Tracking system and marker device tracked by tracking system

Abstract

To provide a tracking system for tracking a marker device attached to a medical device.SOLUTION: A marker device 501 comprises a sensing unit including magnetic substances 507 and 508 causing mechanical vibrations by an external magnetic excitation field or an external electromagnetic excitation field. A tracking system includes a magnetic field generator for generating a prescribed magnetic excitation field or an electromagnetic excitation field to induce mechanical vibrations of the magnetic substances, a transducer for converting the induced mechanical vibrations of the magnetic substances into one or more electrical response signals, and a position determination unit for determining a 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 more particularly minimally invasive, medical procedures are commonly used means to properly assess and / or treat 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 being adapted for example to 3 to 5 degrees of freedom (DoF).

[0004] Also, commonly known electromagnetic marker devices are considerably 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 Trasponder" 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 away. 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] It is therefore an object of the present invention to provide an improved tracking system, an improved marker device, a respective medical device, as well as 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 capable of accurately indicating the position of a human being, in particular 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 is capable of accurately tracking 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 comprises 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 comprises 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, there is provided a tracking system 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 surgery, and more particularly during minimally invasive surgery, as it allows for accurate positioning and / or orientation determination for the medical device using fairly small marker devices.

[0009] For this purpose, the tracking system uses a marker device composed of a sensing unit with a magnetic material having a permanent magnetic moment. Thereby, when the sensing unit of the marker device is subjected to a certain external magnetic excitation field or an external electromagnetic excitation field, the magnetic material starts to vibrate in response to this excitation field. The mechanical vibration of the magnetic material gives rise to a magnetic field or an electromagnetic (response) field, which is then converted by the respective transducer into one or more electric 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 a position response signal and can be used by a position determination device to determine the position of the marker device and thereby the position of the 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 surgery, in particular during minimally invasive surgery. In some embodiments, the medical device may refer to an interventional tool used in an interventional procedure performed on a human being, in particular on a patient.

[0011] Generally, the positioning technique using the tracking system and marker devices proposed herein is used for any medical device for which it is advantageous to perform positioning / localization. Thus, in some embodiments, the term medical device is used for any further medical device that can be useful for localization. As an example, we will mention bandages and patches. In such cases, it may be important for safety reasons to track the position and / or orientation of such kind of bandages and patches, for example after a surgical operation, 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 performing a respective mechanical vibration, in particular a rotational vibration. Such mechanical vibration is used by the 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 comprises 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 over the entire field of effect of the magnetic field.

[0014] Generally, the inventive concept is based on the fact that the reaction of a magnetic body to a magnetic or electromagnetic excitation field (measured in terms of mechanical vibrations) can give information about the position and / or orientation of a marker device having a sensing unit comprising 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 and the magnetic or electromagnetic excitation field.

[0015] Various possible approaches can be used to determine, i.e. locate, 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 respect, two specific location approaches, also referred to as localization approaches, are used. One approach is based on the coil sensitivity of each coil in the coil array. This approach involves determining a spatial sensitivity profile B s、i In this case, the magnetic body of the sensing unit generates a unique 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. For this purpose, a non-uniform magnetic field, ideally with a constant magnetic field gradient over the entire field of action, 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 independent control of the coils as 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 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] Details of these location techniques are provided further 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 a 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. For this purpose, a positioning 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 to determine 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 thus of the medical device to which the marker device is attached, using only one marker device.

[0021] According to some embodiments, the 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, in particular by rotation, such that different resonant frequencies cause 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 the resonant frequencies of the induced signals differ for each marker device, and the position determining unit is adapted to determine also the orientation of the marker devices based on the different frequencies of the emitted induced signals. By using marker devices, the resonant frequencies of which differ for each marker device, 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 orientation with respect to the several marker units. The marker device may in particular determine the shape, position and / or orientation of the medical device. It should be understood that the marker device is in particular used to determine the shape, position and / or orientation of the medical device, but also to determine the shape, position and / or orientation of other elements to which the several marker devices are attached, such as body tissues. In some embodiments, several 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 an 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 another marker device. 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, hi 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 positioning unit, is configured to compensate for the dependence of the 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 input means for inputting the temperature. To compensate for the temperature dependence, an algorithm is provided which takes into account the input temperature and correlates it 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 effect of temperature from the electrical signal. That is, in some embodiments, the temperature compensation is performed by means of a compensation algorithm, i.e. embodied in the program code.

[0028] Alternatively or additionally, temperature compensation can be obtained by various means, such as a physical compensation element. That is to say, 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 vibration of the magnetic body on temperature. For this, 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 higher or lower frequency direction, and the opposite second frequency direction corresponds to a lower or higher frequency direction, respectively.

[0029] The compensation element preferably comprises a magnetic material, the magnetization of which changes and thus the resonant frequency changes according to temperature, and in this case a 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 compensation 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 the undesirable temperature dependence can be significantly reduced or even eliminated in a technically relatively simple manner 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 magnetic background field and the dynamic magnetic background 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 correction values ​​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 can be used, which can be integrated into the tracking system. As an example, we will mention a three-axis Hall sensor. 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 of 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 the frequency shift during the evaluation, the coils of the multi-coil tracking system can be used to generate a small offset magnetic field to counterbalance the background magnetic field and / or the geomagnetic field. If 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 the interpolated background magnetic field mapping derived from these measurements, correction values ​​for sensing units with known positions and orientations can be calculated, 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 effect 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 devised that employs two hanging balls with the same magnetic dipole moment and moment of inertia (or a suitable ratio between the two quantities). Since the counter vibration occurs at a single frequency, the first order effect of static bias magnetic fields, such as the earth's magnetic field, is 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 vibration.

[0037] The positioning unit is configured to compensate for nonlinearities in the system resulting from different vibration amplitudes of the mechanical vibration of the magnetic body of the sensing unit. In some embodiments, this extends to further optional processing steps, in particular to apply an inverse nonlinear filter to counteract the nonlinearities of the tracking system. Thereby, the nonlinearities of the tracking system are measured and a computational filter is constructed that reverses the effect of the nonlinearities. This is particularly useful when low-cost components are used that tend to have more nonlinear behavior.

[0038] Alternatively, a nonlinear filter is used as the first processing stage. If more than one signal is used, the signal processing stages increase. If at least one receiving channel does not detect a response from the sensing unit of the marker device, thereby giving 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 channels bearing the signal. This subtraction can be done in the time domain, the frequency domain or a combination of both. If there are no channels without any sensor signal, a data processing strategy sometimes called "virtual magnetic gradiometer" is used. It resolves many channels in a virtual channel, which is a linear combination of the physical channels, so as to minimize the interference of response signals not caused by detection. The coefficients in 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 device comprises a magnetic field generating array having a plurality of generating units arranged in a predetermined spatial arrangement, where the one or more electrical response signals are indicative of a characteristic mechanical vibration of the magnetic body of the sensing unit induced by each of the plurality of generating units, and 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 vibration. In some embodiments, the position determining unit is adapted to determine an amplitude of the characteristic mechanical vibration of the magnetic body for each unit of the plurality of generating units from the one or more electrical response signals.

[0040] In some embodiments, the magnetic field generating device may include a plurality of generating units spatially arranged in a magnetic field generating array. In some embodiments, this spatial arrangement may be two-dimensional. However, a three-dimensional spatial arrangement is also accepted as a possibility. 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 will be described in more detail below.

[0041] In some embodiments, the tracking system further comprises a control unit, the magnetic field generating device having a magnetic field generating array with a plurality of generating units arranged in a predetermined spatial arrangement, where each unit of the plurality of generating units is adapted to be controlled by the control unit independently of the other units 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 the 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 varying in space and / or time, and the position determining unit is adapted to determine the 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 give independent position information. For this, a non-uniform magnetic field, where the magnetic field gradient is ideally constant over the entire area of ​​effect, is generated, for example, by applying low frequency currents to selected coils of the coil array.

[0043] This additional magnetic field acts on the magnetic material of the sensing unit as a resonant magnetic field B rest , and thus 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 may be deferred from the higher order response of the sensor to the external magnetic field, but at the expense of stronger magnetic field strength, which is required to provide a sufficiently high order involvement. The basic encoding idea 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 provide the desired frequency shift. For the desired spatial resolution, the applied encoding magnetic 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 one readout, i.e., a position-dependent frequency shift is applied with only a short window of opportunity to create a position-dependent signal phase shift. If the phase resolution is not sufficient for accurate position determination / localization, the duration and / or amplitude of the phase encoding pulse can be varied in successive excitations to allow for ambiguities in the phase addition (larger than 2pi). Thus, the 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 the invention, a comparison of the localization results obtained by gradients versus sensitivity encoding can be used to determine systematic errors, for example, caused by background magnetic fields. It should also be noted that the linear response of the sensing unit, which employs, for example, two suspended magnetic spheres as the magnetic body, to low-frequency external magnetic fields is suppressed, and in this case the higher order response of the frequency can be used not only for localization but also for sanitary checks. However, the magnetic field sensitivity of these oscillators is so low that a higher gradient magnetic field is required for gradient magnetic field encoding.

