LCQ position markers

Abstract

The invention relates to device tracking. An improved passive marker device (10) is described. A passive marker device (10) comprises a coil element (12), a mechanical resonator (14) and a circuit element (16). And a coil element coupled to the mechanical resonator for generating a response signal in response to an excitation pulse, the response signal having an operating frequency corresponding to a mechanical resonant frequency of the mechanical resonator and a signal attenuation having an attenuation time constant, the excitation pulse is a magnetic excitation pulse or an electromagnetic excitation pulse. The circuit element is configured to limit power transmitted to the mechanical resonator when at least one excitation pulse has an excitation amplitude equal to or greater than a threshold.

Description

Technical Field

[0001] The present invention relates to a (one or more) passive marker device to be tracked, a (one or more) passive marker device arrangement, a (one or more) tracking system, and related methods thereto. Background Art

[0002] Device tracking is useful, for example, in certain medical procedures. For example, during a procedure, a marker device can be attached to a medical device, such as a medical intervention device. Systems for micro markers and sensors have recently been described in WO2019243098, which are based on the use of so-called micro magnetic oscillators (MMOs). In these systems, mechanical oscillations are induced in the MMOs in response to a magnetic or electromagnetic excitation field for tracking marker devices including these MMOs, and thus, the devices to which these marker devices are attached.

[0003] However, for MMOs, the signal-to-noise ratio (SNR) is proportional to the square of the linear dimension of the MMO device. Thus, as the available space increases, these MMO devices become less and less beneficial. Summary of the Invention

[0004] There is a need to provide an improved passive tracker device.

[0005] The object of the present invention is solved by the subject matter of the independent claims, wherein further embodiments are incorporated in the dependent claims. It should be noted that the aspects described below of the present invention also apply to the passive marker device to be tracked, the passive marker device arrangement, and the tracking system.

[0006] According to a first aspect of the present invention, a passive marker device is provided. The passive marker device includes a coil element, a mechanical resonator, and a circuit element. The coil element is coupled to the mechanical resonator for generating a response signal in response to an excitation pulse, which has an operating frequency corresponding to the mechanical resonance frequency of the mechanical resonator and a signal decay having a decay time constant. The excitation pulse is a magnetic excitation pulse or an electromagnetic excitation pulse. The circuit element is configured to reduce the power transmitted to the mechanical resonator when the at least one excitation pulse has an excitation amplitude equal to or greater than a threshold.

[0007] One of the challenges to be solved may be that a passive marker device near an emitting coil system may be subjected to much stronger excitation. This may result in too large a voltage across the quartz that may damage the quartz, and / or in the receiving mode, where the signal received from a nearby passive marker device is much stronger than the signal from a distant passive marker device, and thus making those weaker signals difficult to detect.

[0008] To this end, the present disclosure provides a passive marker device having a circuit element for restricting or reducing the power transferred to a mechanical resonator at a high excitation amplitude (i.e., when the passive marker device is close to a coil system). This will be described in detail below and in particular with respect to Figure 2 A- Figure 2 the example shown in

[0009] In a more complex scenario, the coupling between the coil element and the mechanical resonator can be reduced or even eliminated for a duration much longer than the oscillation period. This will be described in detail below and in particular with respect to Figure 2 D- Figure 2 the example shown in

[0010] Furthermore, this can be combined with a phase-changing excitation pulse to obtain the desired low amplitude for the near passive marker device while the amplitude of the more distant passive marker device is not significantly affected. Assuming a simple circuit in which the amplitude is capped, the sequence during the transmission phase (i.e., in the excitation time slot) has two phases: first a low amplitude signal is transmitted and then a higher amplitude signal with approximately the same duration but with an opposite phase. This has the following effect: at the close position, due to the capping of the signal, near-zero excitation is achieved. At positions far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel, and thus a relatively high signal amplitude is achieved. This is because at this far position, the excitation is linear with the applied field, and the weak field from the first phase does not cancel the strong field from the second phase, which will be described in detail below and in particular with respect to Figure 9 and Figure 10 the exemplary excitation and recording schemes shown.

[0011] According to an embodiment of the present invention, the circuit element includes at least one of the following:

[0012] a diode configured to limit the voltage across the mechanical resonator;

[0013] a plurality of diodes connected in series, the plurality of diodes being configured to limit the voltage of the mechanical resonator; or

[0014] a PIN diode configured to limit the voltage across the mechanical resonator.

[0015] This will be explained in detail below and in particular with respect to Figure 2 A- Figure 2 the example shown in

[0016] According to an embodiment of the present invention, the circuit element is configured to reduce the coupling between the coil element and the mechanical resonator.

[0017] In other words, the coupling between the coil element and the mechanical resonator can be reduced or even eliminated for a duration much longer than the oscillation period.

[0018] The reasons for changing the coupling between the coil and the quartz are twofold: firstly, not to damage the quartz, and secondly, not to saturate the receiving amplifier (or have too much spectral leakage). For the latter reason, a suitable sequence must be provided. Assuming a simple circuit in which the amplitude is set to an upper limit, the sequence during the transmission phase (i.e., in the excitation time slot) has two phases: firstly, a low-amplitude signal is transmitted, and then a higher-amplitude signal with approximately the same duration but with an opposite phase is transmitted. This has the following effect: at close positions, due to the upper limit setting of the signal, near-zero excitation is achieved. At positions far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel out, and thus a relatively high signal amplitude is achieved. This will be described in detail below and especially with respect to Figure 9 and Figure 10 the exemplary excitation and recording schemes shown.

[0019] According to an embodiment of the present invention, the circuit element includes a varactor diode and a resistor connected in parallel with the varactor diode.

[0020] This will be described in detail below and especially with respect to Figure 2 the example shown in D.

[0021] According to an embodiment of the present invention, the circuit element further includes one of the following:

[0022] A field-effect transistor configured to clamp the charging action;

[0023] A field-effect transistor and a series capacitor connected to the field-effect transistor, the field-effect transistor being configured to clamp the charging action;

[0024] A field-effect transistor and a series capacitor connected to the field-effect transistor, the field-effect transistor being configured to clamp the charging action, wherein the field-effect transistor is connected to a capacitive divider of a capacitive element; or

[0025] A normally-on field-effect transistor as a switching element.

[0026] This will be described in detail below and especially with respect to Figure 4 E- Figure 4 the examples shown in H.

[0027] According to a second aspect of the present invention, a passive marker device is provided. The passive marker device includes a first passive marker device and a second passive marker device. Each of the first passive marker device and the second passive marker device includes a coil element and a mechanical resonator, wherein the coil element is coupled to the mechanical resonator for generating a response signal in response to an excitation pulse, having an operating frequency corresponding to the mechanical resonance frequency of the mechanical resonator and a signal decay having a decay time constant. The excitation pulse is a magnetic excitation pulse or an electromagnetic excitation pulse. The first passive marker device and the second passive marker device are mounted on the mechanical device at an angle relative to each other such that the coil element of the first passive marker device is not parallel to the coil element of the second passive marker device.

[0028] In some examples, it may be preferable to track the movement of a tracking device (e.g., a medical device) in six (6) mechanical degrees of freedom (DOF) in three-dimensional space. However, the efficiency of a single coil is proportional to the sine of the angle between the local excitation field and the coil plane. Thus, if the coil element of the passive marker device is parallel to the local excitation field, the coil element will not be excited and thus the passive marker device cannot generate an oscillatory response. One solution is to switch to a different field direction to excite the coil element. However, such an approach may be slow and inefficient. For example, if an acquisition rate of 40 - 100 Hz is applied and the passive marker device operates at a few kHz, a long and complex excitation sequence may be required. Thus, it may be difficult to obtain the desired temporal resolution for medical applications using a single coil. Additionally, if the angle between the coil and the field is low, the response signal is also low, which may result in a lower SNR.

