LCQ position marker

Abstract

The present disclosure relates to device tracking. An improved passive marker device 10 is described. The passive marker device 10 includes a coil element 12, a mechanical resonator 14, and a circuit element 16. The coil element is coupled to the mechanical resonator and generates a response signal having an operating frequency corresponding to a mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant in response to excitation pulses, the excitation pulses being magnetic or electromagnetic field excitation pulses. The circuit element is configured to limit power delivered to the mechanical resonator when at least one excitation pulse has an excitation amplitude above a threshold.

Description

[Technical Field]

[0001] The present invention relates to tracked passive marker devices, passive marker device configurations, tracking systems, and related methods. [Background technology]

[0002] Device tracking is useful, for example, in certain medical procedures, for example, a marker device can be attached to a medical device, such as a medical interventional device, during the procedure. Summary of the Invention [Problem to be solved by the invention]

[0003] Systems for miniature markers and sensors based on the use of so-called micro-magnetic oscillators (MMOs) are described in WO 2019243098. In these systems, the initiation of mechanical vibrations in the MMOs in response to a magnetic or electromagnetic excitation field is used to track marker devices containing these MMOs and hence the devices to which these marker devices are attached.

[0004] However, for MMOs, the signal-to-noise ratio (SNR) is proportional to the square of the linear dimension of the MMO device, and therefore these MMO devices become less useful as available space increases.

[0005] There is a need to provide an improved passive marker device. [Means for solving the problem]

[0006] The object of the present invention is solved by the subject matter of the independent claims, and further embodiments are incorporated in the dependent claims. It should be noted that the below-described aspects of the present invention also apply to tracked passive marker devices, passive marker device configurations and tracking systems.

[0007] According to a first aspect of the present invention, there is provided a passive marker device. The passive marker device includes a coil element, a mechanical resonator, and a circuit element. The coil element is coupled to the mechanical resonator and generates a response signal in response to excitation pulses, the response signal having an operating frequency corresponding to a mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant. The excitation pulses are magnetic or electromagnetic field excitation pulses. The circuit element is configured to reduce power delivered to the mechanical resonator when at least one excitation pulse has an excitation amplitude equal to or greater than a threshold.

[0008] One of the challenges to overcome is that passive marker devices near the transmit coil system may be subject to very strong excitation fields, which can cause the voltage across the crystal to be too large and destroy it, and also cause signals received from nearby passive marker devices in receive mode to be much stronger than signals received from distant passive marker devices, making these weak signals difficult to detect.

[0009] Therefore, the present disclosure provides a passive marker device having circuit elements that limit or reduce the power transferred to the mechanical resonator when the excitation amplitude is large, i.e., when the passive marker device is in the vicinity of the coil system. The passive marker device is described in more detail below, particularly with reference to the example shown in Figures 2A-2H.

[0010] In more sophisticated ways, the coupling between the coil element and the mechanical resonator can be reduced or eliminated for durations much longer than the vibration period, as will be explained in more detail below, particularly with reference to the examples shown in Figures 2D-2H.

[0011] This, in combination with the phase-varying excitation pulse, can further achieve the desired low amplitude of nearby passive marker devices, while the amplitude of more distant passive marker devices is not significantly affected. Assuming a simple circuit in which the amplitude is capped, the sequence during the transmit phase (i.e., during the excitation time slot) has two phases: first, a low-amplitude signal is transmitted, followed by a higher-amplitude signal of approximately the same duration but with the opposite phase. This has the effect that, due to the signal capping, near-zero excitation is achieved at close positions. At positions far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel out, and therefore a relatively high signal amplitude is achieved. This is because, at this far-away position, the excitation is linear with the applied field, and the weak field from the first phase does not cancel out the strong field from the second phase, as will be explained in more detail below, particularly with reference to the exemplary excitation and recording schemes shown in Figures 9 and 10.

[0012] According to embodiments of the present invention, the circuit element comprises a diode configured to limit the voltage across the mechanical resonator, a plurality of series connected diodes configured to limit the voltage across the mechanical resonator, or a pin diode configured to limit the voltage across the mechanical resonator, as will be described in more detail below, particularly with reference to the examples shown in Figures 2A-2C.

[0013] According to one embodiment of the present invention, the circuit element is configured to reduce coupling between the coil element and the mechanical resonator.

[0014] In other words, the coupling between the coil element and the mechanical resonator can be reduced or eliminated for a period much longer than the oscillation period.

[0015] There are two reasons for varying the coupling between the coil and the crystal: to avoid destroying the crystal and to avoid saturating the receive amplifier (or causing excessive spectral leakage). For the latter reason, it is necessary to provide an appropriate sequence. Assuming a simple amplitude-limited circuit, the sequence during the transmit phase (i.e., during the excitation time slot) has two phases: first, a low-amplitude signal is transmitted, followed by a high-amplitude signal of approximately the same duration but in opposite phase. This achieves near-zero excitation at close locations due to signal capping. At locations far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel out, and therefore a relatively high signal amplitude is achieved. This is explained in more detail below, particularly with reference to the exemplary excitation and recording schemes shown in Figures 9 and 10.

[0016] According to one embodiment of the present invention, the circuit element comprises a varactor diode and a resistor connected in parallel with the varactor diode, which will be explained in more detail below, particularly with reference to the example shown in Figure 2D. According to one embodiment of the present invention, the circuit element further comprises: a field effect transistor configured to clamp the charging operation; a field effect transistor configured to clamp the charging action and a series capacitor connected to the field effect transistor; a field effect transistor configured to clamp the charging action and a series capacitor connected to the field effect transistor, the field effect transistor being connected to a capacitive voltage divider of a capacitive element; or Normally-on field-effect transistors as switching elements, It has one of the following:

[0017] This is explained in more detail below, particularly with respect to the example shown in Figures 4E-4H.

[0018] According to a second aspect of the present invention, there is provided a passive marker device. The passive marker device includes a first passive marker device and a second passive marker device. Each of the first and second passive marker devices includes a coil element and a mechanical resonator, the coil element being coupled to the mechanical resonator and generating a response signal in response to an excitation pulse having an operating frequency corresponding to a mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant. The excitation pulse is a magnetic or electromagnetic field excitation pulse. The first and second passive marker devices are mounted in a mechanical configuration at an angle to each other such that the coil element of the first passive marker device is non-parallel to the coil element of the second passive marker device.

[0019] In some instances, it may be desirable to track six mechanical degrees of freedom (DOF) of movement of a device, such as 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. Therefore, if a coil element of a passive marker device is parallel to the local excitation field, the coil element will not be excited, and therefore no oscillatory response can be generated by the passive marker device. One solution is to switch between different directions of the magnetic field to excite the coil element. However, such an approach can be slow and inefficient. For example, if an application has an acquisition rate of 40-100 Hz and the passive marker device operates at several kHz, a long and complex excitation sequence may be required. Therefore, it may be difficult to achieve the desired time resolution for medical applications with a single coil. Furthermore, a low angle between the coil and the magnetic field will result in a low response signal, which can result in a low SNR.