[0048] In another aspect, a marker device is provided for attachment to a medical device. The marker device includes a casing and a sensing unit having a magnetic body 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 body, and the induced mechanical vibration is independent of an 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 body is disposed within the casing such that the magnetic body is rotatable from an equilibrium orientation when the external magnetic excitation field or the external electromagnetic excitation field acts on the magnetic body. Thus, the sensing unit further includes a restoring torque unit that provides a restoring torque to return the magnetic body to an equilibrium orientation when the external magnetic excitation field or the external electromagnetic excitation field rotates the magnetic body from an equilibrium orientation that allows mechanical vibration of the magnetic body at a resonant frequency.

[0049] According to a further aspect, a marker device is provided which allows to determine the position and / or orientation of a medical device to which it is attached, the marker device comprising a casing and a sensing unit which allows to convert an external magnetic or electromagnetic excitation field generated by a magnetic field generating device into a mechanical, preferably rotational, vibration of a magnetic body with a permanent magnetic moment, which is provided on the sensing unit.

[0050] The sensing unit with the magnetic body in particular comprises a casing or is arranged in a casing. In particular, the magnetic body is arranged in the casing. In particular, the magnetic body is therefore arranged in the casing such that it can be rotated from its equilibrium direction by an external magnetic torque acting on it. 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 comprises a restoring torque unit which provides a restoring torque to return the magnetic body to the equilibrium orientation when the magnetic body is rotated from the equilibrium orientation by the external magnetic or electromagnetic field. This results in a 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 which depends on 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, rotational vibration of the magnetic body is thereby converted into a respective one or more response signals. These response signals thus depend on the resonant frequency of the vibration.

[0052] In some embodiments, the rotational vibrations in particular ultimately result in respective induced signals, since these induced signals depend on the position and orientation of the marker device in the external magnetic or electromagnetic field. These induced signals are in particular emitted in an excitation and induced signal unit of the tracking system. In particular, the excitation and induced signal unit may consist of i) a first coil adapted to generate a magnetic field that exerts a magnetic torque that rotates the magnetic body of the tracking device from its equilibrium direction, thereby exciting a rotational vibration of the magnetic body, and ii) a second coil adapted to emit an induced signal that depends 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, such that it is possible to determine the position and orientation of the medical device equipped with this marker device using only one marker device.

[0053] Furthermore, this arrangement allows the tracking system to perform tracking based on marker devices that are relatively far away, for example, more than 30 cm. Also, the marker devices may 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] It is preferable that the magnetic body is rotatable around a vertical axis of rotation that runs through the center of the magnetic body, and the magnetic body is rotationally symmetrical with respect to the vertical axis of rotation. 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 the restoring torque. Additionally or alternatively, the restoring torque unit may include a further magnetic body that provides the restoring torque.

[0055] In one embodiment, the magnetic body is attached to one end of an attachment such as a filament, the other end of which is attached to the casing. The attachment is adapted to prevent the magnetic body from hitting the further magnetic body embodying the recovery torque unit due to their magnetic force and to rotate the magnetic body. The further magnetic bodies are preferably fixedly attached to the casing in a stationary manner. However, the further magnetic bodies may also be arranged in the casing such that they are capable of rotating and oscillating relative to the casing. In particular, the further magnetic body may be attached to one end of an attachment such as a filament, the other end of which is 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 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. It is further 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 vibration of the magnetic material such that the restoring torque, and therefore the entire marker device, can be relatively small, the resonant frequency of the marker device can be produced as desired, and the construction of the marker device can still be relatively simple.

[0058] Here, in order to determine the 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 instrument from the outside, such as blood pressure or circulatory pressure.

[0059] For this purpose, the marker device is provided with a casing in which the sensing unit is located, because this casing has one or more rigid walls, in particular a hard casing, i.e. a casing with walls that do not change their shape when an external pressure acts on it. This means that the positioning of the magnetic body inside the casing is not significantly affected by the external pressure due to the walls not bending under 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 from the outside on the sensing unit. Thus, the magnetic force acting between the magnetic body and the restoring torque unit does not change due to the spacing change caused by the pressure-induced bending of any wall in this case, and therefore does not depend on any external pressure acting on the sensing unit. Thus, the resonant frequency of the rotational vibration caused by the 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 influenced 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 determine / locate 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 fairly 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, in particular one or more of an electrical implant and / or an orthopedic implant. In some embodiments, the medical device may include in particular one or more of, for example, a surgical instrument, an imaging probe, an endoscope, a bronchoscope, or an ingestible pill. Alternatively or additionally, the medical device may include in particular one or more of a catheter, a wire, in particular 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 hereinbefore, using the tracking system as described hereinbefore. 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 body of the sensing unit, converting the magnetic or electromagnetic field generated by the induced mechanical vibrations of the magnetic body 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 comprising program code means for causing the tracking system as described hereinbefore to perform the steps of the tracking method as described hereinbefore, when the computer program is executed on a computer controlling the tracking system. [Brief description of the drawings]

[0063] In the following drawings: [Figure 1] 1A and 1B are schematic and exemplary diagrams illustrating an embodiment of a marker device according to a first embodiment; [Diagram 2] FIG. 2 shows a schematic and exemplary marker device attached to a medical device. [Diagram 3] 3A-3C show different views of the marker device and the medical device according to FIG. 2. [Figure 4] FIG. 2 illustrates, diagrammatically and exemplarily, a further implementation of a marker device in a tracking system for tracking a medical device. [Figure 5A] FIG. 2 illustrates, diagrammatically and exemplarily, a further implementation of a marker device in a tracking system for tracking a medical device. [Figure 5B] FIG. 2 illustrates, diagrammatically and exemplarily, a further implementation of a marker device in a tracking system for tracking a medical device. [Figure 6] FIG. 1 shows a schematic and exemplary implementation of a marker device and a tracking device 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 show schematic and exemplary embodiments of a medical device corresponding to a wire for the treatment of cerebral aneurysms, to which a marker device is attached. [Figure 9] FIG. 2 shows a schematic and exemplary embodiment 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 illustrates a schematic and exemplary tracking system for tracking a marker device, according to an embodiment of the present invention. [Figure 12] FIG. 1 illustrates a schematic and exemplary tracking system for tracking a marker device, according to an embodiment of the present invention. [Figure 13] FIG. 2 is a schematic and exemplary diagram illustrating an excitation pulse and the resulting induced voltage. [Figure 14] FIG. 1 illustrates a schematic and exemplary multi-coil array integrated into a mattress of a patient bed of an imaging system. [Figure 15] FIG. 2 shows a schematic and exemplary diagram of a receiving coil of a tracking system for detecting magnetic or electromagnetic field variations induced by mechanical vibrations. [Figure 16] FIG. 2 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. 2 is a diagram illustrating an example of a Chebyshev II type bandpass frequency response. [Figure 19] FIG. 13 shows the measured dependence of signal amplitude at different harmonics on sensor orientation for a single transmit / receive coil. [Figure 20] 13A-13D show schematic and exemplary diagrams of further embodiments of the marker device; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 being, 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 body 507 is fixed. However, in alternative 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, 508 are aligned with their magnetizations antiparallel, respectively. 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 allows obtaining 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 construction or coating materials, since the magnetic field is not shielded by the metal. Also, the marker device can be placed 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 like copper, the skin depth is on the order of 1 mm, while for nitrile, the skin depth is about 10 mm.

[0069] Thus, the sensing unit 501 includes two magnetic bodies 507, 508, which in equilibrium are aligned with anti-parallel magnetization. An external magnetic field pulse provided by a respective magnetic field generator can be used to initiate a rotational movement of the 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, both magnetic bodies 507, 508 can undergo resonant counter vibrations if the other magnetic body 507 is also suspended in free space and allowed to undergo a rotational movement.

[0070] The use of magneto-mechanical oscillators is known to determine the position and / or orientation of a marker device relative to a tracking system. 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 Trasponder" 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 the marker devices are made smaller, the accuracy of the measurements also decreases. Thus, position determination measurements using the marker devices listed so far are not ideal, especially for small marker devices.

[0071] In other words, as we reduce the size, 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 formulas overestimate the Q values ​​that are practically achievable. Nevertheless, these values ​​lead to a well-working 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 is scaled according to γ 2 Therefore, the proposal presented here is very well suited for sensor miniaturization.

[0073] The problems mentioned above are therefore avoided, for example, by the design proposed in FIG. 1. Since the energy is mainly stored in the magnetic field, a high Q value can be achieved relatively easily. High vibration amplitudes are also easily possible. The sensing unit usually also employs mountings such as thin filaments that are not subject to severe wear. This allows the resonance to be easily altered by altering the magnetic field through mechanical movement of the magnets relative to each other. This alteration 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 ball, the diameter of the fixed ball may be 620 μm, while the diameter of the vibrating ball 108 may be 500 μm. The magnetic moment of the vibrating ball 108 is m≈70 μA m 2 The fundamental frequency may be f0≈2KHz and the Q factor may be roughly Q≈500. The SNR depends on a) the coil used to read out the resonant frequency and b) the sensing device as well as 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 ​​of demonstrators for fixed spheres may be 10-100 if little measures against background signal suppression are implemented. The noise is therefore mainly determined by the fluctuations of the lead-in power harmonics. At half the sphere diameter, i.e. 250 μm for the vibrating sphere, the magnetic moment is m≈9 μAm 2 , the fundamental frequency may be f0≈4kHz, 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 attachment can be used. In this case, a hole is drilled through the center of gravity, approximately perpendicular to the magnetization. The magnetic material is hard and brittle, but there are several ways to drill holes, such as pulsed laser or Electrical Discharge Machining (EDM). The thread is threaded through the hole and glued in place. Threading is best done using a vacuum suction process. Several types of adhesives can be used. An economical one is a light-curing adhesive. This has a low viscosity and simply pulls the thread through the hole by capillary forces alone. Additionally or alternatively, the attachment portion can be fixed to the magnetic material 508 by mechanical means, for example by tying a knot in the thread or by creating some other thick part on the thread, such as a glue drop or a heat-induced (melted) bead. The latter is particularly easy to do with UHMWPE fibers. This attachment method only reduces the magnetic dipole moment slightly, so it retains a good signal. The shape of the magnetic material cannot be changed significantly, which is important in the case of a sphere.