[0029] For this reason, according to this embodiment, the orientations of the two passive marker devices can be physically linked via a mechanical device (e.g., a housing). The two passive marker devices are arranged such that they are not parallel to each other. Thus, if one coil element of the passive marker device arrangement is parallel to the local excitation field, the other coil element of the passive marker device arrangement is not parallel (e.g., orthogonal or nearly orthogonal) to the local excitation field and can generate a higher response signal to achieve a higher SNR.

[0030] This will be described in detail below and in particular with respect to Figure 3A 、 Figure 3B and Figure 4 the examples shown.

[0031] According to an embodiment of the present invention, at least one of the first passive marker device and the second passive marker device is a passive marker device according to the first aspect and any associated examples.

[0032] This will be described in detail below and in particular with respect to Figure 4A detailed description will be given with reference to the illustrated examples.

[0033] According to one embodiment of the present invention, the angle is in the range from about 45° to about 135°, optionally in the range from about 85° to about 95°, and preferably about 90°.

[0034] According to an embodiment of the present invention, the first passive marker device and the second passive marker device are configured to have operating frequencies with a frequency separation less than a threshold and have different decay time constants. Alternatively, the first passive marker device and the second passive marker device are configured to have operating frequencies with a frequency separation equal to or greater than the threshold.

[0035] In other words, the decay time space and / or the frequency space can be used to distinguish the first passive marker device and the second device. In some examples, the two passive marker devices can have the same or similar frequencies but have different decay time constants. Alternatively, the two marker devices can have different frequencies.

[0036] According to a third aspect of the present invention, a tracking system is provided. The tracking system includes a plurality of passive marker devices, an excitation field generator, and a tracking device. The plurality of passive marker devices includes passive marker devices according to the first aspect and any associated examples and / or passive marker devices according to the second aspect and any associated examples. The excitation field generator is configured to generate at least one excitation pulse within an excitation time slot to simultaneously or sequentially excite at least two passive marker devices. The tracking device is configured to detect response signals generated by the at least two passive marker devices and distinguish the at least two passive marker devices based on the operating frequencies of the response signals and the decay time constants of the response signals.

[0037] For most clinical applications, more than one device must be tracked. This is especially true if a 6DOF sensor is required, as this alone requires two passive marker devices. The classical way to address different passive marker devices is to use frequency-selective excitation pulses and design (i.e., tune) and operate all markers at different frequencies. However, the limitation is that for a desired fast repetition time, the excitation pulses need to be short and significant spectral overlap may occur. When it comes to LC resonators with quartz resonators, an additional limitation is that only a limited number of quartz frequencies are available from off-the-shelf components. Although manufacturers can tune their crystals to any desired frequency, the relatively low market volume makes special designs economically less viable.

[0038] To speed up detection, a tracking system as disclosed herein can excite two or more marker devices in one excitation time slot and use decay time space in addition to frequency space to distinguish passive marker devices. In the following and particularly with respect to Figure 9 and Figure 10 the exemplary excitation and recording schemes shown are described in detail.

[0039] According to an embodiment of the present invention, the tracking device is configured to detect response signals generated by two or more passive marker devices in one or more reception time slots according to the decay time constant of the response signals.

[0040] In some examples, as described in the exemplary excitation and recording schemes with respect to Figure 9 and Figure 10 shown, the response signals can be detected in one reception time slot.

[0041] In some other examples, for a large number of passive marker devices, the excitation phase may be so long that some of the signals have decayed too much before the reception phase (i.e., the reception time slot). In such cases, the response signals generated by two or more passive marker devices can be detected in multiple reception time slots according to the decay time constant of the response signals.

[0042] According to an embodiment of the present invention, the plurality of passive marker devices includes two or more passive marker devices configured to have operating frequencies with a frequency separation less than a threshold and different decay time constants.

[0043] For example, two or more passive marker devices can have substantially the same operating frequency, e.g., the operating frequency is less than a defined threshold. For example, if the coil current patterns are sufficiently different, two passive marker devices can be excited simultaneously. This will be described in detail in the following and particularly with respect to Figure 10 the exemplary excitation and recording scheme shown.

[0044] According to an embodiment of the present invention, the plurality of passive marker devices includes two or more passive marker devices configured to have operating frequencies with a frequency separation equal to or greater than a threshold. The excitation field generator is configured to generate a sequence of frequency-selective excitation pulses within the excitation time slot so as to sequentially excite at least two of the two or more passive marker devices.

[0045] For example, frequency-selective excitation pulses can be used to excite two or more passive marker devices at different frequencies. This will be described in detail in the following and particularly with respect to Figure 9 the exemplary excitation and recording scheme shown.

[0046] According to an embodiment of the invention, the excitation field generator is configured to sort frequency selective excitation pulses according to the decay time constants of at least two passive marker devices to be excited.

[0047] This will be described in detail below and in particular with respect to Figure 9 the exemplary excitation and recording schemes shown.

[0048] According to an embodiment of the invention, the excitation field generator is configured to generate a sequence of phase change excitation pulses to excite at least two passive marker devices having different distances from the excitation field generator. The sequence of phase change excitation pulses includes a first signal having a low amplitude and a second signal having a higher amplitude, wherein the first signal and the second signal have opposite phases.

[0049] For example, phase change excitation pulses can be used to obtain a desired low amplitude for the (one or more) near passive marker devices while the amplitude of the (one or more) more distant passive marker devices is not significantly affected. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] These and other aspects of the invention will be apparent and elucidated with reference to the embodiments described by way of example in the following description and with reference to the drawings, in which

[0051] Figure 1 examples of passive marker devices are schematically shown.

[0052] Figure 2 A - Figure 2 H schematically show some examples of circuit elements.

[0053] Figure 3A - Figure 3B An exemplary passive marker device is shown.

[0054] Figure 4 Another exemplary passive marker device is shown.

[0055] Figure 5A - Figure 5E An exemplary circuit for changing the time constant of signal decay after excitation is shown.

[0056] Figure 6 An example of a tracking system is schematically shown.

[0057] Figure 7 Another example of a tracking system is schematically shown.

[0058] Figure 8 An overall excitation and recording scheme is illustrated.

[0059] Figure 9Illustrates the basic principle of an exemplary improved excitation scheme.

[0060] Figure 10 Illustrates the basic principle of another exemplary improved excitation scheme.

[0061] Figure 11 Illustrates a flowchart depicting a method for tracking multiple marker devices.

[0062] It should be noted that the drawings are purely schematic and not drawn to scale. In the drawings, elements corresponding to elements already described may have the same reference numerals. Examples, embodiments, or optional features, whether indicated as non - restrictive or not, should not be construed as limiting the claimed invention. Detailed Description

[0063] Figure 1 Schematically shows an example of a passive marker device 10 (e.g., a passive marker) that can be attached to a device such as a medical device for tracking by a tracking system. The passive marker device 10 includes a coil element 12 such as a coil and a mechanical resonator 14.

[0064] The coil element 12 may refer to an element that includes and / or corresponds to a magnetic coil device having a specific number of windings. The coil element 12 can be an off - the - shelf magnetic coil having an appropriate number of windings, an appropriate size, and an appropriate distance between windings. The quantity, size, and distance between windings can be determined particularly based on the desired magnetic properties of the coil element. The coil element 12 can include copper or be made of copper. Alternatively or additionally, the coil element 12 can include silver or be made of silver. In some examples where the passive marker device should be transparent to radiation, the coil element 12 can include aluminum or be made of aluminum. In some examples where the passive marker device should be opaque to radiation, the coil element can include gold or be made of gold.