[0020] Therefore, according to this embodiment, the orientations of the two passive marker devices can be physically linked via a mechanical configuration, for example, a housing. The two passive marker devices are arranged non-parallel to each other. Thus, when one coil element of the passive marker device configuration is parallel to the local excitation field, the other coil element of the passive marker device configuration is non-parallel (e.g., perpendicular or nearly perpendicular) to the local excitation field, and can generate a higher response signal to achieve a higher SNR.

[0021] This is explained in more detail below, particularly with respect to the examples shown in Figures 3A, 3B and 4. 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.

[0022] This is explained in more detail below, particularly with reference to the example shown in FIG.

[0023] According to one embodiment of the present invention, the angle is in the range of about 45° to about 135°, optionally in the range of about 85° to about 95°, and preferably about 90°.

[0024] According to one embodiment of the present invention, the first and second passive marker devices are configured to have operating frequencies with a frequency separation below a threshold and to have different decay time constants, or alternatively, the first and second passive marker devices are configured to have operating frequencies with a frequency separation equal to or greater than a threshold.

[0025] In other words, it is possible to use decay time space and / or frequency space to distinguish between a first passive marker device and a second device. In some examples, both passive marker devices can have the same or similar frequency, but different decay time constants. Alternatively, both marker devices can have different frequencies.

[0026] According to a third aspect of the present invention, there is provided a tracking system. The tracking system includes a plurality of passive marker devices, an excitation field generator, and a tracking device. The plurality of passive marker devices include passive marker devices according to the first aspect and any related examples, and / or passive marker configurations according to the second aspect and any related examples. The excitation field generator is configured to generate at least one excitation pulse to excite at least two passive marker devices simultaneously or sequentially within one excitation time slot. The tracking device is configured to detect response signals generated by the at least two passive marker devices and distinguish between the at least two passive marker devices based on the operating frequencies and decay time constants of the response signals.

[0027] In many clinical applications, two or more devices must be tracked. This is especially true when a 6DOF sensor is required, since this alone requires two passive marker devices. The traditional approach to addressing different passive marker devices is to use frequency-selective excitation pulses, designing (i.e., tuning) all markers to operate at different frequencies. However, this is limited by the need for short excitation pulses to achieve the required fast repetition times, which can lead to significant spectral overlap. For LC resonators with quartz resonators, another limitation is the limited number of quartz frequencies available from commercially available components. While manufacturers can tune the quartz crystal to any frequency, the relatively small market size makes specialized designs economically unfeasible.

[0028] To speed up detection, the tracking system disclosed herein can excite two or more marker devices in one excitation time slot and distinguish between passive marker devices using decay time space in addition to frequency space. Exemplary excitation and recording schemes are described in detail below, particularly with reference to the examples shown in Figures 9 and 10. According to one 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 receive time slots according to the decay time constants of the response signals.

[0029] In some examples, the response signal can be detected in one receive time slot, as described with respect to the exemplary excitation and recording schemes shown in FIGS.

[0030] In some other examples, when there are a large number of passive marker devices, the excitation phase may be too long, causing some of the signal to decay excessively before the receive phase (i.e., receive time slot). In such cases, the response signals generated by two or more passive marker devices can be detected in multiple receive time slots according to the decay time constants of the response signals.

[0031] According to one embodiment of the present invention, the plurality of passive marker devices comprises two or more passive marker devices having operating frequencies spaced apart by less than a threshold frequency and configured to have different decay time constants.

[0032] For example, two or more passive marker devices may have essentially the same operating frequency, e.g., such operating frequency is below a defined threshold. For example, if the coil current pattern is sufficiently distinct, it is possible to simultaneously excite two passive marker devices. This is described in more detail below, particularly with respect to the exemplary excitation and recording scheme shown in FIG. 10.

[0033] According to one 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 greater than or equal to a threshold, and the excitation field generator is configured to generate a sequence of frequency-selective excitation pulses within an excitation time slot to sequentially excite at least two of the two or more passive marker devices.

[0034] For example, frequency-selective excitation pulses can be used to excite two or more passive marker devices at different frequencies, as will be described in more detail below, particularly with respect to the exemplary excitation and recording scheme shown in FIG.

[0035] According to one embodiment of the present invention, 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.

[0036] This is explained in more detail below, particularly with respect to the exemplary excitation and recording scheme shown in FIG.

[0037] According to one embodiment of the present invention, the excitation field generator is configured to generate a sequence of phase-varying excitation pulses for exciting at least two passive marker devices at different distances to the excitation field generator, the sequence of phase-varying excitation pulses having a first signal with a low amplitude and a second signal with a higher amplitude, the first signal and the second signal having opposite phases.

[0038] For example, a phase-varying excitation pulse can be used to suppress the amplitude of nearby passive marker devices to a desired low amplitude without significantly affecting the amplitude of more distant passive marker devices.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] These and other aspects of the invention will be apparent from and will be further elucidated with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0041] [Figure 1] 1A and 1B are schematic diagrams illustrating examples of passive marker devices. [Figure 2] 2A to 2H are diagrams schematically showing some examples of circuit elements. [Figure 3A] FIG. 1 illustrates an exemplary passive marker configuration. [Figure 3B] FIG. 1 illustrates an exemplary passive marker configuration. [Figure 4] 10A and 10B illustrate other exemplary passive marker configurations. [Figure 5]5A to 5E show exemplary circuits for varying the signal decay time constant after excitation. [Figure 6] FIG. 1 is a diagram illustrating an example of a tracking system. [Figure 7] FIG. 10 is a diagram illustrating another example of a tracking system. [Figure 8] 1 shows a general excitation and recording scheme. [Figure 9] FIG. 1 illustrates the basic principles of an exemplary improved excitation scheme. [Figure 10] FIG. 1 illustrates the basic principles of another exemplary improved excitation scheme. [Figure 11] 10 is a flow chart illustrating a method for tracking multiple marker devices. DETAILED DESCRIPTION OF THE INVENTION

[0042] It should be noted that the figures are merely schematic and are not drawn to scale. In the figures, elements corresponding to elements already described may have the same reference numerals. Examples, embodiments, or optional features, whether non-limiting or not, should not be understood as limiting the invention as claimed.

[0043] Detailed Description of the Embodiments

[0044] 1 shows a schematic diagram of an example of a passive marker device (passive marker) that can be attached to a device, such as a medical device, to be tracked by a tracking system. The passive marker device 10 has a coil element 12, such as a coil, and a mechanical resonator 14.

[0045] The coil element 12 can refer to an element having and / or corresponding to a magnetic coil configuration having a specific number of windings. The coil element 12 can also be a pre-fabricated magnetic coil having an appropriate number of windings, an appropriate size, and an appropriate distance between the windings. The quantity, size, and distance between the windings can be specifically determined based on the desired magnetic properties of the coil element. The coil element 12 can include or be made of copper. Alternatively or additionally, the coil element 12 can include or be made of silver. In some examples where the passive marker device is intended to be radiation-transparent, the coil element 12 can include or be made of aluminum. In some examples where the passive marker device is intended to be radiation-opaque, the coil element can include or be made of gold.