[0077] Clamp mounting can also be used. In this case, the magnetic body is divided into at least two components. The partition plane is preferably raised perpendicular to the magnetization and parallel to the thread attachment direction. The thread, i.e., the filament, is placed in this plane. Exact alignment is not necessary. The second magnetic part is placed on top. The magnetic parts are usually held together by magnetic forces. Finally, glue is applied to fix everything in place. The glue type is preferably the same as in the through-hole mounting process. It is also possible to groove one or both of the magnetic bodies to narrow the overall gap between the magnetic bodies. This method gives almost the same good results as the through-hole method, and does not require any special equipment for its manufacture. Usually, the magnetic sub-body is 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 laborious, since two magnetic bodies are used in the first place.

[0078] The cheapest method is to attach the end of the thread directly to the magnetic material 508 using a suitable adhesive. The magnetic material 508 is held and aligned by some kind of tool. A suitable magnetic field can achieve both functions. The tool can be in the shape of a funnel through which the thread passes and the magnetic material is attached to the funnel opening by magnetic force. Adhesive is applied to the funnel and allowed to harden. The assembly is then pulled out of the tool and the unnecessary part of the thread is cut off. This method is very cheap 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 with the structure to which it is attached. The thread can be attached to the magnetic body 508 by first attaching the thread to the non-magnetic body and then gluing the non-magnetic body to the magnetic body. The non-magnetic body may be manufactured by joint molding or an equivalent inexpensive process. The shape of the non-magnetic body must be such that it allows for easy thread attachment, i.e. it has holes or fastening mechanisms, perhaps as simple as notches. Thereby, the non-magnetic body is glued to the magnetic body. Alternatively, the non-magnetic body may be clamped or screwed onto the magnetic body. This method is simple and inexpensive, but depending on the application, it may require a significant amount of space.

[0080] In principle, all of the methods mentioned for thread-magnetic attachment apply equally to thread-casing attachment. However, the through-hole method is considered the better choice because the casing material usually works out easier with it. Clamping is also a good choice. This will be cheaper, but the finish can be more difficult to seal.

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

[0082] The marker device described above is adapted to be attached to any kind of medical device to be tracked during a medical procedure. For this purpose, Figs. 2 and 3 show a schematic representation of 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 kind of medical device, in particular any kind of medical instrument, and even more particularly any kind of medical instrument in performing a surgical procedure (minimally invasive) where tracking is advantageous. In some embodiments, the marker device is also used to track various elements such as tissues, bandages, etc. Examples of further devices and / or elements to be tracked are shown in Figs. 4 to 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 506.

[0084] The dimensions shown in Figures 2 and 3 are merely exemplary. The dimensions may vary. However, the illustrated dimensions are highly suitable for tracking during interventional procedures on human patients. Applying scaling laws to the demonstrator SNR, it can be seen that the illustrated dimensions provide sufficient SNR and accuracy for remote surgery far enough away to completely penetrate the patient. Thus, 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 it 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 a 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 a tracking system, but also allows the orientation and / or shape of the stylet 710 to be determined.

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

[0089] FIG. 7 shows, in schematic form, 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, the information provided by the marker device 501, 501', 501", 501"' attached to the patient's prostate 102 is combined with information from further sensors such as pressure, temperature, radiation sensors, etc. This combination makes it possible to determine the position and / or orientation of the prostate 102 with respect to a coordinate system provided by the tracking system and further to measure parameters such as temperature, pressure or radiation using the sensors mentioned above, since these measurements are correlated to specific positions. This allows a spatial mapping of these measured parameters to be obtained. Such a spatial mapping makes it possible to control and / or monitor the course of the treatment procedure.

[0090] In particular, FIG. 7 shows the prostate 102 of a patient. In a particular embodiment according to FIG. 7, it is assumed that an ablation procedure is to be performed on the prostate tissue. For this purpose, a number of markers 501, 501', 501", 501" are provided at several locations on the prostate 102. Also, one or more sensors are provided (not shown) at various locations on the prostate, preferably close to the area of ​​interest where the ablation procedure is to be performed. This allows to provide a spatial mapping of the temperature evolution during the ablation procedure, helping to avoid temperature induced damage and / or over-treatment of healthy tissue during the ablation procedure.

[0091] FIG. 8 shows, in a schematic and exemplary manner, an embodiment of a wire for the treatment of cerebral aneurysms. The wire 910 comprises one or more marker devices 501, 501', 501" according to the above embodiments. In particular, a first marker device 501 is at a first end 911 of the wire 910 on one side of said first end 911. Also, a further marker device 501' may be attached in an intermediate section of the wire 910 to which a further marker device 501" of the second end 912 of the wire 910 may be attached, and the wire 910 may comprise an internal cavity in which the marker device 501" is located.

[0092] FIG. 9 shows an embodiment of an intrahepatic shunt device 1100 having a wire structure 1103 in a schematic and exemplary manner. In this embodiment, the wire structure 1103 has a first part 1101 surrounded by a linear material and a second part 1102 that is bare. 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 in the second part 1102 of the wire structure 1103, the wires are interwoven. The intrahepatic shunt device 1100, which may simply be referred to as an intrahepatic shunt, comprises several marker devices 501, 501', 501'', 501'''. For example, a first marker device 501 is disposed next to each wire of the first part 1101 of the wire structure 1103 inside a PTFE tube. 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] In Figures 4 to 9 the arrangement of the marker devices is merely exemplary, i.e. there may be more or less 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 (>10 readings per second) at intervals of >30 cm with high accuracy.

[0095] The marker device may be attached to a guidewire, for example as described above with reference to Figs. 2 and 3, and used to track such a guidewire. The marker device may also be used to track a catheter. The marker device may be placed on a stent. To minimize disturbances caused by the marker device during stent placement, the marker device must be as small as possible and should not exceed the stent wire diameter. A typical stent wire diameter is 0.2-0.5 mm. This is therefore a useful range for the marker device diameter. The marker device may also be injected by syringe, in which case it may be fitted into smaller blood vessels in the pulmonary or intrahepatic areas without risk to the patient. A typical diameter at attachment is 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 of the resonant frequency will be described below with reference to FIG.

[0097] 10, the marker device 3001 is also composed of a casing 3002 and a magnetic body 3004 arranged in the casing 3002, the magnetic body 3004 being rotatable from an equilibrium orientation when an external magnetic torque is acting on the magnetic body 3004. The marker device 3001 further comprises a restoring torque unit 3003 adapted to provide a restoring torque to return the magnetic body 3004 to an equilibrium orientation when the magnetic body is rotated from an equilibrium orientation by an external magnetic field or an external electromagnetic field, so as to enable a rotational oscillation of the magnetic body 3004 excited by the external magnetic field or the external electromagnetic field providing a respective 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 restoring torque unit 3003 has a further magnetic body 3003 which provides the restoring torque. Specifically, the magnetic body 3004 is attached to one end of an attachment portion 3007, such as a filament, the other end of which 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 rotate and oscillate. 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 has two end faces 3030, 3031, 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 surface 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 a mounting portion 3007 such that the external pressure effect 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, this influence being temperature dependent, so as to change the strength of the magnetic field at the position of the magnetic body 3004 and thus the resonant frequency as the temperature changes. 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. Thus, the magnetic materials 3005, 3006, which are soft magnetic materials, affect the resonant frequency as a function of temperature in opposite frequency directions, i.e., one of the magnetic materials changes toward a higher frequency as temperature increases and the other of the magnetic materials changes toward a lower frequency as temperature increases.

[0103] The marker device 3001 is preferably configured such that the resonant frequency is not temperature dependent. To compensate for any undesirable temperature-dependent frequency shifts, the magnetic materials 3005, 3006 may be arranged to provide the same frequency shift in the opposite frequency direction in response to temperature changes. In particular, the magnetic material 3005 is selected and arranged 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 lowers the resonant frequency as the temperature increases, or a material that increases the resonant frequency as the temperature increases, may be used to suppress 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 as elements that compensate for the temperature-induced shift of the resonant frequency.

