transmitted to a second system that performs the display task. In this way, the user can be specifically indicated which object is disturbing the measurement and may wish to move or remove it. During this process, the combined data of the coils essentially acts as a metal detector array. The incorporation of ferromagnetic material is conceptually equivalent to the conductive material that generates eddy currents. However, ferromagnetic material simulations are somewhat more computationally intensive and may lack a solid reference position defined by a dedicated marker device, and therefore may not yield accurate positions. Again, it is best to model a collection of ferromagnetic materials, such as a thin plate or rod, and deform it around the coil array in the simulation. Here, a model is very convenient if a database of suitable ferromagnetic materials is provided. Furthermore, the mutual coupling measurement process may be augmented by measurements of harmonic generation in the coil environment. The presence of harmonics is a strong indicator of soft ferromagnetic materials, and the measured signal provides useful input into the size and location of the ferromagnetic material.

[0187] In the following, excitation pulse generation will be discussed.

[0188] The tracking system, and optionally the magnetic field generator, preferably has software for generating the timing and shape of the excitation pulses. This excitation pulse generator preferably knows the capabilities of the hardware. There can be various types of amplifiers and filtering. Some types of amplifiers can produce current waveforms that closely follow somewhat arbitrary paths. This is referred to herein as an "analog amplifier."

[0189] Other amplifiers can only increase the current by a certain percentage, decrease the current by a similar percentage, or keep the current nearly constant. Essentially, these amplifiers apply a voltage of either a positive or negative sign to the coil, or act as a short circuit. These are referred to herein as "digital amplifiers." Digital amplifiers can vary their switching rate, i.e., allow several state changes per unit time. When the switching rate becomes significantly higher than the oscillation rate, digital amplifiers behave like analog amplifiers. Therefore, this type of amplifier can conceptually be treated as an analog amplifier.

[0190] If the switching speed remains approximately the same as the marker device frequency, then something needs to be done a little differently. However, this is a more difficult situation, so we will focus all our discussion on this point. This type of amplifier has some advantages over analog amplifiers. The main advantage is that the efficiency of this amplifier is usually very high, with 98% efficiency readily achievable. A further advantage is that it is very easy to interface with a computing system. There may be a matching circuit between the amplifier and the coil. The simplest matching circuit is just a capacitor in series with the coil. The use of a matching circuit increases the maximum current through the coil for a given amplifier supply voltage. However, such a matching circuit has the disadvantage of causing low-frequency blocking.

[0191] Some sequences require low-frequency current. The solution to this problem is twofold. First, a matching circuit can be provided that passes electromagnetic waves at high and low frequencies. An example of such a circuit is a coil or coil-capacitor series circuit in parallel with a first matching capacitor. Another approach is to provide a switch that bypasses the matching circuit, which is closed when near-DC current is required. A capacitor can also be integrated into the bypass path if the resonant frequency is low enough. Similarly, a range of different matching frequencies can be provided using multiple switches and capacitors. Also, note that even if the circuit is tuned near DC, some current at the marker / sensor resonant frequency is still available. Note that DC current is not necessarily available during readout. There are two main factors that provide this capability. First, DC current is not allowed to interface with the readout. This is a major problem when the transmit and receive coils are combined. A DC source can provide a short-circuit path to the signal. This needs to be avoided, and a proper matching circuit prevents this.

[0192] The matching circuit must introduce a sufficiently high impedance between the coil and the DC source. This can be achieved by an additional series coil, whose inductivity is similar to that of the transmit / receive coil. For inductance there is a parallel switch to spare it when it is not needed. There are many other solutions that can be used. 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 marker device. This can be achieved in the case of DC transmission by an appropriate analogue filter.

[0193] This filter is bypassed during AC transmit pulses by an appropriate switch (e.g., a MOSEFT optocoupler). It is also possible to avoid any switching in the DC source during signal reception, using only the slowly decaying current in the coil. It is also possible to perform only a few switching events during reception, rejecting the received data only if it is corrupted. The DC magnetic field source can be a completely separate coil, or the field generator can be a (moving) permanent magnet. This avoids most of the problems. Another problem with the presence of DC current during signal reception is that the coils present a different environment to the sensor. This means, for example, that some coils may be short-circuited with AC current, meaning that the AC magnetic field will no longer penetrate the coil and will alter the magnetic field values ​​in nearby coils. This effect must be taken into account when computing the position and / or orientation. Two main magnetic field components interact with the marker device. One component is the near-DC amplitude of the current, i.e., the current value averaged over a time period on the order of 0.1 seconds (approximately 0.01 seconds to approximately 1 second). The other component is the Fourier amplitude at the resonant frequency of the sensor / marker (as a complex value, since phase is important), so the first task is to map the two values ​​to an occurrence of the sequence.