[0046] The mechanical resonator 14 is connected to the coil element 12 and deforms in response to the voltage output of the coil element in response to an externally applied magnetic or electromagnetic field, initiating mechanical vibration. The mechanical resonator 14 can include or be made of a crystalline material. The crystalline material can be selected to have sufficient piezoelectric properties. The crystalline material can include and / or correspond to quartz crystal. Quartz crystal is a piezoelectric resonator material with a known resonant frequency. Therefore, it is possible to provide a passive marker device having a coil element and a mechanical resonator, and the components can be selected to provide an electric field with a frequency close to or corresponding to the mechanical resonant frequency of the quartz crystal. This can enable a high quality factor to be obtained for the passive marker device. Another advantage of using a quartz crystal is that since both natural frequencies are very close to each other, the quartz crystal can be used in series or parallel resonance. Furthermore, depending on whether the quartz crystals are connected in parallel or series, the resonant frequency can be slightly adjusted by connecting additional elements, such as trimmers, in parallel or series with the quartz crystal. This allows for compensation for manufacturing tolerances, etc. Additional crystalline materials with similar properties can be envisioned as well. The mechanical resonator 14 can have a body and at least one protrusion attached thereto. For example, the resonator element can be fork-shaped, such as a tuning fork-shaped, resonator element having two protrusions attached to the body. A tuning fork-shaped mechanical resonator 14 can improve resonance characteristics and can also be beneficial for using passive marker devices to sense physical properties.

[0047] The coil element 12 can be positioned at a distance from the mechanical resonator 14. The coil element 12 and the mechanical resonator 14 can be connected to each other via their respective connecting portions for this purpose. In some examples, if space needs to be saved, the distance can be saved by providing a winding around the mechanical resonator so as to provide a space between the wings of the mechanical resonator 14 and the coil element 12. The size of this space can be appropriately selected depending on the size of the passive marker device 10. In some examples, the configuration between the coil element 12 and the mechanical resonator 14 can be such that the mechanical resonator 14 is provided within the coil element and extends along its axis.

[0048] When an external magnetic or electromagnetic field is applied, the externally applied magnetic or electromagnetic field can act on the coil element 12. In response, the coil element 12 can convert the externally applied magnetic or electromagnetic field into a respective output voltage. The coil element 12 is electrically connected to the mechanical resonator 14 and provides an output voltage to the input / output terminals of the mechanical resonator 14. The mechanical resonator 14, which has piezoelectric properties, is deformed by the voltage applied from the coil element 12. Thus, the mechanical resonator begins to perform mechanical vibration. 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 excitation magnetic or electromagnetic field. As described above, when the frequency component is provided at or within a range near the mechanical resonant frequency of the mechanical resonator, mechanical vibration is excited in a resonant mode. That is, the mechanical resonator 14 vibrates near its mechanical resonant frequency. Each vibration can persist for some time, regardless of whether or not a voltage is supplied from the coil element 12, thereby causing a signal to decay with a decay time constant. The deformation of the mechanical resonator 12 then generates a piezoelectric voltage that is output to the coil element 12. This causes a current to flow through the coil element. In response, the coil element 12 generates a magnetic field that can then be detected by the tracking system as a vibration response.

[0049] In some examples, as shown in FIG. 1 , the passive marker device 10 can further include a capacitive element 16, such as a capacitor. The capacitive element 16 can be connected in parallel with the coil element 12, such as a coil. This essentially 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 be supplied to the mechanical resonator 16, causing greater deformation in the mechanical resonator and thus increasing the vibration level of the mechanical resonator. This increases the piezoelectric voltage acting on the coil element 12 and improves signal strength. In some examples, the capacitance value of the capacitive element can be selected so that the resonant frequency of the coil element in the LC resonator is equal to or close to the mechanical resonant frequency of the mechanical resonator 16, such as a quartz crystal. This can allow for a reduced amount of windings required, as losses in the oscillator of the resonator circuit are lowered. With fewer windings, manufacturing of the passive marker device can also be simplified. A passive marker device having a coil element 12, a mechanical resonator 14 in the form of a quartz crystal, and a capacitive element 16 can be referred to as an "LCQ" marker. Compared to micro-magnetic oscillator-based markers, LCQ markers can provide a greater operating distance from the coil array and have a better SNR. In the following, an exemplary passive marker device 10 is shown in the form of an LCQ marker by way of example, but it will be understood that the passive marker device 10 can be other types of passive marker devices.

[0050] One problem associated with passive marker devices can be that the signal amplitude of the passive marker device may be too large. This can be caused by two or more passive marker devices operating at the same frequency, with one of them being much closer to the coil array than the others. This problem can also occur if the devices are not tuned to the same frequency and suffer from spectral leakage. This problem can be solved by utilizing a circuit element that limits or reduces the power delivered to the crystal at high excitation amplitudes, i.e., when it is close to the coil system. The circuit element can be a nonlinear circuit element.

[0051] Figures 2A-2H schematically illustrate some examples of circuit elements 18. In particular, Figures 2A-2C illustrate some exemplary circuit elements configured to limit the voltage across the mechanical resonator, and Figures 2D-2E illustrate some exemplary circuit elements configured to reduce or even eliminate coupling between the coil element and the mechanical resonator for durations much longer than the vibration period.

[0052] FIG. 2A illustrates an exemplary passive marker device 10 having 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. When using a diode, the energy stored in the tank circuit can be sufficiently reduced so that only one direction is sufficient. An advantage of this exemplary circuit element can be that it is simple and cost-effective. The use of a PIN diode requires relatively little additional capacitance, so the pre-clamping effect can be relatively small.

[0053] 2B shows an exemplary passive marker device 10 having a circuit element 18b for increasing the voltage at which the voltage limiting effect occurs. In this example, the circuit element 18b includes multiple diodes, each diode in series increasing the voltage by approximately 0.5V.

[0054] 2C illustrates an exemplary passive marker device 10 having an exemplary circuit element 18c. In this example, the exemplary circuit element 18c includes only one (PIN) diode 18c. The Z-diode set can limit the majority of the voltage through its breakdown while avoiding the potential drawback of requiring a larger number of diodes.

[0055] FIG. 2D illustrates an exemplary passive marker device 10 having an exemplary circuit element 18d configured to limit the voltage across the mechanical resonator by detuning the tank circuit. In this example, the exemplary circuit element 18d includes a varactor diode and a resistor in parallel with the varactor diode. In response to an applied magnetic or electromagnetic field, the varactor diode begins to detune the tank circuit when it becomes conductive, charging the series capacitor and itself. The detuning operation is not limited to an instantaneous voltage and may potentially last for a long period of time. The duration can be set to an appropriate duration, i.e., approximately the dead time. A resistor may be provided in parallel and placed in parallel with the varactor diode. A further advantage of this circuit is that during the dead time, the tank circuit is detuned, and thus, stored energy persists in the crystal resonator, increasing the signal during the receive time.