[0104] Fig. 11 shows a schematic and exemplary tracking system 1501 for tracking the marker device described above, i.e. for wirelessly determining the position and / or orientation of the marker device attached to the 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 sensor of the magnetic field, 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 and wide enough to cover the response of several sensors operating in parallel at different frequencies, and possibly even higher harmonics of the sensor resonant frequency, for example to increase the SNR. The transmitted magnetic field amplitude is up to a few millitesla, while the magnetic field amplitude to be detected is between a tenth of nT and a few nT. Many different magnetic field generators (vibrating permanent magnets, cored / coreless coils, magnetostrictive magnetic field modulators, etc.) can work well as many different magnetometers (Hall effect sensors, various kinds of magneto-resistive sensors, magneto-resonant sensors, SQUIDS, etc.). The technically simplest system is an air-core conductor loop for transmitting and receiving magnetic fields. A coil is typically sufficient for sensor applications. The coil that generates the magnetic field can also be used to receive the magnetic field. However, various coils that offer several advantages can be employed for these tasks.

[0106] In Fig. 11, the tracking system 1501 comprises a transmit coil 1503 connected to a microcontroller 1507 via a digital-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 arranged, for example, for signal generation and reception, frequency evaluation and control. Also shown in Fig. 12 is a transmit / receive decoupler.

[0107] The microcontroller 1507 generates a transmit pulse (see top trace 1350 in FIG. 13) that is amplified using an audio amplifier 1502 and then sent to the transmit coil 1503, sometimes referred to as the 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 ADC 1508 of the microcontroller 1507, where a time trace, typically 1 / 20 of a second, is sampled at a rate of about 20 kS / s. In addition to the transmit pulse 1350, sometimes referred to as the excitation pulse, FIG. 13 shows an induced voltage 1351 in the receive coil 1504 due to ball vibrations in the sensor and therefore also due to the sensor response. The spacing of the excitation pulses 1350 can be constantly adjusted by the microcontroller 1507.

[0108] In the embodiment described herein, the tracking system specifically corresponds to a multi-coil system. Using several coils allows for position 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 together with known coil element sensitivities can be fitted to a dipole model for determining position and orientation parameters. An example of a multi-coil system for implementation in a pillow or mattress is displayed in FIG. 14. With many available receiving coils and channels, additional information can also be used to enhance background signal suppression as further described below.

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

[0110] In the following, a coil-based transmission system of a tracking system will be described in more detail and by way of example. The coil-based transmission system comprises a transmission amplifier and a transmission coil, possibly accompanied by a matching circuit and a "mute" circuit. Since the transmission signal shape is not very important in sensor applications, many amplifiers are suitable for this task (Class A, Class B, Class AB, Class D, etc., employing more components such as transistors, vacuum tubes, thyristors, etc.). Since the signal quality is not critical, the lowest loss amplifier topology can be chosen, which is a half-bridge or full-bridge amplifier employing switches with low on-resistance. Preferred switches are MOSFETs or IGBTs. The matching circuit is in the simplest case a simple capacitor in series with an inductor. Provided that the amplifier works with a sufficient supply voltage, such a matching capacitor can be omitted or a capacitance so high that the resonance frequency of the coil with the capacitor is much lower than the operating frequency. The matching circuit is also interesting for another reason. Medical devices must always work safely, and voltage reduction is a challenge. By placing a capacitor in the middle of the coil, so that the current flows through one coil section, then through the matching capacitor, and then through the second coil section, the peak voltage difference can be narrowed. This becomes even more true if the coil is divided into more sections, each connected to a corresponding capacitor. This makes the coil and the matching circuit a combined unit. The magnetic field amplitude is constantly controlled by pulse width modulation, i.e. the amplifier increases / decreases the current through the coil only for a small part of the period, or alternates between increasing / decreasing the current quickly. Since the exact signal shape is not very relevant for sensor applications, this is best achieved by only changing state twice within half a wavelength (or once for full power, where the pulse length is the same as the length of the half wavelength). Ideally, the amplifier not only has the possibility to increase or decrease the current, but also to keep it almost constant or at the level dictated by the matching circuit. This is achieved by the correct switching order of the transistors in the half or full bridge. Usually the supply voltage of the amplifier should be quite low, in the range below 50V.Furthermore, the matching circuit should be set so that at any two points this 50V limit is not exceeded, and even better not to exceed 24V in both cases. 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 the noise from the transmit system, i.e. 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 sensing unit into an electric response signal. Furthermore, the transmit amplifier should not shorten the receive signal or even partially reduce it. There are several possibilities to achieve this. If we have different transmit and receive coils, we can geometrically separate the two coils (see Fig. 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 that allows tracking the marker device to a distance of about 30 cm above the upper coil. The geometric design uses a geometric decoupling method, where the transmit coil loop 1451 is connected to generate a parallel magnetic field, while the receive coil loop 1450 is connected to receive the magnetic field gradient and suppress the uniform magnetic field. This transmit and receive system uses parallel transmit loops and anti-parallel receive loops, resulting in an inherent geometric decoupling, which may be referred to as a magnetic gradiometer configuration. This results in an 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 lower part of FIG. 15 shows a good example of an outer turn of the receive coil 1450 and an inner turn of the transmit coil 1451.

[0113] Specifically, in FIG. 15, the bottom left image is a close-up of the center portion of the upper coil assembly. In the bottom left image, you can actually only see one turn of the transmit coil 1451 peeking out from the bottom. The rest is hidden by the receive coil, which is wrapped with thinner wire. The DC blocking circuit 1455 is just a signal conditioner in front of the audio amplifier, since the signal to the audio amplifier is generated by a simple PWM output. The low pass filter 1553 is a filter that sits between the output of the audio amplifier 1454 and the transmit coil 1451. This filter has two purposes. First, it is 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 of senders and receivers is used. In this case, a transformer can be introduced in the transmitting circuit and at the terminals connected to the receiving circuit. This transformer provides a decoupling of the transmitting and receiving systems. This transformer solution can also be used when a combined transmitting / receiving coil is used. Instead of a transformer, a capacitive (or even resistive) decoupling network can be used with both a combined transmitting / receiving coil and a separate transmitting / receiving coil. The drawback of the compensation method is that it requires a lot of space, adds noise and, in the case of capacitive decoupling, reduces the frequency range of the tracking system. A more robust and cheaper solution is to add a circuit that completely mutes the transmitting amplifier during the receiving time. For this, a cross diode can be added at the output of the amplifier. In particular, a diode with a low capacitance at zero voltage, such as a PIN diode, is useful. This provides a high impedance when no current is flowing. To further enhance this, an electronic switch can be placed at the output of the amplifier that shorts out all residual noise signals while receiving. The diode still provides the desired high impedance. It is also possible to make special amplifiers that completely eliminate noise and provide a high impedance when not working. In half-bridge and full-bridge designs this can be achieved by never switching any components when receiving, using low output capacitance transistors, having around half the supply voltage at the output in receive / 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 any power supply during the receive process).

[0115] In the following we will discuss the coil-based receiving system of the tracking system. The receiving amplifier needs to be of the low noise type. However, the requirements are not so high that rare receiving transistors need to be used. Standard low noise dipole or JFET silicon transistors are usually sufficient. The only special feature is that the amplifier needs to survive the transmit pulse and start working immediately after the transmit pulse. There are several ways to reach 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 reach this goal. In the absence of any decoupling, the amplifier can be made robust against the transmit pulse. This can be done by adding a suitable capacitor to the input of the amplifier and a cross diode to the second terminal. This will provide a suitable high impedance in the transmit case and spare the amplifier all the high voltages up to a level that is harmless to it. Naturally, the added capacitor needs to be rated for the maximum transmit voltage. The capacitance value needs to be high enough that the signal at the amplifier is not reduced too much in the receive case. With JFET-based amplifiers this is usually not a critical issue. The cross diodes 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 receiving amplifier will not go into saturation and will function correctly as soon as the transmit signal has been sufficiently reduced.

[0116] In the following, the interface with the digital system is described in more detail, firstly the digital signal output and its processing. An analog timer system can generate the output signal, but usually a digital system like a DSP or FPGA is used. Different outputs are used for different types of output amplifiers. For analog amplifiers, some kind of ADC is used. Since the output signal quality is not very important, 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 amplifiers, using analog outputs for them. For the best matched amplifiers, half-bridge or full-bridge, it is best to create the switching pattern directly in the digital system. Furthermore, switching patterns for the receive amplifier input protection and the transmit amplifier output noise elimination are also generated immediately by the digital system. A common feature of all output options is that they need to be fast enough to keep the phase tight over the various excitations of one marker device or between various marker devices. Therefore, this output must have the possibility to update the switching 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, and even better than 200 kHz. This does not mean that a change of switching state must be possible at every raster point. It is therefore possible to provide for each amplifier a serial interface that communicates the new switching state to the amplifier and the protocol, for example, so that this change is carried out at a fixed time over the same serial interface. This is particularly useful for amplifiers of the type that are essentially silent during the receiving phase. For this purpose, a 1-bit serial interface can be realized, which requires only one optocoupler in the amplifier. This makes it easier to achieve noise immunity from the digital transmission side, since the stray capacitance in one optocoupler can be very low.