[0194] Below we will discuss the mapping of Fourier amplitudes and currents to specific time-domain pulse patterns when desired.

[0195] It would also be useful to develop a software subsystem that performs just this kind of mapping, i.e., a software product that takes as input the desired near-DC current and the desired Fourier amplitude (and frequency) and generates a time-domain pulse train. It would also be desirable for this software to return information on whether the desired value was reached within hardware-imposed limits, such as maximum coil current or maximum heating, or regulatory limits, e.g., patient heating or peripheral nerve stimulation. Information about the severity of unwanted side effects is provided, rather than simply a yes / no response. This information is provided for each individual transmit channel (each transmit coil). Further returned values ​​are the actual best-fit output DC current and Fourier amplitude. The inputs are not just a combination of a frequency and a Fourier oscillation, but also various Fourier amplitudes at different frequencies. The maximum length of the pulse train is also a parameter that is input to this function. Its internal workings are as follows: For analog amplitudes, the first result is obtained by simply performing an inverse Fourier transform of the desired Fourier amplitude (and DC value) at the desired transmit time. If this process results in a waveform that cannot be realized due to some limitations, this is reported and a plan to possibly create a scaled version is made. The possible filter characteristics are described by the corresponding convolution. If there are several switching filter states, all are examined and the one with the lowest amplitude requirement is chosen. Note that in most cases, some hysteresis is available to avoid evaluating all filter states. For example, if a better filter is available, it is possible to eliminate a filter with a frequency that is far from the resonant frequency. In digital amplifiers, the inverse Fourier transform (including filtering) provides a good starting point for optimization. In this first approximation stage, the resulting peak in the time spectrum is approximated by two (or at most a few) slopes and flat regions between them. Thus, for example, a half-cycle of a sine wave that starts and ends at zero is approximated first by a flat (zero) part, then a rising edge, then a flat part, then a falling edge, and finally a flat (zero) part. The timing of the various parts is adjusted to reach approximately the same range.After this first approximation, in a second step the positions of the slope onset and plateau onset are shifted to reach a best fit with the desired Fourier values. This best fit is at least the minimum sum of squares of the various (complex) values ​​of the desired realized Fourier components. Any of the usual optimization algorithms can be used, such as gradient descent.

[0196] Below we will discuss the mapping of desired Fourier values ​​in the marker device to currents in the coils.

[0197] The next higher level of abstraction for a pulse generation program is a software product that takes as input specific Fourier magnetic field values ​​and directions at specific locations and converts them into demands for coil currents. The evaluation algorithm typically provides some measure of the sensor / marker's position and orientation. The position is not, and need not be, a position in 3D space. However, 3D position is an ideal case. For example, if there is only one coil, one can only determine the magnetic field value in the sensor's sensitive direction. Nevertheless, this translates into some virtual position and virtual orientation in 3D space. Therefore, this situation does not require special handling in the software. The conversion to coil current demands is therefore the result of an optimization process. There is a model that computes the Fourier magnetic field components at specific spatial locations from the currents in the coils. This is the basis for optimization, where the coil current Fourier components are optimized to produce the desired magnetic field components. Usually, there is no obvious way to generate the desired magnetic field components from the coil currents. The desired currents may not fit the constraints of the hardware system. The lower level software returns values ​​describing the side effects, and this software uses this information to optimize the current. The goal of the optimization is to obtain a good compromise between the Fourier components of the magnetic field obtained by the marker device and the side effects. This combines the deviation from the desired field and the side effects into a number for which a maximum or minimum can be found using standard optimization algorithms. The combination of numbers is a weighted sum of squares. Naturally, a huge number of practical mathematical combinations can be determined in this entity. Finally, this part of the program returns to the calling program (higher level) the magnetic field achieved at the position and Q value for which it is optimizing.

[0198] In the following, we will discuss generating the desired magnetic field Fourier values ​​for the marker / sensor.