[0056] FIG. 2E 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) for clamping the charging operation. This exemplary circuit can also extend the switching pattern of the excitation sequence. Thus, after the critical voltage is reached, the circuit is muted for a long period of time. In this circuit, there are two elements that store charge to perform the delay operation. First, there is a capacitor in series with the Schottky diode. This charge is removed by a resistor in parallel with this capacitor, forming a resistor-capacitor (RC) circuit. The second RC circuit is formed by the gate resistor and gate capacitance (shown as a dotted capacitor). Depending on the excitation sequence, only one RC circuit for the delay operation is required; 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, this voltage can become high enough to switch the transistor, thereby shorting the tank circuit. Practical implementations of field effect transistors, typically metal oxide semiconductor field effect transistors (MOSFETs), have a body diode (dotted diode) that may limit the tank circuit voltage to 0.5V peak.

[0057] FIG. 2F shows an exemplary passive marker device 10 with exemplary circuit element 18f, which can circumvent the problem by adding a series capacitor to the transistor. Diode rectification creates a voltage at the transistor drain that eventually stops the DC current flow. When the transistor "on" state, i.e., an appropriately high gate voltage, is reached, the transistor series capacitor effectively becomes a parallel capacitor to the tank circuit, detuning it. The operating voltage can be determined by the selected transistor (threshold voltage) or by a circuit such as that shown in FIG. 2G.

[0058] FIG. 2G shows an exemplary passive marker device 10 with exemplary circuit element 18g. In this example, the tank circuit capacitance is replaced by a capacitive voltage divider. A charge pump (e.g., two diodes) provides the desired gate voltage. A remaining problem with transistors is their voltage-dependent output capacitance. This is largely avoided in the circuit shown in FIG. 2H.

[0059] FIG. 2H shows an exemplary passive marker device 10 with exemplary circuit element 18h. In this example, a normally-on field-effect transistor (here, an n-channel junction field-effect transistor (jFET)) is used as the switching element. A series capacitor is parallel to the remainder of the tank circuit capacitance until the gate voltage becomes negative enough to render the transistor non-conductive. The diode for generating the negative voltage typically has a series resistance in the megaohm range. Therefore, the diode's nonlinear behavior does not significantly alter the resonance in the circuit. The parallel RC circuit between the gate and source governs the discharge time constant and, therefore, the duration of detuning. This circuit also allows for a long mute duration by applying power at a higher frequency after the jFET is switched off.

[0060] The passive marker device shown in Figures 1 and 2A-2H can be used to track six degrees of freedom (DOF) of movement of a device, such as 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. Therefore, if a coil element of a passive marker device is parallel to the local excitation field, the coil element will not be excited, and therefore no oscillatory response can be generated by the passive marker device. One solution is to switch the direction of the magnetic field to excite the coil element. However, such an approach can be slow and inefficient. For example, if an application has an acquisition rate of 40-100 Hz and the passive marker device operates at several kHz, a long and complex excitation sequence may be required. Therefore, it may be difficult to achieve the desired time resolution for medical applications with a single coil. In addition, if the angle between the coil and the magnetic field is small, the response signal will be similarly low, resulting in a lower SNR.

[0061] 3A illustrates an exemplary passive marker arrangement 20 for addressing one or more of the above-mentioned problems. The passive marker arrangement 20 includes a first passive marker device 10a and a second passive marker device 10b.

[0062] 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 and generates a response signal in response to an excitation pulse, the response signal having an operating frequency corresponding to the mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant.

[0063] 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 and generates a response signal in response to an excitation pulse, the response signal having an operating frequency corresponding to the mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant.

[0064] The first passive marker device 10a and the second passive marker device 10b are mounted in a mechanical configuration (not shown) at an angle to each other so that the first coil element 12a of the first passive marker device 10a is non-parallel to the second coil element 12b of the second passive marker device 10b.

[0065] FIG. 3B illustrates an exemplary arrangement of the first coil element 12a of the first passive marker device 10a relative to the second coil element 12b of the second passive marker device 10b. As shown in FIG. 3B, 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 disposed at an angle α relative to each other. Because the first coil element 12a of the first passive marker device 10a is non-parallel to the second coil element 12b of the second passive marker device 10b, the angle α is not equal to 0° or 180°. For example, the angle α may be greater than 0° and less than 180°. For example, the angle α may range from about 45° to about 135°. Preferably, the angle α is in the range of about 85° to about 95°, and more preferably about 90°.

[0066] 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 non-parallel, so that 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 non-parallel to the local excitation field. Therefore, at least one coil element in the passive marker configuration 20 is excited to provide a vibration response that is tracked by the tracking system. In this way, there is no need to switch the direction of the excitation field to excite 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 orthogonal (approximately 90°) or nearly orthogonal (e.g., approximately 85° to approximately 95°) to each other to generate a higher response signal. For example, if one coil element of the passive marker device configuration is parallel to the local excitation field, the other coil element of the passive marker device configuration is perpendicular to the local excitation field and can generate a higher response signal to achieve a higher SNR.

[0067] In some examples, one or both of the first passive marker device 10a and the second passive marker device 10b may have circuit elements that limit power when the local excitation field has an excitation amplitude above a threshold.

[0068] FIG. 4 illustrates another example of a passive marker device configuration 20 in which both the first passive marker device 10a and the second passive marker device 10b have circuit elements. While FIG. 4 illustrates that both the first passive marker device 10a and the second passive marker device 10b have the circuit element 18d shown in FIG. 2D, it will be understood that the first passive marker device 10a and the second passive marker device 10b can have other circuit elements, such as the circuit elements 18a-18c and 18e-18g shown in FIG. 2, to limit power. In some examples, as shown in FIG. 4, the first passive marker device 10a and the second passive marker device 10b can have the same circuit elements. In some other examples (not shown), the first passive marker device 10a and the second passive marker device 10b can have different circuit elements. For example, a first passive marker device 10a may have the circuit element 18a shown in Figure 2D, while a second passive marker device 10b may have the circuit element 18f shown in Figure 2F.

[0069] 3A and 4 may have different operating frequencies. For example, the first mechanical resonator 14a of the first passive marker device 10a has a mechanical resonant frequency of f1, and the second mechanical resonator 14b of the second passive marker device 10b has a mechanical resonant frequency of f2. The frequency interval, i.e., |f1-f2|, may be equal to or greater than a threshold value. Therefore, 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.

[0070] In some examples, the first passive marker device 10a and the second passive marker device 10b shown in FIGS. 3A and 4 can have essentially the same operating frequency but different decay time constants. For example, the first mechanical resonator 14a of the first passive marker device 10a can have a mechanical resonant frequency f1, and the second mechanical resonator 14b of the second passive marker device 10b can have a mechanical resonant frequency f2. The frequency interval, |f1-f2|, is less than a threshold. For example, the first passive marker device 10a can have a first decay time constant τ1, and the second passive marker device can have a second decay time constant τ2. The difference between the two decay time constants, |τ1-τ2|, is equal to or greater than a defined threshold. In this way, the vibration decay behavior of the first passive marker device 10a and the second passive marker device 10b can be used as a discrimination factor. Therefore, even if two passive marker devices operate at the same frequency, they can be distinguished by fitting known responses to recorded data, for example. The damping constant can be adjusted by proper matching of the LC resonator with the mechanical resonator. In this matching, the product of inductance and capacitance is more or less fixed and is given by the mechanical resonant frequency. However, the ratio L / C is a freely selectable design parameter. If the LC parallel resonator has a high resistance (at resonance), the damping time constant will be low, but the initial transmit amplitude will be high. Overall, if the quality factor of the coil element remains constant, the total SNR will not change, which means that the physical size of the coil element will not change. Figures 5A-5E show exemplary circuits for varying the time constant of signal damping after excitation. The exemplary circuit shown in Figure 5A can be implemented, for example, as the first passive marker device 10a. The passive marker device 10b may have any one of the circuits shown in Figures 5B to 5E to achieve different time constants of signal decay after excitation.