[0117] In the following, we will discuss the analog-to-digital interface. Analog-to-digital conversion is quite standard. Since the signal is low bandwidth, it is possible to mix down the signal to near DC and sample this signal if only at least one marker device is used. However, the frequency of the signal from the marker device is somewhat low, usually below 10kHz. Today there are many suitable ADC chips that sample it directly. It is best to use significant oversampling for the ADC, especially since the digital signal processing is trivial compared to the analog filters. At least 10 times the frequency of the marker device should be used, with 100 times or even 1000 times being valid options. Significant oversampling makes the design of the ADC input filter easier and cheaper, since it only needs to pass the frequency of the sensor signal while not passing signals above the Nyquist frequency. However, filtering below the sensor frequency is also useful to ensure that the usual high background signal is not there. A high background signal reduces the possible amplification before the ADC and increases the ADC noise contribution. The ADC noise (effective number of bits) and samples must match the required dynamic range and noise expectation. This means that the ADC must not be saturated while the maximum expected signal and all noise components are present. At the same time, the quantization noise of the ADC must be low enough that the overall noise is not raised. Here, noise means all the undesirable components in the recorded signal due to real noise sources such as coil resistance and receiving amplifier behavior. This also includes interference components that cannot be removed by proper filtering and background signal subtraction. Usually, with modern ADC chips, this requirement can be met, for example, by a 2MS / s 18Bit ADC. To save costs, it is beneficial to employ an ADC with lower specifications and adding gain control, and still reach a good overall performance.

[0118] In the following, data processing will be described. Before data evaluation, the raw ADC data needs to be processed. Since a significant oversampling is desired, the first processing stage is a decimation stage. This has the main advantage of reducing the data size, thereby reducing the required computational power for further stages. In some cases, the decimation stage may include another filter, namely a band-pass 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 to perform inverse nonlinear filtering to counteract the nonlinearity of the receiving system. This means that the nonlinearity of the entire receiving system is measured and a computational filter is constructed that counteracts the effect of the nonlinearity. This is particularly helpful when low-cost components are used, since such components tend to be more nonlinear in behavior.

[0119] This nonlinear filter can be used instead 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 components are subtracted from the channels bearing the signal. This subtraction can be done in the time domain or in 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 "virtual magnetic gradiometer" can be used. This resolves the multiple channels in a virtual channel, which is a linear combination of the physical channels, so as to minimize the interference of 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, in the following, data evaluation will be described. The frequency is the main parameter extracted from the acquired signal from the sensing unit. Due to the high Q value of the resonator (time constant up to seconds), the subsequent excitation pulse usually runs out before the vibrations have completely decayed (see FIG. 13) and therefore needs to 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-computed 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 use time-domain zero padding or frequency-domain interpolation, as well as 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 in this evaluation (see the top right spectrum of FIG. 16), for example, using a weighted frequency estimation based on several harmonics or by determining the consistency of the frequency determination between several harmonics.

[0121] In the example mentioned in Fig. 16, the signal at the second harmonic is an order of magnitude smaller than the fundamental frequency signal. Hence, better filtering is required. Various filter stages such as analog excitation filters like DC block or low pass, analog receive filters like band pass filters, and also digital receive filters like ITR resonator filters for real time processing (6th order Chebychev type II) can be used to optimize the signal at the resonant frequency and its higher harmonics. In Fig. 16, the central position of the f0 resonant peak is determined from the most significant peak in the filtered spectrum. From f0, the timing of the next in-phase excitation pulse is calculated. The repetition frequency of the system is 5-30 Hz, resulting in a real time trace of the frequency response.

[0122] In Fig. 16 the signal spectrum is displayed with and without digital bandpass filtering (1051 vs. 1050). The dotted lines are the ranges selected for evaluation. The various dot symbols correspond to different filter types that can be ignored since they do not actually show any difference. In Fig. 17 the bandpass is attached to a commercial low noise audible range amplifier, the amplifier type being DLPVA-100-BUN-S from FEMTO Messtechnik GmbH. In Fig. 18 the actual 40 dB suppression spectrum of the digital filter is traced over the range of the selected band. There is no noticeable difference between the two implementations. The filters shown are applied to the data shown in Fig. 16, resulting in the difference between 1050 and 1051.

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

[0124] In the following, a process will be detailed in which a tracking system is used to determine the position of a marker device, and thus to localize 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 act on the sensor frequency, these compensations are not necessary, since only frequency changes over time periods of less than one second need to be determined. The changes are not significantly dependent on the 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 a suspended magnetic ball 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 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 a further approximation C≈0, allowing the calculation of the angular response frequency as follows:

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[0130] This approximation is not valid in the typical case, 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 magnetizations 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 from zero at an oscillation amplitude of 90° to going from attractive to repulsive at larger angles.

[0133] If the excitation magnetic field generated by the transmitting coil has a constant amplitude, the oscillation amplitude φ max becomes narrower (reducing the excitation field) as the coil-sensor spacing increases, and so the frequency decreases. As shown in Figure 10, the amplitude is also determined by the relative orientation of the coil and the sensor.

[0134] In a tracking system, the orientation and 3D position of the marker device described so far in this specification needs to 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 lead to 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 unique mechanical vibration, and this vibration is called B S、i (γ) versus the dynamic dipole moment of the magnetic material

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[0137] For this reason, a mapping between the six position and orientation coordinates of the marker device in the coordinate system given by the tracking system and the voltage amplitudes at the fundamental frequency or higher harmonics on all receive channels is desired. In the following equations we describe how to get rid of the time dependence 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, the 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 it may be determined by a mixture of both, i.e. in a model that can fit experimental results with well-fitting parameters. For an oscillation of the magnetization, an explicit description in terms of frequency ω and amplitude α0, in the frame of the marker, is:

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[0139] From the forward function set (14), (15) and the measured response amplitudes, the marker device position and marker device orientation can be calculated by solving the simultaneous equations using standard mathematical nonlinear solutions. 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 of 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 / location determination is also based on gradient magnetic field encoding. While coil sensitivity localization is based on the amplitude distribution picked up by the coil array, the frequency of one or more marker devices can be manipulated to obtain independent location information. For this, the tracking system is equipped with a control unit that can independently control each coil in the coil array to produce a non-uniform magnetic or electromagnetic excitation field, ideally with a constant magnetic field gradient over the entire area of ​​action. This is achieved, for example, by applying low frequency currents to selected coils of the coil array. This additional magnetic field creates a recovery magnetic field B acting on the oscillating magnetic body. rest changes, and so does its frequency (Equation 9).

[0141] Due to the non-uniformity of the 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, applying magnetic field gradients in 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 it is required to provide a sufficiently high order involvement at the expense of a stronger magnetic field strength. The basic encoding idea 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 magnetic field strength needs to be matched to the frequency sensitivity of the marker device and the frequency resolution that the system produces. The frequency sensitivity of a NdFeB marker device with a magnetic sphere with a sphere diameter of 0.5 mm as the magnetic material is

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[0143] In phase encoding, a non-uniform encoding field is applied before one readout, i.e., only during a short window of opportunity for position-dependent signal phase shifts. If the phase resolution is not sufficient for precise localization, the duration and / or amplitude of the phase encoding pulses can be varied in successive excitations to allow ambiguities in phase occurrence to be resolved. Thus, full 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 another non-uniform magnetic field pattern and frequency encoding (e.g., encoding an orthogonal spatial axis) for effective localization. If the rough marker device location is already known from the 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 full spatial information.

[0144] As described below, the comparison of the localization results obtained by gradient vs. sensitivity encoding can be used to determine the systematic error caused by, for example, background magnetic field. It should also be noted that the linear response of the sensor employing two hanging balls to low-frequency external magnetic fields is suppressed, and in this case, the higher order response of frequency can be used for localization or sanitary check. However, the magnetic field sensitivity of these oscillators is so low that a higher gradient magnetic field is required for gradient magnetic field encoding.

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

[0146] Determining the position (meaning three position parameters and three orientation parameters) and measuring further parameters (such as pressure and temperature) becomes difficult, especially when only a few coils are used. However, using only a few coils is cost-effective and, in some applications, preferable due to space limitations. It is therefore desirable to modify the detection procedure and hardware to work with only 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 that several sensor / marker devices, each operating at a well-known frequency, are combined in a fixed relative orientation into an assembly. Usually the sensors are mounted on a rigid frame, but in technology, 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 the 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 is determined and whose orientation is evaluated. However, the rotation of the coil around its axis (the axis of the dynamic dipole moment) cannot be detected due to the rotational symmetry of the coil.

[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 find the sensor / marker device array somewhere in a ring around the dynamic dipole axis of the transmit coil. Note that if the coils were not round, the ring would not be a perfect circle in space, but this does not change the logic. Thus, with one coil the position cannot be determined, but with two coils (with non-parallel dynamic dipole moments) the symmetry is broken and the position and orientation of the sensor / marker array can be determined.

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

[0150] It is also possible to incorporate prior knowledge into this procedure (i.e. only allow the maximum displacement speed of the sensor relative to the coil). Here, the only difference with the methods described so far is that this step is not performed for one 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 to employ a full parametric method or at least a zero amplitude frequency extrapolation method, since it is difficult to run many sensors simultaneously with all the desired amplitudes. However, the full model approach is somewhat computationally intensive. To reduce the required computational power, it is advantageous to first use the already described single sensor / marker estimation methods separately and use their results as starting values ​​towards a final full model-based reconfiguration of positions and values.