[0199] At this level of abstraction, the software system actually handles the measurements that need to be made. Therefore, the input to this program is the current requirements for what to measure, how accurately, and how quickly. These requirements depend on the actual application in which the sensors / markers are used, and are not part of this specification. These requirements can vary widely. For example, if only one sensor is involved, the requirement might be to measure just one quantity as accurately as possible, say every 0.1 seconds. If the application is a tracking solution for multiple coupled markers, the desired outcome is a position update (based on coil sensitivities) for the entire marker assembly, say every 0.1 seconds, regardless of which markers / sensors in the marker assembly are involved in the signal, and an independent position determination every second using a gradient method is required. The program also has access to the current state of the sensors / markers (e.g., position / vibration parameters), as well as the simulation model described elsewhere in this specification. From this, the optimal excitation field Fourier value, including orientation, can be computed for each marker device. These parameters can then be sent to the lower software level above (hopefully somewhere down the line), which will ultimately generate the current. In the case of a single sensor, this works well, and the plan can be written to a hardware output buffer. However, when tracking an assembly of marker devices, for example, it is unlikely that there will be a pulse shape that fully excites every individual marker device. In particular, this step is not suitable for every individual marker device. Therefore, the software must try to focus the optimal excitation on only a subset of the current marker devices, and try to find a solution that gives a usable pulse sequence. This is a general principle for optimizing this software: it tries to vary and slightly target the desired excitation of various sensors while still achieving the desired outcome. The conceptually simplest approach is to go through a subset of all possible marker devices and determine which subset of excitations gives the best information about the desired parameters.Because there are many possible solutions, the program needs to add some heuristics to reduce complexity. For example, if a given marker device is excited and can necessarily be grouped, it can first observe which other marker devices are excited. Once a suitable solution is found, it can be written to an output buffer. The inclusion of near-DC fields requires additional logic circuitry depending on the hardware implementation. If the hardware can apply a DC field while the signal is being recorded, the software does not need to do anything very special except apply one or several gradients during readout. However, if the DC gradient and readout are incompatible, an additional optimization step is required to produce the correct DC field or DC gradient at some point between the excitation pulses. The logic behind the optimization remains the same: the parameters are varied in the simulation until the application predicts a sufficiently good measurement.

[0200] The following describes the generation of the startup sequence.

[0201] Algorithms typically assume a significant degree of knowledge about the marker devices that can be used to optimize the sequence. This knowledge is usually not fully available at the start of the sequence. For example, the application can tell how many marker devices are needed and what frequency range they will be at that time. However, the exact locations and frequencies are unknown. Therefore, a spatial startup sequence is required that attempts to find all possible marker devices at all possible locations. The simplest startup sequence is as follows: The available volume is divided into a spatial 3D grid, or abstract grid. The abstract grid is used when there are not enough coils to perform full 3D encoding. Each spatial point is divided into various directions. The program goes through each location and each angle of that location, applying the highest transmit power at a given frequency for a preset transmit time. The system then records the possible signals from the sensors / markers. Typically, a single transmit pulse excites not just one marker device but many others simultaneously. However, this procedure ensures that even marker devices with what may be considered the weakest signals are detected. An optional next step is to individually excite each sensor with various amplitudes, from which nonlinearities can be derived. A further optional step is to excite the marker device in the presence of a DC magnetic field or measure the signal phase after a DC magnetic field (again, in various directions) to determine the sensitivity of the sensor / marker to the DC field. These basic procedures can be greatly sped up by using some knowledge of the system. For example, if a volume far from the sensor / marker has already been examined, it can be assumed that many or all closer volumes received what may be considered the largest amplitude, at least from some perspective. Therefore, fewer remaining parameters need to be applied to the closer volumes. The same logic can be used to determine the nonlinear characteristics of the sensors / markers and their response to DC magnetic fields.

[0202] In the following, a strategy for high time resolution measurements will be explained.

[0203] For many applications, it is desirable to have high time resolution. Therefore, strategies for achieving high time resolution with magneto-mechanical oscillators are desirable, whether for position determination or parameter determination. The simplest approach to high time resolution is simply to shorten the repetition time. The repetition time refers to the time period between subsequent excitation pulses. As described elsewhere herein, each excitation pulse determines a frequency and amplitude from which physical values ​​and position can be computed. However, the Q factor of marker devices tends to be relatively high, and the oscillation amplitude does not narrow significantly during the next excitation pulse. To ensure the desired marker device excitation, the phase of the next excitation must be considered. Typically, one desires "in-phase excitation," i.e., excitation in which the marker device gains energy immediately after the onset of the excitation pulse. How timing is optimized is described elsewhere. In-phase excitation minimizes the transmitted energy, thereby allowing the excitation pulse length to be kept to a minimum. This increases the overall signal-to-noise ratio.