[0071] FIG. 5A shows a basic circuit for comparing the following circuits. As shown in FIG. 5A, the basic circuit is also referred to as LCQ1. The exemplary passive marker device shown in FIG. 5A includes a coil element with inductance L1, a capacitance element with capacitance C1, and a mechanical resonator, such as a quartz crystal resonator, with a quality factor Q. In this example, the LC resonator formed by the inductor, capacitor, and stray capacitance of the inductor and the quartz crystal can be tuned to the same frequency as the natural frequency of the quartz crystal. In all subsequent circuits, the mechanical resonator remains the same, i.e., Q1 = Q2 = Q3 = Q4 = Q5. For example, after a fixed excitation period with the same frequency, length, and amplitude, the signal decays (to a first approximation) with a characteristic decay time constant τ1.

[0072] Figure 5B shows what happens if the inductance is replaced with a larger value (L2 > L1) while maintaining the coil's quality factor (i.e., the ratio of inductance to series resistance at a given frequency). The exemplary passive marker device shown in Figure 5B is also referred to as LCQ2. Typically, this means that L1 and L2 can have approximately the same physical dimensions. Increasing the inductance increases the impedance of the tank circuit formed by L and C, thus coupling the crystal to the tank circuit more strongly. This results in a shorter time constant (τ2 < τ1) and a larger amplitude at onset (A2 > A1). The overall SNR remains unchanged. Therefore, adjusting the inductance can change the shape of the response. The shape of the response is inherent to the passive marker device and can be known in advance or measured during acquisition.

[0073] FIG. 5C shows a circuit that reduces the coupling between the tank circuit and the crystal using a capacitive voltage divider. The exemplary passive marker device shown in FIG. 5C is also referred to as LCQ3. In this implementation, the inductance remains unchanged, i.e., L3 = L1, while maintaining the quality factor of the coil. The capacitive voltage divider formed by the two capacitors C31 and C32 reduces the coupling between the tank circuit and the crystal. This results in an extended time constant, i.e., τ3 > τ1, and A3 <A1となる。

[0074] FIG. 5D shows an example circuit for reducing the coupling between the crystal and the tank circuit by slightly detuning the tank circuit. The example passive marker device shown in FIG. 5D is also called an LCQ4. This means that the LC resonator formed by the inductor, capacitor, and stray capacitance of the inductor and the crystal is not tuned to the same frequency as the crystal's natural frequency. Here, capacitance C4, unlike capacitance C1 shown in FIG. 1, maintains the same inductance, i.e., L4 = L1. Because the LC resonator is not tuned to the same frequency as the crystal's natural frequency, the signal will (to a first approximation) have a longer decay time constant τ4, i.e., τ4 > τ1, and will have a reduced initial amplitude (i.e., A4 <A1)。

[0075] FIG. 5E shows another example circuit for achieving a shorter τ by using two or more quartz crystals in parallel. The example passive marker device shown in FIG. 5E is also referred to as LCQ5. While FIG. 5E shows two quartz crystals, Q5 and Q5', as an example, it will be appreciated that other implementations may involve connecting three, four, or more quartz crystals in parallel. Such a configuration may be useful for increasing the inductance of the coil indefinitely. In practice, the inductance may be limited by impractically thin conductors and manufacturing issues such as coil self-resonance. Instead of placing quartz crystals in parallel, a larger quartz crystal (not shown) may also be used.

[0076] 6 schematically illustrates an example of a tracking system 100. The tracking system 100 includes a plurality of passive marker devices 10, an excitation field generator 30, and a tracking unit 40. As shown in FIG. 6, in some examples, the tracking system 100 may further include a position determination unit 50.

[0077] As shown in FIG. 6 , the plurality of passive marker devices 10 can include passive marker devices 10a, 10b, ..., 10n (n is 2 or greater). In some examples, the plurality of passive marker devices 10, such as passive marker devices 10a, 10b, ..., 10n shown in FIG. 6 , can include one or more of the exemplary passive marker devices shown in FIG. 1. In some examples, the plurality of passive marker devices 10, such as passive marker devices 10a, 10b, ..., 10n shown in FIG. 6 , can include one or more of the exemplary passive marker devices shown in FIGS. 2A-2H. In some examples, the plurality of passive marker devices 10, such as passive marker devices 10a, 10b, ..., 10n shown in FIG. 6 , can include one or more of the exemplary passive marker device configurations shown in FIGS. 3A and 4. For example, two of the multiple passive marker devices may be attached to a rigid mechanical arrangement, such as a housing, to track 6 DOF of movement of a device (e.g., a medical device) in three-dimensional space. In some examples, the multiple passive marker devices 10 may include any combination of the exemplary passive marker devices shown in Figures 1, 2A-2H, 3A, and 4. In some examples, the multiple 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 multiple devices relative to the excitation field generator 30 and the tracking device 40.

[0078] The excitation field generator 30 is configured to generate at least one excitation pulse to excite at least two passive marker devices simultaneously or sequentially within one excitation time slot.

[0079] The tracking device 40 is configured to detect the response signals generated by the at least two passive marker devices and to distinguish between 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. The excitation and recording schemes are described below with particular reference to the examples shown in Figures 9 and 10.

[0080] In some examples, as shown in FIG. 6 , the tracking system 100 may further include a positioning 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 positioning device 50 may determine the positions of the multiple passive marker devices 10 relative to the excitation field generator 30 and the tracking device 40, and thus the multiple passive marker device positions. In some examples, the positioning device 40 may implement a gradient-based tracking approach that uses saturation of coil elements in the passive marker devices. For example, the excitation field generator 30 may generate an external saturation magnetic field that saturates the coil elements. This imparts a gradient to this saturation magnetic field. This gradient means that the amplitude picked up by the tracking device 40 will vary depending on the position and orientation of the passive marker device 10 in the gradient field. The measured amplitude may therefore enable the position of the passive marker device to be restricted to a particular region, e.g., a particular plane, within the excitation field by correlating the position information provided by the excitation field generator 30 and the tracking device 40.

[0081] In some examples, as shown in Figure 6, the tracking system 3 may further comprise a display configured, for example, to generate a graphical representation of the tracking and output this graphical representation to a user. Using this device, it is possible to provide tracking and systems with high frequency resolution and high quality factor for both small and large sizes, i.e., regardless of the sizing of the device.