[0065] The mechanical resonator 14 is an element connected to the coil element 12 so as to respond to the voltage output of the coil element in response to an externally applied magnetic field or electromagnetic field by respectively deforming and thus starting to perform mechanical oscillations. The mechanical resonator 14 may comprise or be made of a crystal material. The crystal material may be selected to have sufficient piezoelectric properties. The crystal material may comprise and / or correspond to a quartz crystal. Quartz is a material used for piezoelectric resonators and the resonance frequency is known. Thus, a passive marker device comprising a coil element and a mechanical resonator may be provided, wherein components may be selected such as to provide an electric field having a frequency close to or corresponding to the mechanical resonance frequency of a quartz crystal. This may allow obtaining a high quality factor for the passive marker device. Another advantage of using quartz is the fact that quartz can be used for series or parallel resonance since the two eigenfrequencies are very close to each other. Furthermore, the resonance frequency may be slightly adjusted respectively by providing additional elements, e.g., a trimmer in parallel or in series with the quartz crystal, depending on whether the quartz crystal is connected in parallel or in series. This may allow compensating manufacturing tolerances etc. Also other crystal materials with similar properties may be foreseen. The mechanical resonator 14 may comprise a body and at least one pin attached to the body. For example, the resonator element may be fork-shaped, such as a tuning fork shape, having two forks attached to the body. A mechanical resonator 14 having a tuning fork shape may improve the resonance properties and may also be beneficial in terms of using the passive marker device for sensing physical properties.

[0066] The coil element 12 may be arranged at a certain distance from the mechanical resonator 14. For this purpose, the coil element 12 and the mechanical resonator 14 may be connected via respective connecting parts. In some examples where space must be saved, the distance may be saved by providing a winding around the mechanical resonator such that there is space between the winding of the coil element 12 and the mechanical resonator 14. The size of this space may be appropriately selected according to the size of the passive marker device 10. In some examples, the arrangement between the coil element 12 and the mechanical resonator 14 may be such that the mechanical resonator 14 is provided within the coil element and extends along its axis.

[0067] When an external magnetic field or electromagnetic field is applied, the externally applied magnetic field or electromagnetic field can act on the coil element 12. In response thereto, the coil element 12 can convert the externally applied magnetic field or electromagnetic field energy into a corresponding output voltage. The coil element 12 is electrically connected to the mechanical resonator 14 to feed the output voltage to the input / output terminals of the mechanical resonator 14. Then, the mechanical resonator 14 having piezoelectric properties is deformed by the voltage applied from the coil element 12. Thus, the mechanical resonator starts to perform mechanical oscillations. Thereby, the deformation depends on the frequency component of the applied output voltage provided by the coil element 12, which in turn depends on the frequency component of the externally applied magnetic or electromagnetic excitation field. As described above, if the frequency component is provided as the mechanical resonance frequency of the mechanical resonator or within a range around the mechanical resonance frequency of the mechanical resonator, the mechanical oscillations are excited in the resonance mode. That is, the mechanical resonator 14 oscillates close to its mechanical resonance frequency. The corresponding oscillations can continue for some time, regardless of whether a voltage is provided from the coil element 12, thereby resulting in signal attenuation with a decay time constant. Then, the deformation of the mechanical resonator 12 causes a piezoelectric voltage to be generated and output to the coil element 12. This generates a current through the coil element. In response thereto, the coil element 12 generates a magnetic field, which can then be detected by the tracking system as an oscillation response.

[0068] In some examples, such as Figure 1As shown, the passive marker device 10 can also be provided with a capacitive element 16, such as a capacitor. The capacitive element 16 can be connected in parallel to the coil element 12, such as a coil. This substantially provides a resonator circuit having an LC resonator and a mechanical resonator combined with each other. The capacitive element 16 can be selected to have a capacitance that amplifies the output voltage provided by the coil element 12 in response to an externally applied magnetic or electromagnetic excitation field. The amplified output voltage can then be provided to the mechanical resonator 16 to cause a stronger deformation therein, and thus a higher level of oscillation in the mechanical resonator. This in turn can generate a higher piezoelectric voltage acting on the coil element 12 to increase the signal strength. In some examples, the capacitance value of the capacitive element can be selected such that the resonance frequency of the coil element in the LC resonator is equal to or close to the mechanical resonance frequency of the mechanical resonator 16 (such as a quartz crystal). This can allow for a reduction in the amount of winding required, since the losses during the oscillation of the resonator circuit become lower. With less winding, the manufacture of the passive marker device can also be simplified. The passive marker device including the coil element 12, the mechanical resonator 14 in the form of a quartz crystal, and the capacitive element 16 can also be referred to as an "LCQ" marker. Compared with markers based on micro magnetic oscillators, the LCQ marker can provide a greater operating distance from the coil array and has a better SNR. Hereinafter, the exemplary passive marker device 10 is illustrated in the form of an LCQ marker by way of example, but it should be understood that the passive marker device 10 can be other types of passive marker devices.

[0069] One problem associated with passive marker devices can be the possibility of an overly high signal amplitude of the passive marker device. This can be due to two or more passive marker devices operating at the same frequency, where one passive marker device is closer to the coil array than the other passive marker devices. This can also occur if they are not tuned to the same frequency but are subject to spectral leakage. This problem can be overcome by using circuit elements to limit or reduce the power transmitted to the crystal at a high excitation amplitude (i.e., when close to the coil system). The circuit elements can be non-linear circuit elements.

[0070] Figure 2 A to Figure 2 H schematically show some examples of the circuit element 18. In particular, Figure 2 A - Figure 2 C show some exemplary circuit elements configured to limit the voltage across the mechanical resonator, while Figure 2 D - Figure 2 E show some exemplary circuit elements configured to reduce or even eliminate the coupling between the coil element and the mechanical resonator over a duration much longer than the oscillation period.

[0071] Figure 2 A shows an exemplary passive marker device 10 with an exemplary circuit element 18a. In this example, the exemplary circuit element 18a is a diode configured to limit the voltage across the mechanical resonator. With this diode, only one direction may be required, as this may have sufficiently reduced the energy stored in the tank circuit. An advantage of this exemplary circuit element may be that it is simple and cost effective. The use of a PIN diode may allow for relatively low additional capacitance, so that the effect before clamping may be relatively small.

[0072] Figure 2 B shows an exemplary passive marker device 10 having a circuit element 18b to increase the voltage at which the voltage limiting effect occurs. In this example, the circuit element 18b comprises a plurality of diodes, and each diode in series increases the voltage by approximately 0.5V.

[0073] Figure 2 C shows an exemplary passive marker device 10 with an exemplary circuit element 18c. In this example, the exemplary circuit element 18c comprises only one (PIN) diode 18c. The Z diode set can largely limit the voltage through its breakdown while avoiding the disadvantages of a potentially large number of required diodes.

[0074] Figure 2 D shows an exemplary passive marker device 10 with an exemplary circuit element 18d, which is configured to limit the voltage at the mechanical resonator by detuning the tank circuit. In this example, the exemplary circuit element 18d includes a varactor and a resistor in parallel with the varactor. In response to the applied magnetic field or electromagnetic field, the varactor begins to detune the tank circuit when it becomes conductive and charges the series capacitor and itself. The detuning action can not be limited to the instantaneous voltage, but can potentially last for a long time. The duration can be set to the correct duration, i.e., approximately the dead time. The parallel resistor can be provided and arranged in parallel with the varactor. Additional advantages of this circuit are as follows: During the dead time, the tank circuit is detuned, and the stored energy is therefore maintained in the quartz resonator, thereby increasing the signal during the reception time.

[0075] Figure 2Figure E shows an exemplary passive marker device 10 with an exemplary circuit element 18e. In this example, the exemplary circuit element 18e includes a field effect transistor (FET) to clamp the charging operation. This exemplary circuit can also allow for a switched mode of an extended excitation sequence. Thus, after reaching a critical voltage, the circuit is muted for an extended period of time. In this circuit, there are two elements to store charge to perform a delayed action. First, there is a capacitor in series with a Schottky diode. The charge is removed by a resistor in parallel with this capacitor, thus forming a resistor-capacitor (RC) circuit. A second RC circuit is formed by a gate resistor and a gate capacitance (depicted as a dashed capacitor). Depending on the excitation sequence, only one RC circuit may be needed for the delayed action, and in this case, the gate resistor and capacitor in series with the diode can be omitted. The diode generates a direct current (DC) voltage, and at some point, the voltage can be high enough to switch the transistor. This then shorts the energy storage circuit. The actual implementation of a field effect transistor (commonly a metal oxide semiconductor field effect transistor (MOSFET)) has a body diode (dashed diode) that can limit the energy storage circuit voltage to a 0.5V peak.