[0204] High repetition rates have several drawbacks. First, the system typically cannot receive values ​​during or immediately after the excitation pulse, resulting in a suboptimal signal-to-noise ratio. Second, each transmit pulse destroys some knowledge of the phase of the sensor's vibration. Phase information can only persist to a certain extent if the excitation pulse and sensor orientation are kept tightly controlled and precisely known, a technical challenge. Long-term phase information is useful because it encodes information about the average frequency (and thus the average physical quantity). Measurements of the average physical quantity are significantly more accurate if an interval twice as long is evaluated than if the first and second halves were evaluated independently and then averaged. Therefore, rather than using the same number of excitation pulses as measurements, it is worthwhile to derive more than one measurement from a single signal pulse. This can be done simply by dividing the signal into several subsections and evaluating each subsection independently.

[0205] This simple approach does not take into account that measurements improve when longer data sets are used. To incorporate this, the data set can be divided into a hierarchy of subsets, and each subset within each hierarchy is evaluated, with the average scaled to fit the longer data set. So, for example, first, the data set (one unperturbed decaying signal) is evaluated as a whole. Next, the data set is split into two, and the two separate data sets are evaluated separately. Then, for each result, the same number is added so that their average matches the average of the full set. This process can be repeated until there are four, eight, etc. subsets. This approach can be mathematically refined up to a full model-based evaluation. For this, a model of the evolution of the physical parameters is created (and possibly also the spatial translation of the sensor). This model is a polynomial of a suitable mathematical function, such as a certain order. This function must represent the properties of the measured quantity so that fewer parameters are used. So, for example, if the parameter is blood pressure, the model can be a better Fourier series since this parameter represents the pressure waveform of the heartbeat better than a polynomial.Finally, if discrete measurement points are needed, they can be computed simply by using the output of the model at a given time.

[0206] Other variations to the disclosed embodiments will be apparent to and can be effected by those skilled in the art when practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0207] In the claims, the word "comprising" does not exclude other elements or steps and the word "a" or "an" does not exclude a plurality.

[0208] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0209] Determinations such as determination of the resonant frequency based on the induced signal, determination of the position and / or orientation based on the resonant frequency, determination of the calibration curve etc. performed by one or several units or devices may be performed by any number of units or devices. The control of the tracking system may be embodied as program code means of a computer program and / or as dedicated hardware.

[0210] A computer program provided together with or as part of other hardware may be stored / distributed on a suitable medium, such as an optical storage medium or a solid-state medium, but may also be distributed in other forms, such as via a wired or wireless telecommunications system such as the Internet.

[0211] Any reference signs in the claims should not be construed as limiting the scope.

[0212] The present invention relates to a tracking system and a marker device, the tracking system being used to track a marker device, the marker device being adapted to be attached to a medical device. The tracking system is provided for use during surgery, wherein the marker device has a sensing unit made of a magnetic material that produces a permanent magnetic moment, the sensing unit being configured to convert an external magnetic or electromagnetic excitation field into mechanical vibrations of the magnetic material, the tracking system having a magnetic field generator that generates a predetermined magnetic or electromagnetic excitation field to induce mechanical vibrations of the magnetic material of the sensing unit, a transducer that converts the magnetic or electromagnetic field generated by the induced mechanical vibrations of the magnetic material into one or more electrical response signals, and a position determination unit that determines the position of the marker device based on the one or more electrical response signals.

Claims

1. 1. A surgical tracking system for tracking a marker device attached to a medical device, comprising: The marker device a sensing unit having a magnetic body that produces a permanent magnetic moment, the sensing unit converting an external magnetic or electromagnetic excitation field into mechanical vibrations of the magnetic body; The tracking system comprises: a magnetic field generating device that generates a predetermined magnetic or electromagnetic excitation field to induce mechanical vibration of the magnetic body of the sensing unit; a transducer that converts a magnetic or electromagnetic field generated by the induced mechanical vibration of the magnetic body into one or more electrical response signals; a position determining unit that determines a position of the marker device based on the one or more electrical response signals.

2. 2. The tracking device of claim 1, wherein the position determination unit determines at least five degrees of freedom for the marker 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 a position and at least two orientation angles of the marker device relative to the tracking system.

3. the tracking system determines the positions of the plurality of marker devices, each of the plurality of marker devices having a respective sensing unit; the magnetic material of each of the sensing units is vibrable to generate different magnetic or electromagnetic fields at different resonant frequencies which translate into one or more respective electrical response signals characteristic of each of the marker devices; The tracking system of claim 1 or 2, wherein the position determining unit determines the position of one or more of the plurality of marker devices based on the respective one or more electrical response signals.

4. A tracking system according to any one of claims 1 to 3, which compensates for the dependence of the one or more electrical response signals on temperature.