[0082] FIG. 7 shows, in a schematic and exemplary manner, another example of a tracking system 100. In general, the exemplary tracking system 100 according to FIG. 7 operates in the same manner as the embodiment according to FIG. 6. For the sake of brevity, reference shall be made to FIG. 6. The difference between the embodiment according to FIG. 7 and the embodiment according to FIG. 6 described above lies in the fact that in the example according to FIG. 7, the excitation field generator 30 and the tracking array 40 are provided as a single magnetic array 70. That is, in the example according to FIG. 7, the same magnetic array used to generate the excitation and / or saturation fields is also used to pick up vibration responses by a plurality of passive marker devices 10, e.g., passive marker devices 10a, 10b, ..., 10n, as shown in FIG. 7. For this purpose, the magnetic array 70 may have circuitry that allows it to switch between a transmit mode, in which excitation and / or saturation fields are generated and provided to act on a plurality of passive marker devices 10, and a receive mode, in which the magnetic fields generated by a plurality of passive marker devices 10 are picked up. As mentioned above, the positioning device and display 60 are the same as those described in connection with FIG. 6, and it only means that communication between the two separate units, i.e., the excitation field generator 30 and the tracking device 40, is not required, since the magnetic array 70 already knows the value and positioning of the excitation field and / or the generated saturation field.

[0083] FIG. 8 illustrates a typical excitation and recording scheme. As shown in FIG. 8, multiple excitation pulses 22 form an excitation sequence. While FIG. 8 illustrates two excitation pulses 22 as an example, it is understood that an excitation sequence can have more excitation pulses 22. Due to technical difficulties, excitation and reception typically do not occur simultaneously. Rather, as shown in FIG. 8, there may be a dedicated time for transmission followed by a dedicated time for reception. Hereinafter, the dedicated time for sensing is also referred to as an excitation time slot or transmission phase, while the dedicated time for reception is also referred to as a reception time slot or reception phase. As shown in FIG. 8, there may be unavoidable dead time between the excitation time slot and the reception time slot. Due to this dead time, it may be impossible to switch very quickly between transmission and reception. The excitation time slot, dead time, and reception time slot form an excitation block. In the embodiment illustrated in FIG. 8, two consecutive excitation blocks are shown, e.g., an nth excitation block and an (n+1)th excitation block. The dead time can be a significant limitation and prevent the use of other excitation schemes. Finally, note that there may be an additional dead time between the receive time slot and the subsequent excitation time slot. However, it is possible to configure the electronics to reduce this second dead time. The switching time is on the order of approximately 1 millisecond. In some simple electronic circuits, it may be longer. In finely tuned, advanced electronic circuits, 100 μs is achievable.

[0084] FIG. 9 illustrates the basic principles of an exemplary improved excitation scheme. In this example, the improved excitation scheme is described with respect to two passive marker devices 10a and 10b. The two passive marker devices 10a and 10b may have different operating frequencies and / or damping time constants. For example, the mechanical resonator of the passive marker device 10a has a mechanical resonant frequency f1, and the mechanical resonator of the passive marker device 10b has a mechanical resonant frequency 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 greater than or equal to a specified threshold. Due to the dead time, it may be most efficient to have only one excitation phase acting on both passive marker devices simultaneously and to measure both markers in one receive phase. For example, as shown in FIG. 8, two frequency-selective excitation pulses 22a and 22b can be generated to sequentially excite two passive marker devices 10a and 10b. The frequency component of the frequency-selective excitation pulse 22a is at or close to f1, and can excite the mechanical vibration of the mechanical resonator of the passive marker device 10a in a resonant mode. The frequency component of the frequency-selective excitation pulse 22b is at or close to f2, and can excite the mechanical vibration of the mechanical resonator of the passive marker device 10b in a resonant mode. Both 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. In addition, it can be efficient to sort the excitations so that the longer-lasting passive marker device is excited first, followed by the shorter-lasting passive marker device. This can ensure a better overall SNR. 8, passive marker device 10a is the longer lasting passive marker device because the time constant of passive marker device 10a is greater than the time constant of passive marker device 10b, i.e., τ1 > τ2. Thus, passive marker device 10a is first excited by frequency-selective excitation pulse 22a, and then passive marker device 10b is excited by frequency-selective excitation pulse 22b.Because the passive marker device 10b is excited later, the response signal generated by the passive marker device 10b has less attenuation in the receive time slot, thereby improving the SNR.

[0085] FIG. 10 illustrates the basic principle of another exemplary improved excitation scheme. Generally, the exemplary improved excitation scheme according to FIG. 10 operates in the same manner as the embodiment according to FIG. 9. The difference between the embodiment of FIG. 10 and the embodiment of FIG. 9 described above is that the mechanical resonator of passive marker device 10a has a mechanical resonant frequency f1, and the mechanical resonator of passive marker device 10b also has a mechanical resonant frequency f1 or close to f1. The two passive marker devices 10a and 10b have different decay time constants. In other words, this example uses two passive marker devices 10a and 10b with the same frequency but different decay times, so that at least one of the passive marker devices, e.g., passive marker device 10b shown in FIG. 10, decays significantly before re-excitation. In this example, both passive marker devices 10a and 10b can be efficiently excited using the same excitation pulse 22, and the phase of the excitation only needs to match the resonator with the longer time constant, e.g., passive marker device 10a shown in FIG. 10. This can achieve a very efficient excitation mode. This exemplary excitation scheme can be combined with frequency selective excitation to yield a very efficient excitation scheme.

[0086] In some examples, the two passive marker devices 10a and 10b shown in Figures 9 and 10 can be configured in a similar manner to the exemplary passive marker device configurations shown in Figures 3A and 4 attached to a single device.

[0087] 9 and 10 illustrate the basic principles of an exemplary improved excitation scheme for two passive marker devices as an example, but it will be understood that the excitation scheme can also be applied to tracking three or more passive marker devices. However, when more than two passive marker devices are present, it may be necessary not to excite some passive marker devices in one joint excitation phase (i.e., within one excitation time slot) because significant attenuation may already occur during the excitation phase. Therefore, it may be beneficial to combine only some of them into an excitation pulse and proceed in a predetermined "round-robin" manner. If the frequencies are close to each other, the first excitation pulse used to excite the first passive marker device will also affect the second passive marker device. This can be considered and used as an advantage by timing the excitation to constructively interfere. This may be possible if the second excitation pulse starts at a very low level of vibration amplitude, i.e., its corresponding τ is low compared to the repetition time of the complete excitation sequence. The phase of the excitation then becomes a free parameter, making phase matching easier. This can be a special case where two or more passive marker devices operate at the same frequency within one excitation block. Here, it is required to set the repetition time long enough to shorten the τ constants of all passive marker devices except one, so that the phase of only one LCQ is not free. This passive marker device will be excited with the correct phase, and all other passive marker devices will automatically be excited with the correct phase since their phases do not matter.

[0088] Previously, it was implicitly assumed that all passive marker devices were aligned so that they could be excited with the same coil array configuration, e.g., a fixed amplitude and phase correlation across all excitation coils. However, in tracking systems, several passive marker devices exist so that excitation vectors at the passive marker device locations can be generated in all spatial directions. Therefore, there is more freedom in generating sequences. Thus, for example, if the coil current pattern is sufficiently clear, it is possible to simultaneously excite two passive marker devices. However, in most cases, this is unreliable. Nevertheless, in some cases, it is possible to rely on the orientations of two passive marker devices being physically linked. One example of this is a 6DOF sensor. Two examples of a 6DOF sensor are shown in Figures 3A and 4. For simplicity, we will refer to Figures 3A and 4. Here, the two passive marker devices are positioned so that their magnetization vectors are non-parallel to each other, preferably orthogonal or nearly orthogonal to each other (e.g., within a range of about 85° to about 95°). This configuration, like that shown in Figures 3A and 4, can be excited simultaneously in one excitation time slot, and the two directions can have different frequencies if the application requires different frequencies.