[0076] Figure 2 Figure F shows an exemplary passive marker device 10 with an exemplary circuit element 18f, which can avoid this problem by adding a series capacitor to the transistor. The rectification of the diode causes the voltage at the transistor drain to eventually stop the DC current flow. In the transistor "on" state, i.e., when a suitably high gate voltage is reached, the transistor series capacitor effectively becomes a parallel capacitor to the energy storage circuit that detunes it. The voltage of the action can be determined by the selected transistor (threshold voltage) or by the circuit depicted as in Figure 2 Figure G.

[0077] Figure 2 Figure G shows an exemplary passive marker device 10 with an exemplary circuit element 18g. In this example, the energy storage circuit capacitance is replaced by a capacitive voltage divider. A charge pump (e.g., two diodes) provides the desired gate voltage. The remaining issue with transistors is their voltage-dependent output capacitance. This is largely avoided in the circuit shown in Figure 2 Figure H.

[0078] Figure 2FIG. H shows an exemplary passive marker device 10 having an exemplary circuit element 18h. In this example, a normally-open field-effect transistor (in this case an n-channel junction field-effect transistor (jFET)) is used as a switching element. A series capacitor is in parallel with the rest of the energy storage circuit capacitance until the gate is negative enough to turn the transistor off. The diode used to generate the negative voltage has a series resistor typically in the megohm range. Thus, the non-linear action of the diode hardly changes the resonance in the circuit. The parallel RC circuit between the gate and the source dominates the discharge time constant and thus the duration of the detuning. This circuit also allows for an extended silence duration by adding power at a higher frequency after the jFET turns off.

[0079] Figure 1 and Figure 2 A- Figure 2 The passive marker device shown in FIG. H can be used to track the 6DOF movement of a device (e.g., a medical device) in three-dimensional space. However, the efficiency of a single coil is proportional to the sine of the angle between the local excitation field and the coil plane. Thus, if the coil element of the passive marker device is parallel to the local excitation field, the coil element will not be excited and thus the passive marker device cannot generate an oscillatory response. One solution is to switch to a different field direction to excite the coil element. However, such an approach may be slow and inefficient. For example, if an acquisition rate of 40 - 100 Hz is applied and the passive marker device operates at a few kHz, a long and complex excitation sequence may be required. Thus, it may be difficult to obtain the desired temporal resolution for medical applications using a single coil. Additionally, if the angle between the coil and the field is low, the response signal is also low, which may result in a lower SNR.

[0080] Figure 3A FIG. shows an exemplary passive marker apparatus 20 for solving one or more of the above problems. The passive marker apparatus 20 includes a first passive marker device 10a and a second passive marker device 10b.

[0081] The first passive marker device 10a includes a first coil element 12a, a first mechanical resonator 14a, and a first capacitive element 16a. The first coil element 12a is coupled to the first mechanical resonator 14a to generate a response signal in response to an excitation pulse, having an operating frequency corresponding to the mechanical resonance frequency of the mechanical resonator and a signal decay with a decay time constant.

[0082] The second passive marker device 10b includes a second coil element 12b, a second mechanical resonator 14b, and a second capacitive element 16b. The second coil element 12b is coupled to the second mechanical resonator 14b for generating a response signal in response to an excitation pulse, having an operating frequency corresponding to the mechanical resonance frequency of the mechanical resonator and a signal decay having a decay time constant.

[0083] The first passive marker device 10a and the second passive marker device 10b are mounted on a mechanical device (not shown) at an angle with respect to each other such that the first coil element 12a of the first passive marker device 10a is not parallel to the second coil element 12b of the second passive marker device 10b.

[0084] Figure 3B An exemplary arrangement of the first coil element 12a of the first passive marker device 10a with respect to the second coil element 12b of the second passive marker device 10b is shown. As Figure 3B shown, the first coil element 12a of the first passive marker device 10a and the second coil element 12b of the second passive marker device 10b are arranged at an angle α with respect to each other. Since the first coil element 12a of the first passive marker device 10a is not parallel to the second coil element 12b of the first passive marker device 10a, the angle α is not equal to 0° or 180°. For example, the angle α is greater than 0° and less than 180°. For example, the angle α can be in the range of about 45° to about 135°. Preferably, the angle α is in the range of about 85° to about 95°, and more preferably about 90°.

[0085] Since the first coil element 12a of the first passive marker device 10a and the second coil element 12b of the second passive marker device 10b are arranged in a non-parallel manner, if one coil element (e.g., the first coil element 12a) is parallel to the local excitation field, the other coil element (e.g., the second coil element 12b) is not parallel to the local excitation field. Thus, at least one coil element in the passive marker device 20 will be excited to provide an oscillatory response to be tracked by the tracking system. In this way, there is no need to switch the direction of the excitation field for exciting the coil element, thereby achieving fast and efficient acquisition. Preferably, the first coil element 12a of the first passive marker device 10a and the second coil element 12b of the second passive marker device 10b are arranged to be orthogonal (about 90°) or nearly orthogonal (e.g., in the range from about 85° to about 95°) with respect to each other in order to generate a higher response signal. For example, if one coil element of the passive marker device is parallel to the local excitation field, the other coil element of the passive marker device is perpendicular to the local excitation field, and a higher response signal can be generated to achieve a higher SNR.

[0086] In some examples, one or both of the first passive marker device 10a and the second passive marker device 10b may include circuit elements to limit power when the local excitation field has an excitation amplitude equal to or greater than a threshold.

[0087] Figure 4 Another example of the passive marker device arrangement 20 is shown, in which both the first passive marker device 10a and the second passive marker device 10b include circuit elements. Although Figure 4 it may be shown that both the first passive marker device 10a and the second passive marker device 10b include Figure 2 the circuit element 18d as shown in D, it should be understood that the first passive marker device 10a and the second passive marker device 10b may include other circuit elements, such as Figure 2 the circuit elements 18a - 18c and 18e - 18g as shown, to limit power. In some examples, as Figure 4 shown, the first passive marker device 10a and the second passive marker device 10b may include the same circuit elements. In some other examples (not shown), the first passive marker device 10a and the second passive marker device 10b may include different circuit elements. For example, the first passive marker device 10a may include Figure 2 the circuit element 18a as shown in D, while the second passive marker device 10b may include Figure 2 the circuit element 18f as shown in F.

[0088] In some examples, Figure 3A and Figure 4 the first passive marker device 10a and the second passive marker device 10b shown in may be different in operating frequency. For example, the first mechanical resonator 14a of the first passive marker device 10a may have a mechanical resonance frequency f1, and the second mechanical resonator 14b of the second passive marker device 10b may have a mechanical resonance frequency f2. The frequency separation (i.e., |f1 - f2|) may be equal to or greater than a threshold. Thus, frequency - selective excitation pulses can be used to operate the first passive marker device 10a and the second passive marker device 10b at different frequencies.

[0089] In some examples, Figure 3A and Figure 4The first passive marker device 10a and the second passive marker device 10b shown may have substantially the same operating frequency and different decay time constants. For example, the first mechanical resonator 14a of the first passive marker device 10a may have a mechanical resonance frequency f1, and the second mechanical resonator 14b of the second passive marker device 10b may have a mechanical resonance frequency f2. The frequency separation, i.e., |f1 - f2|, is less than a threshold. For example, the first passive marker device 10a may have a first decay time constant τ1, and the second passive marker device may have a second decay time constant τ2. The difference between the two decay time constants, i.e., |τ1 - τ2|, may be equal to or greater than a defined threshold. In this way, the decay behavior of the oscillations of the first passive marker device 10a and the second passive marker device 10b can be used as a differentiating factor. Thus, even if they operate at the same frequency, two passive marker devices in a passive marker device can be distinguished, for example, by fitting a known response to the recorded data. The decay constant can be tuned by appropriate matching of the LC resonator with the mechanical resonator. In this matching, the product of the inductance and the capacitance is more or less fixed and is given by the mechanical resonance frequency. However, the ratio L / C is a design parameter that can be freely chosen. When the LC parallel resonator has a high resistance (at resonance), the decay time constant is low, however, the initial transmission amplitude is high. Overall, if the quality factor of the coil element remains constant, the total SNR remains the same, which means that the physical size of the coil element remains unchanged.