[0151] Several calibration aspects are described below, 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, the accuracy of compensation for amplitude effects can be reduced, so that the sensor readings can also change, to a lesser extent. Therefore, a calibration procedure for the magnetic field is desirable. Furthermore, it is also preferable to have a measure to determine whether a magnetic field disturbance is currently occurring. Therefore, first, a method for detecting disturbance problems is described.

[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 any case, in this configuration, one coil can transmit and all other coils receive the transmitted signal directly. 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 alarm for inaccuracy, triggering a self-calibration process, or a calibration process with user interaction, or a combination of these things. It is also possible for several coils to transmit simultaneously. The transmitted pulse must contain several frequencies. This can be achieved by pulsing, by frequency sweeping, or by using several vectors, as is well understood herein. The frequency analysis is important, since the eddy currents that run through the conductive structures are mainly frequency dependent. Therefore, a noticeable change is that the ratio of the received target signal 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. So, for example, if the receive amplitude is configured such that a gain change can occur, this effect can be used to set a new gain value in the software that compensates for this gain change. This logic applies as well if a gain change is expected to occur in the transmit amplitude but not in the receive path. Now, 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 also theoretically possible to measure the impedance of one coil and use this change as an indication for changes in the eddy current environment. However, the ability to measure impedance would necessarily not have electronics and would require special equipment. The known properties of the sensor / marker in the operating range as well as the coil coupling can be used to detect environmental changes in the eddy currents.

[0154] In particular, sensors can be integrated into the transmit / receive coil array itself. Even a single sensor / marker is useful. For example, if only one marker is integrated into the system, at a fixed position relative to the coil, the change in the marker's response is an indication of eddy current environmental changes. It is even more preferable to integrate 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 not only an indication of the static magnetic field, but also of the presence of ferromagnetic material. When detecting ferromagnetic material, the coil is filled not only with a current at the frequency of the sensor / marker device vibration, but also with a current at a much lower frequency. The current filling can be done coil by coil or using several coils. If the measured sensor response (i.e. the frequency change due to the applied low-frequency magnetic field) is not the same as the stored prediction, it is possible that the ferromagnetic material distorts the magnetic field. If there are enough coils in the system, the position of the magnetic 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 are different, the eddy current (or ferromagnetic) environment has changed. But it is better if not only one such marker device is incorporated into the system, but many. It is also better to have them in known positions rather than unknown positions. But it is also useful to know only some properties of the position, rather than having no position information at all. A practical way of 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 put into the computational budget of the tracking system. If the tracking system finds relative positions and orientations that deviate from expectations, the system is disturbed by eddy currents or ferromagnetic materials.

[0156] Also, if the sensor / marker device is 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 disturb the magnetic field, and at the sensor / marker resonant frequency, where both ferromagnetic and eddy currents cause magnetic field distortions. Thus, information about the nature of the disturber can be elicited, for example, if ferromagnetic materials are involved in this disturbance.

[0157] Also, the best way to detect disturbances is a full mathematical model of the transmit / receive amplitudes, the coils, and the marker device / sensor. This model also includes known positions and orientations, both relative and absolute. In the first stage, all positions / orientations and physical parameters are optimized to minimize errors. This stage includes, for example, fixed position markers attached to the coil array and prior knowledge of their relative positions in a potential difference frame. As an aside, the "frame" does not have to be introduced only for calibration, a marker device consisting of many oscillators can naturally act as a frame. In the second stage, the total weighted error between the expected signal and the delivered signal is calculated by a 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. if the error occurs in the AC or DC sensitivity components), the nature of the disturber can be inferred.

[0158] In the following, we will explain the dependence of the signal amplitude at various harmonics on the sensor orientation for one coil. In particular, FIG. 19 is a good example of the measured dependence of the signal amplitude at various harmonics on the sensor orientation for one transmit / receive coil. When the excitation field is aligned parallel to the magnetic dipole orientation, no excitation occurs and the signal is zero. The highest vibration amplitude is achieved at the 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 seen 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 the orientation dependence, where the ratio of the second harmonic to the first harmonic goes from zero to a maximum value (or singularity), while the ratio of the third harmonic to the first harmonic is 1:1. 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] Either sensitivity encoding or gradient encoding can provide the position and / or orientation of the marker device and / or the medical device to which it is attached, or 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. Being a nonlinear vibration, magneto-mechanical oscillators generate harmonics of the resonant frequency at the dynamic dipole moment. These harmonics are picked up at the receiving coil. Care should be taken not to suppress these multiple fundamental frequencies during the sampling and filtering stage. The spectrum of harmonics depends on the details of the sensor. Some sensors produce predominantly odd harmonics (3ω0, 5ω0...), others even and odd harmonics (2ω0, 3ω0, 4ω0...). However, mixed types can also be built. 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 at right angles to the dynamic dipole moment of the fundamental frequency and at right angles to the axis of rotation. Therefore, odd harmonics are conceptually the easiest to use. Odd harmonics are conceptually the easiest to use, since the ratio of the dynamic dipole moment of, for example, the third harmonic to the dipole moment of the fundamental frequency is reflected as a corresponding ratio in the recording voltage in one coil, and is therefore considered as, for example, a spectral peak amplitude. However, since 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 given integration period. A correction is applied, since for each sensor a calibration of this ratio to the vibration amplitude or to the direct frequency shift can be given. In the case of even harmonics, the situation is somewhat more complicated, since the direction of the dynamic dipole moment does not align with the fundamental frequency dynamic dipole moment. Here, therefore, it is usually necessary to employ more than one coil, and the orientation of the coils relative to the sensor needs to 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 few coils (e.g., 3-5), it should be possible to at least reconfigure the sensor orientation relative to the coils using methods similar to the position determination methods detailed 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 coils 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 vibration form 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 can be the stretching 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 almost constant, but at higher amplitudes it is no longer constant. This force variation is governed by a first approximation in the square of the vibration amplitude, which corresponds to the approximation of the cosine function by a parabola. This square dependence is the reason for the nonlinearity in the dissipation. The periodically changing force between the magnetic pair stretches the filament, which leads to dissipation participation. Other effects can also lead to a nonlinear behavior. Altogether these effects lead to a situation where the envelope shape of the decay curve over a given time 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 also its shape. 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 as mentioned above or the possibility of controlled constant amplitude excitation. However, this also changes the shape of the envelope and is somewhat sensitive to the movement of the sensing unit during the recording. If it is known that the sensing unit of the marker device does not undergo rapid accelerations, for example, 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 reaction of the sensor signal to various strengths of the excitation field. In this case, the current pulse is systematically varied and the response of the sensor to the various excitation pulses is evaluated. The current, duration and phase of the transmitted pulse are varied or a combination thereof. For example, assume there are two excitation pulses. If the spacing is wide and the local magnetic field amplitude is low, two pulses are specified that cause an amplitude twice that of one pulse. However, if the spacing is narrow and the local magnetic field at the sensor is large, the amplitude falls below this amplitude twice. This results in a characteristic reduction of the received voltage to the expected double. Therefore, the ratio of the received signal (Fourier) amplitude of the sensor 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 to this, 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, some of which require modification of the magnetic field pulse to be transmitted. No modifications to the hardware system are required to perform this evaluation. It is therefore reasonable to implement all of them. This is done by simply proceeding with the evaluation in parallel and in conjunction with the results, so as to minimize the noise, i.e., to perform 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, summarized below. On the flip side, mathematically advanced methods can be significantly more difficult to implement and require significantly more computational resources to run on cost-effective computer hardware. The basis for a correct mathematical approach is a mathematical model for the sensing unit. This model predicts the response of the sensing unit to the excitation magnetic field and the state of the sensing unit at that time. The sensing unit state is the current deflection angle and rotation speed of the hanging ball, which corresponds to the magnetic body.

[0170] In some embodiments, a model of the transmit and receive coils needs to be generated, including the 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 may simply be a set of spatial points with sensitivities and an interpolation algorithm between the points. It may also be based on a simulation of the coil based on the Biot-Savart law. This model can now predict the voltage response of the sensor at any time, location and orientation with a certain history of the excitation pulse and external parameters. The procedure therefore involves varying the sensor position and orientation, as well as the sensor, which affects the physical parameters in the simulation so that the recorded signal and the simulation match as well as possible. Many well-known optimization methods are used, such as gradient descent or random walk. This fit can be defined as the root mean square of the sum of the differences between the measured sample points and the simulated sample points. The fit is best if this quantity is the lowest. This best fit can be varied by introducing further constraints, for example by models of the expected relative positions and relative orientations, or by constraints on the maximum expected sensor acceleration and / or models of the measured quantities, for example giving constraints on the maximum rate of change of these quantities. For at least one input that is independent of the spacing and orientation changes, further sensor inputs such as an accelerometer are also used in the handheld coil system. 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 gravitational effects.

[0173] The processor, and more particularly the positioning 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 a 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) and therefore the oscillator frequency changes, resulting in a source of error in detection. For a 0.5 mm diameter magnetic sphere made from NdFeB with saturation magnetization of 1.3 T / μ0, the magnetic field generated by a fixed sphere in the center of the vibrating sphere is 16.1 mT at 0.75 mm center-to-center spacing and 6.8 mT at 1.0 mm center-to-center spacing, respectively. The geomagnetic field is 25 to 65 μT. The frequency difference between parallel and anti-parallel alignment of the static magnetic field component of the maximum geomagnetic field of 65 μT results in a frequency difference of around 5 Hz for the 0.75 mm spacing mentioned above and around 9 Hz for the 1.0 mm spacing mentioned above. Various mitigation measures for this are introduced below.