[0089] As mentioned above, one challenge is the possibility of excessively high signal amplitudes from passive marker devices close to the excitation field generator. This can occur when two or more passive marker devices operate at the same frequency, with one passive marker device much closer to the coil array than the other. This can also occur when two non-parallel passive marker devices, such as the passive marker device configurations shown in Figures 3A and 4, operate with one passive marker device parallel to the local excitation field and the other passive marker device perpendicular or nearly perpendicular (e.g., in the range of about 85° to about 95°) to the local excitation field. This can also occur when the passive marker devices are not tuned to the same frequency but are susceptible to spectral leakage. This must be resolved by hardware modifications and sequence adaptations of the passive marker devices.

[0090] Examples of hardware modifications to the passive marker device 10 are shown in Figures 2A-2H, and reference will be made to Figures 2A-2H for the sake of brevity.

[0091] As described with respect to Figures 2D-2H, it is possible to reduce or even eliminate the coupling between the coil elements and the mechanical resonator for durations much longer than the oscillation period. The reasons for varying the coupling between the coil elements and the mechanical resonator are twofold: first, to avoid destroying the mechanical resonator, and second, to avoid saturating the receiving amplifier (or having excessive spectral leakage). For the latter reason, an appropriate array must be provided. Assuming a simple circuit with an upper limit on amplitude, the sequence of transmit phases (i.e., excitation time slots) has two phases: first, a low-amplitude signal is transmitted, followed by a high-amplitude signal of approximately the same duration but opposite phase. This has the effect of achieving nearly zero excitation at nearby locations due to the signal capping. At locations far from the coil array, the relative amplitudes of the two 180° phase-shifted signals do not cancel out, and therefore, a relatively high signal amplitude is achieved. This is because, at this distance, the excitation is linear with the applied field, and the weak field from the first phase does not cancel out the strong field from the second phase. The exact time ratio of the low-amplitude signal to the high-amplitude signal can depend on the desired residual level of excitation and the residual amplitude achieved in the mechanical resonator between phases with different amplitudes. In these circuits, the amplitude in the mechanical resonator can still increase slightly when the excitation amplitude increases, even when in clipping mode. Therefore, in these circuits, relatively long periods of time can be wasted during ineffective excitation. Transistor- (or varactor-) based circuits can offer better efficiency, since they can further reduce the effective excitation level, especially when frequency components that are not resonant with the mechanical resonant frequency keep them deeply switched off.

[0092] Here, there is a set of excitations that is sufficient if the readout phase starts immediately. However, if there are a large number of passive marker devices, the excitation phase may be too long, causing parts of the signal to decay too quickly before the receive phase (i.e., the receive time slot). To avoid this, the first step is to sort the excitations according to their decay times, so that the shortest decay times are the slowest. If this is not sufficient, the excitation phase must be split into several parts. During sorting and splitting, simultaneous excitations are maintained as a single unit. When splitting a sequence, components are selected in order from the source sequence and then placed in the next subsequence. Thus, for example, if the initial pulse sequence is "22a, 22b, 22c, 22d, 22e, 22g, 22i, 22j, 22k," the split into two sequences would be "22a, 22c, 22e, 22g, 22i, 22k" and "22b, 22d, 22f, 22h, 22j." The distribution into three sequences is "22a, 22d, 22g, 22j", "22b, 22e, 22h, 22k", and "22c, 22f, 22i". Since the 180° phase flip excitations are kept within the same excitation window, this division rule must be adapted accordingly.

[0093] Note that the calculation must take into account the initial state of the mechanical resonator. For passive marker devices, especially those with long decay times, excitation gradually builds up. A further consideration is that the initial buildup process may prevent the desired signal level from being met, requiring the excitation to be split into more excitation windows. The software must allow for this before concluding that the excitation cannot meet the desired quality criteria. The excitation should be aligned to constructively add more energy to the crystal. While the final excitation level may not be exactly desired, in most applications, deviations of several tens of percent in the signal amplitude and phase are acceptable. For most applications, the structure of the sequence does not need to be calculated every time. In most applications, the passive marker device may not move very fast, so the structure remains constant. Only the specific amplitude and duration can be varied. This also allows the calculation of the sequence structure to take much longer than the repetition time, allowing less powerful computers to be used.

[0094] 11 shows a flow chart illustrating a method for tracking multiple marker devices. The method steps are described with reference to the tracking system shown in FIGS.

[0095] In step 210, the method includes generating at least one excitation pulse to excite at least two passive marker devices simultaneously or sequentially within one excitation time slot. The at least one excitation pulse can be generated by the excitation field generator 30 shown in Figure 6 or by the magnetic array 70 shown in Figure 7.

[0096] In some examples, the multiple passive marker devices may include two or more passive marker devices having operating frequencies with a frequency interval equal to or greater than a threshold value. The excitation field generator may be configured to generate a sequence of frequency-selective excitation pulses within an excitation time slot to sequentially excite at least two of the two or more passive marker devices. For example, FIG. 9 schematically illustrates an exemplary excitation scheme for generating two frequency-selective excitation pulses 22a and 22b within an excitation time slot to sequentially excite two passive marker devices 10a and 10b. The excitation field generator may 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 the example illustrated in FIG. 9, the time constant of passive marker device 10a is greater than the time constant of passive marker device 10b, i.e., τ1 > τ2, so passive marker device 10a is the longer-lasting passive marker device. For this reason, the passive marker device 10a is first excited by the frequency-selective excitation pulse 22a, and then the passive marker device 10b is excited by the frequency-selective excitation pulse 22b. Because the passive marker device 10b is excited later, the response signal generated by the passive marker device 10b has less attenuation in the receive time slot, thereby improving the SNR. For simplicity, reference will be made to FIG. 9.

[0097] In some examples, the multiple passive marker devices can have operating frequencies with a frequency separation less than a threshold, and can include two or more passive marker devices configured to have different decay time constants. For example, the exemplary excitation scheme shown in FIG. 10 uses two passive marker devices 10a and 10b with the same frequency but different decay times, such that at least one of the passive marker devices, e.g., passive marker device 10b shown in FIG. 10, decays significantly before re-excitation. In this case, both passive marker devices 10a and 10b can be efficiently excited using the same excitation pulse 22, and the phase of the excitation only needs to coincide with the resonator with the longer time constant, e.g., passive marker device 10a shown in FIG. 10. For simplicity, reference will be made to FIG. 10.

[0098] In addition, as mentioned above, if more than one passive marker device is present, it may be necessary not to excite them within one joint excitation phase (i.e., within one excitation time slot) because excessive attenuation may occur for some passive marker devices during the excitation phase. It may therefore be more beneficial to combine only some of them into an excitation pulse and let them proceed in a default "round robin" fashion.