[0090] Figure 5A - Figure 5E An exemplary circuit that changes the time constant of signal decay after excitation is shown. For example, Figure 5A The exemplary circuit shown can be implemented as the first passive marker device 10a. The passive marker device 10b can include Figure 5B to Figure 5E one of the circuits shown to achieve a different time constant of signal decay after excitation.

[0091] Figure 5A A basic circuit that should be compared with the following circuits is shown. As Figure 5A shown, the basic circuit is also referred to as LCQ1. Figure 5A The exemplary passive marker device shown includes a coil element of inductance L1, a capacitance element of capacitance C1, and a mechanical resonator, such as a quartz resonator having a quality factor Q. In this example, the LC resonator formed by the inductor, the capacitor, and the stray capacitance of the inductor and the quartz can be tuned to the same frequency as the natural frequency of the quartz. In all subsequent circuits, the mechanical resonator remains the same, i.e., Q1 = Q2 = Q3 = Q4 = Q5. After a fixed excitation period, for example, with the same frequency, length, and amplitude, the signal decays (in a first approximation) with a characteristic decay time constant τ1.

[0092] Figure 5B Shows the situation when the inductance ratio is replaced by a larger inductance ratio (i.e., L2 > L1) while maintaining the quality factor of the coil - i.e., the ratio of inductance to series resistance at a given frequency - constant. Figure 5B The exemplary passive marker device shown is also referred to as LCQ2. Generally, this means that L1 and L2 can have approximately the same physical dimensions. The increase in the inductance ratio increases the impedance of the energy storage circuit formed by L and C, and thus couples the quartz more strongly to the energy storage circuit. The result is a shorter time constant, i.e., τ2 < τ1, and an increased amplitude at the start, i.e., A2 > A1. At the same time, the total SNR changes little. Therefore, by adjusting the inductance, the shape of the response can be changed. The shape of the response is inherent to the passive marker device and can be known in advance or measured during acquisition.

[0093] Figure 5C Shows a circuit for reducing the coupling between the energy storage circuit and the quartz by means of a capacitive voltage divider. Figure 5C The exemplary passive marker device shown is also referred to as LCQ3. In this example, the inductance ratio remains constant, i.e., L3 = L1, while maintaining the quality factor of the coil. The capacitive voltage divider formed by two capacitors C 31 and C 32 reduces the coupling between the energy storage circuit and the quartz. The result is an extended time constant, i.e., τ3 > τ1, and a reduced amplitude at the start, i.e., A3 < A1.

[0094] Figure 5D Shows an exemplary circuit for reducing the coupling between the quartz and the energy storage circuit by slightly detuning the energy storage circuit. Figure 5D The exemplary passive marker device shown is also referred to as LCQ4. This means that the LC resonator formed by the inductor, capacitor, and the stray capacitance of the inductor and quartz is not tuned to the same frequency as the natural frequency of the quartz. In this example, the capacitance C4 is different from Figure 1 the capacitance C1 shown, while maintaining the same inductance, i.e., L4 = L1. Since the LC resonator is not tuned to the same frequency as the natural frequency of the quartz, the signal decays (in a first approximation) with an extended decay time constant τ4 (i.e., τ4 > τ1) and a reduced amplitude at the start (i.e., A4 < A1).

[0095] Figure 5E Shows another exemplary circuit for achieving a shorter τ by using two or more quartz crystals in parallel. Figure 5E The exemplary passive marker device shown is also referred to as LCQ5. Although Figure 5ETwo quartz crystals, namely Q5 and Q5', may be shown, but by way of example, it should be understood that for some other embodiments, three, four or more quartz crystals may be connected in parallel. Such a configuration may be beneficial for increasing the inductance of the coil indefinitely. In practice, the inductance may be limited by manufacturing problems such as impractically thin conductors and the self-resonance of the coil. An alternative to connecting quartz crystals in parallel is to use a larger quartz crystal (not shown).

[0096] Figure 6 An example of a tracking system 100 is schematically shown. The tracking system 100 includes a plurality of passive marker devices 10, an excitation field generator 30, and a tracking device 40. In some examples, as Figure 6 shown, the tracking system 100 may further include a position determination device 50.

[0097] As Figure 6 shown, the plurality of passive marker devices 10 may include passive marker devices 10a, 10b, …, 10n, where n is equal to or greater than 2. In some examples, the plurality of passive marker devices 10 (such as Figure 6 the passive marker devices 10a, 10b, ……, 10n shown) may include Figure 1 one or more exemplary passive marker devices shown. In some examples, in some examples, the plurality of passive marker devices 10 (such as Figure 6 the passive marker devices 10a, 10b, ……, 10n shown) may include Figure 2 A- Figure 2 H shown one or more exemplary passive marker devices. In some examples, the plurality of passive marker devices 10 (such as Figure 6 the passive marker devices 10a, 10b, ……, 10n shown in) may include Figure 3A and Figure 4 one or more exemplary passive marker device arrangements shown in. For example, two of the plurality of passive marker devices may be mounted on a rigid mechanical device (e.g., a housing) to track the 6DOF movement of a device (e.g., a medical device) in three-dimensional space. In some examples, the plurality of passive marker devices 10 may include Figure 1 、 Figure 2 A- Figure 2 H、 Figure 3A and Figure 4 any combination of the exemplary passive marker devices shown. In some examples, the plurality of passive marker devices 10a, 10b, …, 10n may be attached to one or more devices (not shown), such as one or more medical devices, to track the positions of the plurality of devices relative to the excitation field generator 30 and the tracking device 40.

[0098] The excitation field generator 30 is configured to generate at least one excitation pulse within an excitation time slot to simultaneously or sequentially excite at least two passive marker devices.

[0099] The tracking device 40 is configured to detect response signals generated by at least two passive marker devices and distinguish the at least two passive marker devices based on the operating frequency and decay time constant of the response signals. The excitation and recording schemes will be discussed hereinafter and in particular with respect to Figure 9 and Figure 10 the examples shown.

[0100] In some examples, as Figure 6 shown, the tracking system 100 may further include a position determination device 50 configured to receive response signals from the tracking device 40 and position information from the excitation field generator 30. Based on the received response signals and the position information from the excitation field generator 30, the position determination device 50 may determine the positions of the plurality of passive marker devices 10 and thus determine the position of the (one or more) devices to which the plurality of passive marker devices are attached relative to the excitation field generator 30 and the tracking device 40. In some examples, the position determination device 40 may implement a gradient-based tracking method that employs the saturation of the coil elements in the passive marker devices. For example, the excitation field generator 30 may generate an external saturation field that saturates the coil elements. Thereby, the saturation field is provided with a gradient. This gradient means that the amplitude picked up by the tracking device 40 will vary according to the position and orientation at which the passive marker device 10 is provided in the gradient field. Thus, by correlating the position information provided from the excitation field generator 30 and the tracking device 40, the measured amplitude may allow the position of the passive marker device to be restricted to a certain region, such as a certain plane, in the excitation field.

[0101] In some examples, as Figure 6 shown, the tracking system 3 may further include a display configured to generate, for example, a graphical representation of the tracking and output the graphical representation to a user. Using such an arrangement, a tracking system can be provided that has high frequency resolution and high quality factor for both small and large sizes, i.e., independent of the size of the device.