[0175] Mitigation on the part of the marker device 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. Since counter vibrations occur 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 method described above, a frequency or magnetic field correction value can be calculated to achieve a positioning improvement. For the detection of 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 3-axis Hall sensor. An alternative is a temperature-compensated micro robotic 3-axis array with a well-defined zero-field frequency.

[0177] From their respective changes in frequency, the magnitude and orientation of the background magnetic field can be determined. Ideally, their resonant frequencies are chosen so that they do not interfere with the frequencies of the sensing unit of interest. Rather than correcting for frequency shifts during evaluation, the coils of the 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 inhomogeneous magnetic fields due to the presence of ferromagnetic materials are in view, several sets of three-axis magnetic field sensors can be employed to characterize the spatial magnetic field variations.

[0178] The marker device needs to be high Q and have a large frequency sweep that is sensitive to the quantity being measured over the range required for the particular application. A high Q is especially important for high vibration amplitudes where the highest signals are emitted. There is a strong attraction between the two magnetic bodies, and as this attraction increases drastically with decreasing separation (up to the fourth power of this separation, see equation (9)), both properties can be impaired. 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 pull on the attachment portion is periodically weakened by weakening the force between the magnetic bodies. This can result in a periodic expansion and contraction of the attachment portion, which usually leads to heat generation. Hence, power is extracted from the oscillator. This force also depends strongly 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, a method of weakening the forces and the changes in forces is described, which simply consists of magnetizing one part of a magnetic body in the opposite direction next to another magnetic body, as shown in FIG.

[0180] In FIG. 20, the sensing unit 4001 comprises a magnetic body 4008, which is a permanent magnet, suspended from a rigid wall 4010 of a casing 4002 via a mounting 4006, such as a filament, preferably a high strength wire. The rigid wall 4010 is preferably made of a metal or polymer, which is insensitive to the effects of external pressure. The remaining parts of the casing 4002 may also be made of metal or polymer. The casing 4002 may be filled with gas or may have a vacuum space. Another magnetic body 4007 is fixed to the inner end face of the casing 4002 by adhesive 4011. The two magnetic bodies 4007, 4008 are usually magnetized in opposite directions. However, the fixed magnetic body 4007 also has a portion with reverse magnetization 4012.

[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 the opposite direction. The cap is located next to the other magnetic sphere. If one magnetic sphere is fixed and the other is vibrating, it is best to have the cap on the fixed sphere. In this way the dynamic dipole moment of the sensor is weakened. The frequency is only slightly lowered. However, it is also possible to reverse the roles of the spheres. The oppositely magnetized parts are so small that at all usable spacings the net force between the magnetic bodies is still attractive. If the oppositely magnetized parts are small enough, the attraction condition can be fulfilled right up to the point of contact of the magnetic bodies.

[0182] There are several ways to make a reverse magnetized cap. One involves simply adding some 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 body, a magnetic paint, or something in between. The magnetic material naturally tends to align to form counter magnetizations, and also tends to stick to the magnetic body. Nevertheless, this additional material needs to be bonded to the magnetic body, especially if the two main magnetic bodies may occasionally come into contact. To preserve the original desired shape, some material can be removed from the magnetic body to be changed, for example by grinding.

[0183] There are alternative ways of creating a zone of inverse magnetization. This can be achieved by a strong pulse of current through a conductor close to the magnetic material. However, this is not very practical due to excessive heating. This can be achieved more easily by simply heating the affected part of the magnetic material until it is close to 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 a somewhat harder or softer magnetic material close to the affected zone. Since the heating has to be fairly localized, the total energy stored in the magnetic material is low and the temperature rise must be very rapid so as not to approach the Curie temperature as a whole. A suitable heating source can be a laser. Resistive and inductive heating methods would also work well.

[0184] Some of the methods for determining the onset of magnetic field disturbances are also a good starting point for methods to compensate for the effects of magnetic field disturbances. Such an exemplary method can be most easily explained under the assumption that there are conductive materials that induce eddy currents, but no ferromagnetic materials. Applying the above model, it is possible to obtain the correct position from determining the dependent signal near DC (gradient magnetic field encoding), but obtain the wrong position and local magnetic field amplitude at the sensor frequency and its harmonics (coil sensitivity encoding). Therefore, the higher frequency magnetic field can be distorted to match this prediction. After the distortion is applied, all position and sensing unit readouts are improved. Since AC sensitivity encoding is fairly fast, it is beneficial not to rely only on the position estimation based on the magnetic field near DC.

[0185] The most important part for 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 with a simple 3D polynomial. This means that the magnetic field values ​​at the actual positions are not used, but rather the magnetic field values ​​at the positions transformed by the 3D polynomial. This is computationally efficient, but lacks physical insight, and it is not clear how, for example, measurements of coil coupling are incorporated into this scheme. Therefore, it is better to use a model closer to the physical reality, for example to use a conductive plate of the magnetic field model close to the coil system that induces the desired magnetic field disturbance.

[0186] Therefore, the basic position, angle, thickness, and size of some virtual plates are changed until the model predictions match the measured data. How to model such conductive plates is well known in the electromagnetic simulation literature. This kind of modeling has the additional advantage that it easily incorporates objects of a shape that is likely to occur in a particular environment. Therefore, when a spatial device, for example an X-ray C-arm, comes close to the field of view, this device can be modeled before it is known and only the exact orientation and position is optimized by the system software. A further advantage is that the position of the supposed disturbing object can be displayed by the system or the data is sent to a second system that performs the display task. In this way, the user can be specifically pointed to an object that disturbs the measurement and the user would like 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 the same as a conductive material that creates 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 material such as a thin plate or rod and deform it in the simulation around the coil array. 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 for soft ferromagnetic materials, and the measured signals provide useful inputs 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 to generate 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. One type of amplifier can produce a current waveform that closely follows a somewhat arbitrary path. This is referred to herein as an "analog amplifier."

[0189] Other amplifiers can only increase the current by a certain percentage and decrease it by a similar percentage, keeping the current approximately constant. Essentially, these amplifiers apply a voltage of positive or negative sign to the coil or act as a short circuit. These are referred to here as "digital amplifiers". Digital amplifiers can vary their switching rate, i.e., they are allowed to change state several times per unit time. When the switching rate becomes much higher than the oscillation rate, the digital amplifier also behaves like an analog amplifier. Therefore, this type of amplifier can be conceptually treated as an analog amplifier.

[0190] If the switching speed remains approximately the same as the marker device frequency, then it needs to be treated a bit differently. However, this is a more difficult situation, so we will focus on this point in its entirety. This type of amplifier has some advantages over analogue amplifiers. The main advantage is that the efficiency of this amplifier is usually very high, with 98% efficiency readily achieved. 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 that low frequency blocking occurs.

[0191] Some sequences require low frequency currents. The solution to this problem consists of two things. 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 way is to have a switch that bypasses the matching circuit, which is closed when a near DC current is required. In the bypass path, a capacitor can also be integrated 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 a DC current is not necessarily available at the readout. There are two main factors that give this capability. First, DC current is not allowed to interface with the readout. This is the main problem when the transmit and receive coils are combined. A DC source can provide a short path to the signal. This needs to be avoided, and a proper matching circuit avoids this.

[0192] The matching circuit has to introduce a sufficiently high impedance between the coil and the DC source. This can be achieved by a further series coil, whose inductivity is similar to that of the transmit / receive coil inductance. The inductance has 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 by a suitable analog filter in the case of DC transmission.

[0193] This filter is bypassed during the AC transmit pulse by a suitable switch (e.g. MOSEFT optocoupler). It is also feasible to avoid any switching in the DC source during signal reception and only use the slowly decaying current in the coil. It is also feasible to perform only a few switching actions during reception and only reject the data to be received if it is corrupted. The DC magnetic field source can be a completely separate coil or the magnetic field generator can be a (moving) permanent magnet. This avoids most of the problems. Another problem with the presence of DC currents during signal reception is that the coils present a different environment to the sensor. This means, for example, that some coils can be shorted to AC currents, meaning that the AC magnetic field will not penetrate the coils and will change the magnetic field values ​​in nearby coils. This effect needs to 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 of 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 discuss the mapping of Fourier magnitudes and currents to specific time-domain pulse patterns when desired.

[0195] It would also be useful to generate a software subsystem that does 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 whether the desired value was reached within the limits imposed by the hardware, such as the maximum coil current or maximum heating or regulatory limits, e.g. patient heating or peripheral nerve stimulation. Information about the severity of undesirable side effects is provided, rather than just a yes / no information. This information is provided for each individual transmit channel (for each transmit coil). A further return value is the actual best fit output DC current and Fourier amplitude. The input is not only a combination of a frequency and a Fourier oscillation, but also different Fourier amplitudes at different frequencies. The maximum length of the pulse train is also a parameter that is an input to this function. Its inner workings are as follows: In the case of analog amplitude, a 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 back and a possibly scaled down version is planned. 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 requirements for the amplitude is chosen. It should be noted that in most cases there is some hysteresis available so that it is not necessary to evaluate all filter states. For example, it is possible to omit filters with a far off resonance frequency if a better filter is available. In digital amplifiers, the inverse Fourier transform (including filtering) gives a good starting point for optimization. In this first approximation step, the resulting peak in the time spectrum is approximated by two (at most a few) slopes and flat regions between them. Thus, for example, a half-cycle of a sine wave starting and ending 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, which 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 discuss the mapping of desired Fourier values ​​in the marker device to currents in the coils.