5. The position determination unit: static background magnetic field, and dynamic background magnetic field 5. The tracking system of claim 1, further comprising a compensation algorithm that compensates for one or more of:

6. 6. The tracking system of claim 1, wherein the position determining unit applies a compensation algorithm to compensate for nonlinearities resulting from the mechanical vibrations of various vibration amplitudes.

7. the magnetic field generating device includes a magnetic field generating array having a plurality of generating units arranged in a predetermined spatial arrangement; the one or more electrical response signals indicative of characteristic mechanical vibrations of the magnetic material of the sensing unit induced by each of the plurality of generating units; The tracking system of claim 1 , wherein the position determining unit determines the position of the marker device based at least in part on the one or more electrical response signals indicative of the characteristic mechanical vibrations.

8. 8. The tracking system of claim 7, wherein the position determining unit determines, from the one or more electrical response signals, an amplitude of the characteristic mechanical vibration of the magnetic body for each of the plurality of generating units.

9. the tracking system further comprises a control unit; the magnetic field generating device includes a magnetic field generating array having a plurality of generating units arranged in a predetermined spatial arrangement; each of the plurality of generating units is controlled independently of the other of the plurality of generating units by the control unit, the control unit controlling at least some of the generating units such that at least one spatial excitation field component of the magnetic or electromagnetic excitation field is modifiable by said control; 9. The tracking system of claim 1, wherein the position determination unit determines the marker device position based at least in part on the one or more electrical response signals indicative of the modification of the at least one spatial excitation field component.

10. the magnetic field generator sequentially generates a series of different additional magnetic or electromagnetic encoding fields that vary in space and / or time; 10. The tracking system of claim 1, wherein the position determining unit determines the position of the marker device based at least in part on the one or more electrical response signals converted by the transducer based on magnetic or electromagnetic fields generated by induced mechanical vibrations of the magnetic body in response to each of the series of various further magnetic or electromagnetic encoding fields.

11. A marker device attached to a medical device, comprising: the marker device comprises a casing and a sensing unit having a magnetic body that produces a permanent magnetic moment, the sensing unit converting a magnetic or electromagnetic excitation field into mechanical vibrations of the magnetic body; A marker device, wherein the induced mechanical vibration is independent of an external pressure experienced by the sensing unit.

12. The marker device of claim 11 , wherein the casing is a hard casing.

13. 13. A marker device according to claim 11 or 12, having an elongated shape with a maximum dimension of 5 mm or less and a minimum dimension of 1 mm or less.

14. The magnetic body is arranged in the casing so as to be rotatable from a balance direction when the external excitation field or the external electromagnetic excitation field acts on the magnetic body, and the sensing unit A marker device described in any one of claims 11 to 13, further comprising a restoring torque unit that provides a restoring torque to return the magnetic body to the equilibrium orientation when the external magnetic excitation field or external electromagnetic excitation magnetic field rotates the magnetic body from the equilibrium orientation so as to enable the mechanical vibration of the magnetic body at a resonant frequency.

15. A surgical medical device fitted with a marker device according to any one of claims 10 to 13, said device being tracked by a tracking system according to any one of claims 1 to 10.

16. The medical device of claim 15 , comprising a tip to which the marker device is attached.

17. 17. The medical device according to claim 15 or 16, comprising one or more of the following interventional devices or implants, in particular electrical implants and / or orthopedic implants.

18. 18. The medical device of any one of claims 15 to 17, comprising one or more of a surgical instrument, a diagnostic imaging probe, an endoscope, a bronchoscope, or an ingestible tablet.

19. 19. The medical device of any one of claims 15 to 18, comprising one or more of a catheter, a wire, particularly a guidewire, a stent, one or more aneurysm coilings, one or more vena cava filters, a heart valve, a shunt, a needle, a wire, a tube, a stylet, or a radioactive seed.

20. A medical device as described in any one of claims 15 to 19, wherein the medical device has a longitudinal shape and is equipped with a plurality of marker devices as described in any one of claims 10 to 13 arranged along the longitudinal axis of the medical device.

21. 15. A method of tracking a marker device according to any one of claims 11 to 14 attached to a medical device using a tracking system according to any one of claims 1 to 10, said method comprising: generating a magnetic or electric excitation field to induce mechanical vibration of the magnetic body of the sensing unit; converting a magnetic or electromagnetic field generated by induced mechanical vibration of the magnetic material of the sensing unit into one or more electrical response signals; and determining a position of the marker device based on the one or more electrical response signals.

22. 22. A computer program comprising program code means for causing a tracking system according to claim 1 to perform the steps of the method according to claim 21 when said computer program is running on a computer controlling said tracking system.

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