[0102] Figure 7 Another example of the tracking system 100 is schematically and exemplarily shown. Generally, the exemplary tracking system 100 according to Figure 7 works in the same manner as the embodiment according to Figure 6 For the sake of brevity, reference will be made to Figure 6 . Figure 7 The embodiment of Figure 6The difference between the embodiments lies in the fact that in accordance with Figure 7 In the example of, the excitation and field generator 30 and the tracking array 40 are provided as a single magnetic array 70. That is, in accordance with Figure 7 In the example of, the same magnetic array used to generate the excitation field and / or the saturation field is also used to pick up the oscillatory response made by a plurality of passive marker devices 10 (e.g., passive marker devices 10a, 10b,..., 10n as Figure 7 shown). For this purpose, the magnetic array 70 may include a circuit that allows switching between a transmit mode, in which the excitation field and / or the saturation field is generated and provided to act on the plurality of passive marker devices 10, and a receive mode, in which the magnetic field generated by the plurality of passive marker devices 10 can be picked up. As described above, the position determination device and the display 60 are the same as those described with respect to Figure 6 , except that there is no necessary communication between the two separate units (i.e., the excitation field generator 30 and the tracking device 40) because the magnetic array 70 already knows the values and positions of the excitation field and / or the generated saturation field.

[0103] Figure 8 shows an overall excitation and recording scheme. As Figure 8 shown, a plurality of excitation pulses 22 form an excitation sequence. Although Figure 8 two excitation pulses 22 may be shown by way of example, it should be understood that the excitation sequence may include more excitation pulses 22. Since it is technically challenging, excitation and reception are usually not performed simultaneously. Instead, as Figure 8 shown, there may be dedicated times for transmitting and then for receiving. Hereinafter, the dedicated time for sensing is also referred to as the excitation time slot or the transmission phase, and the dedicated time for receiving is also referred to as the reception time slot or the reception phase. As Figure 8 shown, there may be an inevitable dead time between the excitation time slot and the reception time slot. Due to this dead time, it may not be feasible to switch very quickly between transmission and reception. The excitation time slot, the dead time, and the reception time slot form an excitation block. In the Figure 8 shown example, two consecutive excitation blocks are shown, e.g., the nth excitation block and the (n + 1)th excitation block. The dead time may be a major limitation and may prevent the use of other excitation schemes. It should also be noted that finally, there may be an additional dead time between the reception time slot and the subsequent excitation time slot. However, an electronic circuit can be constructed to reduce this second dead time. The switching time is approximately 1 millisecond. For some simple electronic devices, it may be longer. With precisely tuned and thus improved electronic devices, 100 μs can be achievable.

[0104] Figure 9Illustrates the basic principle of an exemplary improved excitation scheme. In this example, the improved excitation scheme will be explained with respect to two passive marker devices 10a and 10b. The two passive marker devices 10a and 10b may differ in terms of operating frequency and / or decay time constant. For example, the mechanical resonance frequency of the mechanical resonator of the passive marker device 10a is f1, and the mechanical resonance frequency of the mechanical resonator of the passive marker device 10b is f2. The time constant of the passive marker device 10a may be greater than the time constant of the passive marker device 10b, i.e., τ1>τ2. For example, the difference between the time constants of the two passive marker devices 10a and 10b may be equal to or greater than a defined threshold. Due to the presence of dead time, it may be most effective to have only one excitation phase that acts on both passive marker devices jointly and then measure both markers during one reception phase. For example, as Figure 8 shown, two frequency-selective excitation pulses 22a and 22b can be generated to sequentially excite the two passive marker devices 10a and 10b. The frequency components of the frequency-selective excitation pulse 22a may be f1 or close to f1 to excite the mechanical oscillation of the mechanical resonator of the passive marker device 10a in the resonance mode. The frequency components of the frequency-selective excitation pulse 22b may be f2 or close to f2 to excite the mechanical oscillation of the mechanical resonator of the passive marker device 10b in the resonance mode. Two frequency-selective excitation pulses 22a and 22b are generated within one excitation time slot to sequentially excite the two passive marker devices 10a and 10b. Additionally, it may be effective to order the excitation in such a way that the more persistent passive marker device is excited first and then the less persistent passive marker device. This can ensure a better overall SNR. In Figure 8 the example shown, the passive marker device 10a is the more persistent passive marker device because the time constant of the passive marker device 10a can be greater than the time constant of the passive marker device 10b, i.e., τ1>τ2. For this purpose, the passive marker device 10a can be first excited by the frequency-selective excitation pulse 22a, and then the passive marker device 10b can be excited by the frequency-selective excitation pulse 22b. When the passive marker device 10b is excited later, the response signal generated by the passive marker device 10b has less attenuation in the reception time slot, thereby improving the SNR.

[0105] Figure 10 Shows the basic principle of another exemplary improved excitation scheme. Generally, the exemplary improved excitation scheme according to Figure 10 works in the same way as the embodiment according to Figure 9 the embodiment of Figure 10 is the same as that described above in Figure 9The difference between the embodiments lies in the fact that the mechanical resonance frequency of the mechanical resonator of the passive marker device 10a is f1, and the mechanical resonance frequency of the mechanical resonator of the passive marker device 10b is also f1 or close to f1. The two passive marker devices 10a and 10b differ in terms of the decay time constant. In other words, this example uses two passive marker devices 10a and 10b that are at the same frequency but have different decay times, such that at least one of the passive marker devices, such as Figure 10 the passive marker device 10b shown, has decayed significantly before being re-excited. In this case, the same excitation pulse 22 can be used to effectively excite both passive marker devices 10a and 10b, and the phase of the excitation may only need to match (if at all) the resonator with the longer time constant, such as Figure 10 the passive marker device 10a shown. This can achieve a very effective excitation pattern. This exemplary excitation scheme can be combined with frequency-selective excitation to produce a very effective excitation scheme.

[0106] In some examples, Figure 9 and Figure 10 the two passive marker devices 10a and 10b shown can be arranged in a manner similar to the exemplary passive marker device arrangement shown in Figure 3A and Figure 4 and are attached to a single device.

[0107] Although Figure 9 and Figure 10The basic principle of an exemplary improved excitation scheme for two passive marker devices is illustrated by way of example, but it should be understood that the excitation scheme can also be applied to tracking three or more passive marker devices. However, if there are more than two passive marker devices, it may be necessary not to excite them all in a combined excitation phase (i.e., in one excitation time slot), because for some passive marker devices, too much attenuation may have occurred during the excitation phase. Therefore, it may be more beneficial to only combine some of them into excitation pulses and then traverse them in a default "round robin" manner. If the frequencies are close to each other, the first excitation pulse for exciting the first passive marker device will also affect the second passive marker device. This can be taken into account and used as an advantage by timing the excitations in a way of constructive interference. This can be possible if the second excitation pulse starts at a very low oscillation amplitude level, i.e., when its corresponding τ is low compared to the repetition time of the complete sequence of excitations. Then, the phase of the excitation becomes a free parameter and phase matching can be easily performed. This may be a special case when two or more passive marker devices operate at the same frequency in one excitation block. In such a case, it may be desirable to have such a long repetition time that all but one passive marker device have short τ constants, such that the phase of only that one passive marker device is not free. That passive marker device can be excited in the correct phase and all other passive marker devices are automatically excited in the correct phase because their phases are uncorrelated.

[0108] So far, it has been implicitly assumed that all passive marker devices are aligned in such a way that they can be excited using the same coil array setup (e.g., fixed amplitude and phase correlations in all excitation coils). However, for a tracking system, there are several passive marker devices in such a way that excitation vectors can be generated at the positions of the passive marker devices in all spatial directions. Therefore, there are more degrees of freedom in sequence generation. Thus, for example, if the coil current patterns are sufficiently different, two passive marker devices can be excited simultaneously. However, in most cases this cannot be relied upon. However, in some instances, when the orientations of two passive marker devices are physically linked, this can be relied upon. An example for this is a 6DOF sensor. Figure 3A and Figure 4 Two examples of 6DOF sensors are shown in Figure 3A and Figure 4。Here, two passive marker devices are arranged such that their magnetization vectors are not parallel to each other, preferably such that their magnetization vectors are orthogonal or nearly orthogonal to each other (e.g., in the range from about 85° to about 95°). Such an arrangement (such as the arrangement shown in Figure 3A and Figure 4 ) can be excited simultaneously in one excitation time slot, and if different frequencies are required in the application, the two orientations can have different frequencies.