[0197] The next higher abstraction level of the pulse generation program is a software product that takes as input specific magnetic field Fourier values ​​and directions at specific positions and converts them into demands for currents in the coils. The evaluation algorithm usually gives some measure of the position and orientation of the sensor / marker. The position is not and does not need to be a position in 3D space. However, the 3D position is an ideal case. For example, if there is only one coil, it is only possible to determine the magnetic field value in the sensitive direction of the sensor. Nevertheless, this translates into some virtual position and virtual orientation in 3D space. Therefore, such a situation does not require special treatment in the software. The conversion to the demands for coil currents is thereby the result of an optimization process. There is a model that computes the Fourier magnetic field components at specific spatial positions from the currents in the coils. This is the basis for the optimization, where the coil current Fourier components are optimized to produce the desired magnetic field components. Usually there is no obvious way to form the desired magnetic field components from the coil currents. The desired currents may not fit the constraints in the hardware system. The lower level software returns values ​​describing the adverse 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 at the marker device and the adverse effects. This combines the deviations from the desired field and the side effects into a number for which a maximum or minimum is found using standard optimization algorithms. The combination of numbers is a weighted sum of squares. Naturally, in this entity a huge number of practical mathematical combinations can be determined. 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 performs the optimization.

[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 deals with the measurements that need to be made. The input to this program is therefore the current requirements of what to measure, how accurately and how quickly. These requirements are not part of this document, as they depend on the actual application in which the sensors / markers are used. These requirements can be very varied. For example, if only one sensor is involved, the requirement is, for example, to measure just one quantity as accurately as possible, for example every 0.1 seconds. If the application is a tracking solution for multiple coupled markers, the desired outcome is that a position update is made for the entire marker assembly, for example every 0.1 seconds (based on coil sensitivity), regardless of which of the markers / sensors in the marker assembly is involved in the signal, and an independent position determination every second using a gradient method is required. This program also has access to the current state of the sensors / markers (position / vibration parameters, etc.), as well as the simulation models described elsewhere in this document. From this, the optimal excitation field Fourier values, including orientation, can be computed for each marker device. These parameters can be sent to the lower software level above (hopefully done somewhere down the line) to finally generate a current. In the case of only one sensor, this works immediately and the plan can be written to a hardware output buffer. However, for example in tracking an assembly of marker devices, it is unlikely that there is a pulse shape that fully excites all the individual marker devices. In particular, this step is not suitable for all the individual marker devices. Therefore, the software needs to try to focus the optimal excitation only on a portion of the current marker devices and try to find a solution that gives a usable pulse sequence. This is a general principle of optimization of this software, which tries to vary and slightly target the desired excitation of the various sensors, while still obtaining the desired outcome. The simplest approach conceptually is to go through a portion of all possible marker devices and determine which portion of the excitation gives the best information about the desired parameters.Since there are many possible parts, the program needs to add some heuristics to reduce the complexity. For example, if a given marker device is excited and can necessarily be grouped together, first it can observe which other marker devices are also excited. Once a suitable solution is found, it can be written to the output buffer. The inclusion of a near DC field requires more 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 to apply one or several gradients at the time of readout. However, if the DC gradient and the readout are incompatible, a further optimization step is needed 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 changed until the simulation predicts a good enough measurement at application.

[0200] In the following, the generation of the startup sequence will be described.

[0201] The algorithm typically assumes that there is already a fair amount of knowledge about the marker devices that can be used to optimize the sequence. Usually, at the start of the sequence, this is not fully exploited. For example, from the application, it is known how many marker devices should be present at the time of application, and what the frequency range is. However, the exact locations and frequencies are not known. Therefore, a spatial startup sequence is needed that tries to find all possible marker devices at all possible locations. The simplest possible startup sequence is as follows: The available volume is divided into a spatial 3D grid, or abstract grid. The abstract grid is the grid used when there are not enough coils to do full 3D encoding. Each spatial point is divided into various directions. The program goes through each location and each angle of that location and applies the highest transmit power at a given frequency with a pre-set transmit time. The system then records the possible signals from the sensors / markers. Usually, one transmit pulse excites not only one marker device but also many others at the same time. However, this procedure ensures that even the marker devices with what may be considered the weakest signals are detected. In an optional next step, each sensor is excited individually with different amplitudes. From this, nonlinearities can be derived. In a further optional step, the marker devices are excited in the presence of a DC magnetic field or the signal phase is measured after the DC magnetic field (again in different directions) to determine the sensitivity of the sensor / marker to the DC magnetic field. These basic procedures can be greatly sped up by using some knowledge of the system. For example, if a volume far away from the sensor / marker has already been examined, it may be assumed that many or all of the closer volumes have received an amplitude that may be considered the largest from at least some point of view. Therefore, only a few remaining parameters are applied to the closer volumes. The same logic can be used to determine the nonlinear properties of the sensors / markers and their response to the DC magnetic field.

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

[0203] For many applications it is desirable to have a high time resolution. Therefore, whether for position determination or parameter determination, a strategy for reaching high time resolution with magneto-mechanical oscillators is desirable. The simplest approach for high time resolution is to simply shorten the repetition time. By repetition time we mean the time period between subsequent excitation pulses. As described elsewhere in this document, each time an excitation pulse occurs, a frequency and amplitude are determined from which physical values ​​and positions can be computed. However, the Q factor of the marker device tends to be relatively high, and the oscillation amplitude is not as narrowed during the next excitation pulse. To ensure that the desired marker device excitation is obtained, the phase of the next excitation must be considered. Usually we want an "in-phase excitation", i.e. an excitation where the marker device gets energy immediately after the start of the excitation pulse. How the timing is optimized is described elsewhere. In-phase excitation minimizes the transmitted energy, so that the excitation pulse length can be kept to a minimum. This increases the overall signal-to-noise ratio.

[0204] A high repetition rate has some drawbacks. First, the signal-to-noise ratio is not optimal, since the system cannot usually receive values ​​during or immediately after the excitation pulse. Second, each transmitted pulse destroys some knowledge of the phase of the sensor vibration. Phase information can only hold up to some extent if the excitation pulse and the sensor orientation are kept tightly controlled and are precisely known, which is technically challenging. Phase information over a long period is useful, since it encodes information about the average frequency (and thus the average physical quantity). Measurement of the average physical quantity is much more accurate if an interval twice as long is evaluated, rather than evaluating the first and second halves independently and averaging the two results. Therefore, instead of having the same number of excitation pulses as measurements, it is worthwhile to derive more than one measurement from one signal pulse. This can be done simply by dividing the signal into several subsections and evaluating each subsection independently of each other.

[0205] This simple approach does not take into account that measurements get better when longer data sets are used. When incorporating this, the data set can be split into a hierarchy of subsets, each hierarchy is evaluated for each subset, and the average is scaled to fit the longer data set. So, for example, first the data set (one unperturbed decaying signal) is evaluated as a whole. Then the data set is split into two and the two split 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 finally 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 movement 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 quantities so that fewer parameters are used. So, for example, if the parameter is blood pressure, the model can be a Fourier series since this 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 can be understood and effected by those skilled in the art in 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 singular does not exclude the plural.

[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 the marker device, the marker device being adapted to be attached to a medical device. The tracking system is provided for use during surgery, where 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 vibration of the magnetic material, the tracking system having a magnetic field generator that generates a predetermined magnetic or electromagnetic excitation field to induce mechanical vibration of the magnetic material of the sensing unit, a transducer that converts the magnetic or electromagnetic field generated by the induced mechanical vibration 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 is a sensing unit having a magnetic body providing a permanent magnetic moment, the sensing unit converting an external magnetic or electromagnetic excitation field into a mechanical vibration of the magnetic body; The tracking system comprises: a magnetic field generating device for generating a predetermined magnetic or electromagnetic excitation field for inducing 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; and a position determining unit for determining 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 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 being vibrable to generate different magnetic or electromagnetic fields with different resonant frequencies which translate into a respective one or more 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 claim 1 , which compensates for dependence of the one or more electrical response signals on temperature.

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

6. The tracking system according to claim 1 , wherein the position determining unit applies a compensation algorithm to compensate for non-linearities 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 a characteristic mechanical vibration of the magnetic body 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 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.

8. The tracking system of claim 7 , wherein the position determining unit determines, 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.

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 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 the control; 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. said magnetic field generating device sequentially generating a series of different further 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 a magnetic or electromagnetic field generated by induced mechanical vibration 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 the 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 a mechanical vibration 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 as claimed in claim 11 or 12, having an elongated shape, a maximum dimension of 5mm or less and a minimum dimension of 1mm 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 is A marker device as 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 having attached thereto 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, an 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 the following: 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 an elongate 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 the steps of: 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 the induced mechanical vibration of the magnetic body 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 executed on a computer controlling said tracking system.

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