[0109] As mentioned above, one problem is the possibility of too high a signal amplitude of the passive marker device close to the excitation field generator. This can be due to the operation of two or more passive marker devices at the same frequency, where one passive marker device is closer to the coil array than the other passive marker devices. This can also occur due to the operation of two non-parallel passive marker devices (e.g., the passive marker device arrangement shown in Figure 3A and Figure 4 ), where one passive marker device is parallel to the local excitation field and the other passive marker device is orthogonal or nearly orthogonal to the local excitation field (e.g., in the range from about 85° to about 95°). It can also occur if they are not tuned to the same frequency but are subject to spectral leakage. This must be solved by hardware modifications to the passive marker device and adjustment of the sequence.

[0110] In Figure 2 A- Figure 2 H, an example of a hardware modification to the passive marker device 10 is shown. Thus, for the sake of brevity, reference will be made to Figure 2 A- Figure 2 H.

[0111] As regarding Figure 2 D to Figure 2As discussed in H, the coupling between the coil element and the mechanical resonator can be reduced or even eliminated for a duration much longer than the oscillation period. The reasons for changing the coupling between the coil element and the mechanical resonator are twofold: firstly, not to damage the mechanical resonator, and secondly, not to saturate the receiving amplifier (or have excessive spectral leakage). For the latter reason, a suitable sequence must be provided. Assuming a simple circuit in which the amplitude is set with an upper limit, the sequence during the transmission phase (i.e., in the excitation time slot) has two phases: first, a low-amplitude signal is transmitted, and then a higher-amplitude signal with approximately the same duration but with the opposite phase is transmitted. This has the following effects: at close positions, due to the upper limit set on the signal, near-zero excitation is achieved. At positions far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel each other out, and thus a relatively high signal amplitude is achieved. This is because at this far position, the excitation is linear with the applied field, and the weak field from the first phase does not cancel the strong field from the second phase. The exact time ratio of the low-amplitude signal and the high-amplitude signal can depend on the desired residual level of excitation during the phases with different amplitudes and the residual amplitude achieved at the mechanical resonator. In these circuits, when the excitation amplitude increases, even in the clipping mode, the amplitude at the mechanical resonator can still rise a little. Therefore, in these circuits, a relatively long time may be consumed during ineffective excitation. Transistor (or varactor)-based circuits can provide better efficiency because they can further reduce the effective excitation level, especially when frequency components that are off-resonance from the mechanical resonance frequency keep them in a deep-off state.

[0112] Now, if the readout phase is to start immediately, a single set of excitations will be sufficient. However, for a large number of passive marker devices, the excitation phase may be so long that some of the signals have decayed too much before the reception phase (i.e., the reception time slot). The first step to avoid this can be to sort the excitations according to the decay time, where the shortest decay time comes first. If this is not enough, the excitation phase must be split into several. During sorting and splitting, the simultaneous excitations are kept as a single unit. When splitting a sequence, the components are picked in order from the source sequence and then placed into the next subsequence. So, for example, if the initial pulse sequence is like "22a, 22b, 22c, 22d, 22e, 22f, 22g, 22h, 22i, 22j, 22k", the distribution in two sequences will be "22a, 22c, 22e, 22g, 22i, 22k" and "22b, 22d, 22f, 22h, 22j". The distribution in three will result in "22a, 22d, 22g, 22j", "22b, 22e, 22h, 22k" and "22c, 22f, 22i". The 180° phase flip excitations are kept within the same excitation window and thus the splitting rule must be adjusted accordingly.

[0113] Note that the calculation needs to take into account the initial state of the mechanical resonator. Especially for passive marker devices with long decay times, the excitation builds up gradually. An additional consideration may be that there is an initial build-up process and for the start, it may not be possible to meet the desired signal level or the splitting may need to be in many more excitation windows. The software must allow for this before concluding that the excitation does not meet the desired quality criteria. The excitation should be aligned in a way that constructively adds more energy to the quartz. The final excitation level may not be exactly the desired excitation level, but in most applications, a small 10% deviation in the amplitude and phase of the signal will not cause harm. For most applications, it may not be necessary to calculate the structure of the sequence every time. In an application, the passive marker device may not move very fast and thus the structure remains constant. Only the specific amplitude and duration may be changed. This also allows the calculation of the sequence structure to take much more time than the repetition time and thus a less powerful computer can be used.

[0114] Figure 11 A flowchart depicting a method for tracking multiple marker devices is illustrated. The method steps will be described in conjunction with Figure 6 and Figure 7 the tracking system shown.

[0115] In step 210, the method includes generating at least one excitation pulse within an excitation time slot to simultaneously or sequentially excite at least two passive marker devices. This can be done by the Figure 6 excitation field generator 30 shown or byFigure 7 The magnetic array 70 shown generates the at least one excitation pulse.

[0116] In some examples, the plurality of passive marker devices can include two or more passive marker devices having operating frequencies with a frequency separation equal to or greater than a threshold. The excitation field generator can be configured to generate a sequence of frequency-selective excitation pulses within an excitation time slot so as to excite at least two of the two or more passive marker devices in a sequential manner. For example, Figure 9 An exemplary excitation scheme is schematically shown for generating two frequency-selective excitation pulses 22a and 22b within an excitation time slot so as to excite two passive marker devices 10a and 10b in a sequential manner. The excitation field generator can sort the frequency-selective excitation pulses according to the decay time constants of the at least two passive marker devices to be excited. For example, in Figure 9 the example shown, the passive marker device 10a is the more persistent passive marker device because the time constant of the passive marker device 10a can be greater than the time constant of the passive marker device 10b, i.e., τ1 > τ2. To this end, the passive marker device 10a can be first excited by the frequency-selective excitation pulse 22a, and then the passive marker device 10b can be excited by the frequency-selective excitation pulse 22b. When the passive marker device 10b is excited later, the response signal generated by the passive marker device 10b has less attenuation in the reception time slot, thereby improving the SNR. Thus, for the sake of brevity, reference will be made to Figure 9 .

[0117] In some examples, the plurality of passive marker devices can include two or more passive marker devices configured to have operating frequencies with a frequency separation less than a threshold and having different decay time constants. For example, Figure 10 the exemplary excitation scheme shown uses two passive marker devices 10a and 10b that are at the same frequency but have different decay times such that at least one of the passive marker devices, e.g., Figure 10 the passive marker device 10b shown, has significantly decayed before being re-excited. In this case, the same excitation pulse 22 can be used to effectively excite both passive marker devices 10a and 10b, and the phase of the excitation may only need to match (if at all) the resonator with the longer time constant, e.g., Figure 10 the passive marker device 10a shown. Thus, for the sake of brevity, reference will be made to Figure 10 .

[0118] In addition, as described above, if there are more than two passive marker devices, it may be necessary not to activate them during a combined activation phase (i.e., during an activation time slot), because for some passive marker devices, too much attenuation may have occurred during the activation phase. Therefore, it may be more beneficial to only combine some of them into activation pulses and then iterate through them in a default "polling" manner.

[0119] In step 220, the method includes detecting one or more response signals generated by a plurality of passive marker devices. The one or more response signals can be detected by Figure 6 tracking device 40 as shown or Figure 7 magnetic coil 70 as shown. The plurality of passive marker devices can be distinguished based on the operating frequency of the response signal and the decay time constant of the response signal.

[0120] In some examples, as described in the exemplary activation and recording schemes with respect to Figure 9 and 10 shown, the response signal can be detected in a reception time slot.

[0121] In some other examples, for a large number of passive marker devices, the activation phase may be so long that some of the signals have decayed too much before the reception phase (i.e., the reception time slot). In such a case, the response signals generated by two or more passive marker devices can be detected in a plurality of reception time slots according to the decay time constant of the response signal.

[0122] In step 230, the method includes determining the positions of the plurality of passive marker devices based on the one or more response signals. The positions can be determined by Figure 6 and Figure 7 position determination device 40 as shown. In some examples, position determination device 40 can implement a gradient-based tracking method that employs the saturation of the coil elements in the passive marker devices. For example, excitation field generator 30 can generate an external saturation field that saturates the coil elements. Thus, the saturation field has a gradient. This gradient means that the amplitude picked up by tracking device 40 will be different according to the following position and orientation, i.e., the position and orientation at which passive marker device 10 is provided in the gradient field. Therefore, by correlating the position information provided from excitation field generator 30 and tracking device 40, the measured amplitude can allow the position of the passive marker device to be restricted to a certain region in the excitation field, such as a certain plane.

[0123] In another exemplary embodiment of the present invention, there is provided a computer program or a computer program unit, characterized in that it is adapted to execute the method steps of the method according to one of the previous embodiments on a suitable system. The computer program or computer program unit may include instructions executable by a computer unit.

[0124] Therefore, the computer program unit may be stored on a computer unit such as a computer, which may also be part of an embodiment of the present invention. For example, the computer program unit may be stored in a storage unit such as a memory. Such a computing unit may be adapted to execute the steps of the method described above or induce the execution of the steps of the method described above. In addition, it may be adapted to operate the components of the above-described device. The computing unit may be adapted to operate automatically and / or execute the commands of a user. In some embodiments, the computing unit includes one or more processors. The one or more processors may be data processors. The computer program may be loaded into the working memory of the data processor or processing circuit. Therefore, the data processor may be equipped to execute the method of the present invention.

[0125] This exemplary embodiment of the present invention covers both computer programs that use the present invention from the beginning and computer programs that transform existing programs into programs that use the present invention through updates.

[0126] Furthermore, the computer program unit is capable of providing all the necessary steps to implement the process of the exemplary embodiment of the method described above.

[0127] According to another exemplary embodiment of the present invention, there is provided a computer-readable medium, such as a CD-ROM, wherein the computer-readable medium has a computer program unit stored thereon, and the computer program unit is described in the previous part. In some embodiments, the computer-readable medium is non-transitory.

[0128] The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium provided together with other hardware or as part of other hardware, but the computer program may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0129] However, the computer program may also exist on a network such as the World Wide Web and be downloadable from such a network into the working memory of a data processor. According to another exemplary embodiment of the present invention, there is provided a medium for making a computer program unit available for download, and the computer program unit is arranged to execute the method according to one of the previous embodiments of the present invention.

[0130] It must be noted that embodiments of the present invention are described with reference to different subject matters. Specifically, some embodiments are described with reference to claims of the method type, while other embodiments are described with reference to claims of the device type. However, those skilled in the art will understand from the above and the following descriptions that, unless otherwise indicated, any combination between features related to different subject matters is also considered to be disclosed by this application, in addition to any combination of features belonging to one type of subject matter. However, all features can be combined to provide a synergistic effect that exceeds the simple sum of the features.

[0131] Although the present invention has been described in detail and illustrated in the accompanying drawings and the foregoing description, such description and illustration shall be considered illustrative or exemplary rather than restrictive. The present invention is not limited to the disclosed embodiments. By studying the drawings, the disclosure, and the dependent claims, those skilled in the art can understand and implement other variations of the disclosed embodiments when practicing the claimed invention.

[0132] In the claims, the word "comprising" does not exclude other elements or steps, and the words "a" or "an" do not exclude a plurality. A single processor or other unit may perform the functions of several items recited in the claims. Although specific measures are recited in mutually different dependent claims, this does not indicate that a combination of these measures cannot be used advantageously. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A passive marker device (10) comprising: A coil element (12); A mechanical resonator (14); And A circuit element (16); Wherein, the coil element is coupled to the mechanical resonator for generating a response signal in response to an excitation pulse, the response signal having an operating frequency corresponding to the mechanical resonance frequency of the mechanical resonator and a signal decay having a decay time constant, wherein the excitation pulse is a magnetic excitation pulse or an electromagnetic excitation pulse; and Wherein, the circuit element is configured to limit the power transmitted to the mechanical resonator when at least one excitation pulse has an excitation amplitude equal to or greater than a threshold.

2. The passive marker device according to claim 1, Among them, The circuit element comprises at least one of the following: A diode configured to limit the voltage across the mechanical resonator; A plurality of diodes connected in series, the plurality of diodes being configured to limit the voltage of the mechanical resonator; Or A pin diode configured to limit the voltage across the mechanical resonator.

3. The passive marker device according to claim 1, Among them, The circuit element is configured to reduce the coupling between the coil element and the mechanical resonator.

4. The passive marker device according to claim 3, Among them, The circuit element comprises a varactor diode and a resistor connected in parallel with the varactor diode.

5. The passive marker device according to claim 4, Among them, The circuit element further comprises one of the following: A field effect transistor configured to clamp a charging operation; A field effect transistor and a series capacitor connected to the field effect transistor, the field effect transistor being configured to clamp a charging operation; A field effect transistor and a series capacitor connected to the field effect transistor, the field effect transistor being configured to clamp a charging operation, wherein the field effect transistor is connected to a capacitive voltage divider of a capacitive element; Or A normally-on field effect transistor as a switching element.

6. A passive marker device (20) comprising: A first passive marker device (10'), which is the passive marker device according to any one of claims 1 to 5; And A second passive marker device (10"), which is the passive marker device according to any one of claims 1 to 5; Wherein, the first passive marker device and the second passive marker device are mounted on a mechanical device at an angle relative to each other such that the coil element of the first passive marker device is not parallel to the coil element of the second passive marker device.

7. The passive marker device according to claim 6 or 7, Among them, The angle is in the range from about 45° to about 135°, optionally in the range from about 85° to about 95°, preferably about 90°.

8. The passive marker device according to any one of claims 6 and 7, Among them, The first passive marker device and the second passive marker device are configured to: have operating frequencies with a frequency separation less than a threshold and have different decay time constants, or have operating frequencies with a frequency separation equal to or greater than the threshold.

9. A tracking system (100), comprising: A plurality of passive marker devices (10a, 10b,..., 10n), wherein the plurality of passive marker devices include the passive marker device according to any one of claims 1 to 5 and / or the passive marker device according to any one of claims 6 to 8; An excitation field generator (30) configured to generate at least one excitation pulse within one excitation time slot to simultaneously or sequentially excite at least two passive marker devices; and A tracking device (40) configured to: detect response signals generated by the at least two passive marker devices and distinguish the at least two passive marker devices based on the operating frequencies and the decay time constants of the response signals.

10. The tracking system according to claim 9, Among them, The tracking device is configured to detect the response signals generated by the two or more passive marker devices in one or more reception time slots according to the decay time constants of the response signals.

11. The tracking system according to claim 9 or 10, Among them, The plurality of passive marker devices include two or more passive marker devices configured to have operating frequencies with a frequency separation less than a threshold and have different decay time constants.

12. The tracking system according to any one of claims 9-11, Among them, The plurality of passive marker devices include two or more passive marker devices configured to have operating frequencies with a frequency separation equal to or greater than a threshold; And wherein the excitation field generator is configured to generate a sequence of frequency-selective excitation pulses within the excitation time slot to sequentially excite at least two of the two or more passive marker devices.

13. The tracking system according to claim 12, Among them, The excitation field generator is configured to sort the frequency-selective excitation pulses according to the decay time constants of the at least two passive marker devices to be excited.

14. The tracking system according to any one of claims 8 to 13, Among them, The excitation field generator is configured to generate a sequence of phase-altered excitation pulses to excite at least two passive marker devices at different distances from the excitation field generator; And wherein the sequence of phase-altered excitation pulses includes a first signal with a low amplitude and a second signal with a higher amplitude, wherein the first signal and the second signal have opposite phases.

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