[0099] In step 220, the method includes detecting one or more response signals generated by the plurality of passive marker devices. The one or more response signals can be detected by the tracking unit 40 shown in Figure 6 or by the magnetic array 70 shown in Figure 7. The plurality of passive marker devices can be distinguished based on the operating frequency of the response signals and the decay time constant of the response signals.

[0100] In some examples, the response signal can be detected in one receive time slot, as described with respect to the exemplary excitation and recording schemes shown in FIGS.

[0101] In some other instances, for a large number of passive marker devices, the excitation phase was too long, causing some of the signals to decay too much before the receive phase (i.e., receive time slots). In such cases, the response signals generated by two or more passive marker devices can be detected in multiple receive time slots according to the decay time constants of the response signals.

[0102] In step 230, the method includes determining the positions of the multiple passive marker devices based on the one or more response signals. The positions can be determined by a positioning device 40, as shown in FIGS. 6 and 7. In some examples, the positioning device 40 can implement a gradient-based tracking approach that takes advantage of saturation of coil elements within the passive marker devices. For example, the excitation field generator 30 can generate an external saturation magnetic field that saturates the coil elements. This saturation magnetic field is thus imparted with a gradient. This gradient means that the amplitude picked up by the tracking device 40 varies depending on the position and orientation of the passive marker device 10 in the gradient field. Thus, the measured amplitude can be used to restrict the position of the passive marker device to a specific region, e.g., a specific plane, within the excitation field by correlating the position information provided by the excitation field generator 30 and the tracking device 40.

[0103] In another exemplary embodiment of the invention, a computer program or a computer program element is provided, characterized in that it is adapted to perform, on a suitable system, the method steps of the method according to one of the previous embodiments. The computer program or computer program element may comprise instructions executable by a computer unit.

[0104] Thus, a computer program can be stored in a computing unit, such as a computer, which may be part of an embodiment of the present invention. For example, computer program elements can be stored in a storage unit, such as a memory. Such a computing unit can be adapted to perform or trigger the performance of the steps of the above-mentioned methods. Furthermore, it can be configured to operate components of the above-mentioned apparatus. The computing unit can be configured to operate automatically and / or to execute user instructions. In some embodiments, the computing unit comprises one or more processors. The one or more processors can be data processors. The computer program can be loaded into a separate working memory of a data processing or processing circuit. Thus, the data processor can be equipped to perform the method of the present invention.

[0105] This exemplary embodiment of the present invention encompasses both computer programs that use the present invention for the first time, and computer programs that, through updates, turn existing programs into programs that use the present invention.

[0106] Furthermore, a computer program may provide some or all of the steps necessary to carry out the procedures of the exemplary embodiments of the methods described above.

[0107] According to another exemplary embodiment of the present invention, a computer-readable medium, such as a CD-ROM, is presented, the computer-readable medium having stored thereon a computer program, the computer program being described by the preceding section. In some embodiments, the computer-readable medium is non-transitory.

[0108] The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms or via the Internet or other wired or wireless telecommunications systems, etc.

[0109] However, the computer program may also be presented over a network such as the World Wide Web and downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the invention, a medium for making a computer program available for downloading is provided, the computer program being configured to perform a method according to one of the aforementioned embodiments of the invention.

[0110] It should be noted that the embodiments of the present invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method-type claims, and other embodiments are described with reference to apparatus-type claims. However, those skilled in the art will understand from the above and below description that, unless otherwise specified, any combination of features belonging to one type of subject matter, as well as any combination between features relating to different subject matters, is disclosed in the present application. However, all features can be combined to provide a synergistic effect that is greater than the simple sum of the features.

[0111] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered exemplary or explanatory and not restrictive. The invention is not limited to the disclosed embodiments. 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 dependent claims.

[0112] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit 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. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. 1. A passive marker device comprising: A coil element; a mechanical resonator; a circuit element; the coil element is coupled to the mechanical resonator and generates a response signal having an operating frequency corresponding to a mechanical resonant frequency of the mechanical resonator and a signal decay having a decay time constant in response to an excitation pulse, the excitation pulse being a magnetic or electromagnetic field excitation pulse; The passive marker device, wherein the circuit element is configured to limit power delivered to the mechanical resonator when at least one of the excitation pulses has an excitation amplitude above a threshold.

2. The circuit element is a diode configured to limit the voltage across the mechanical resonator; a plurality of series-connected diodes configured to limit the voltage of the mechanical resonator; and a pin diode configured to limit the voltage across the mechanical resonator; The passive marker device of claim 1 , comprising at least one of:

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

4. 4. The passive marker device of claim 3, wherein the circuit element comprises a varactor diode and a resistor in parallel with the varactor diode.

5. The circuit element further comprises: a field effect transistor configured to clamp the charge action; a field effect transistor configured to clamp the charging action and a series capacitor connected to the field effect transistor; a field effect transistor configured to clamp the charging action and a series capacitor connected to the field effect transistor, the field effect transistor being connected to a capacitive voltage divider of a capacitive element; and Normally-on field-effect transistors as switching elements, 5. The passive marker device of claim 4, comprising one of:

6. a first passive marker device, which is a passive marker device according to any one of claims 1 to 5; a second passive marker device, which is a passive marker device according to any one of claims 1 to 5; A passive marker arrangement having A passive marker configuration, wherein the first passive marker device and the second passive marker device are mounted in a mechanical configuration at an angle to each other such that coil elements of the first passive marker device are non-parallel to coil elements of the second passive marker device.

7. 7. The passive marker arrangement of claim 6, wherein the angle is in the range of about 45 degrees to about 135 degrees, or in the range of about 85 degrees to about 95 degrees, or about 90 degrees.

8. A passive marker configuration as described in claim 6 or 7, wherein 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 to have different decay time constants, or to have operating frequencies with a frequency separation greater than or equal to a threshold.

9. 1. A tracking system comprising: a plurality of passive marker devices, said plurality of passive marker devices comprising a passive marker device according to any one of claims 1 to 5 and / or a passive marker arrangement according to any one of claims 6 to 8; an excitation field generator configured to generate at least one excitation pulse to excite at least two passive marker devices simultaneously or sequentially within one excitation time slot; a tracking device configured to detect response signals generated by the at least two passive marker devices and to distinguish between the at least two passive marker devices based on operating frequencies of the response signals and decay time constants of the response signals; A tracking system having:

10. 10. The tracking system of claim 9, wherein the tracking apparatus is configured to detect the response signals generated by the two or more passive marker devices within one or more receive time slots according to a decay time constant of the response signals.

11. 11. The tracking system of claim 9 or 10, wherein the plurality of passive marker devices comprises two or more passive marker devices having operating frequencies with a frequency spacing less than a threshold and having different decay time constants.

12. A tracking system as described in any one of claims 9 to 11, wherein the plurality of passive marker devices comprises two or more passive marker devices configured to have operating frequencies having a frequency spacing equal to or greater than a threshold, and 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.

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

14. 14. A tracking system as described in any one of claims 8 to 13, wherein the excitation field generator is configured to generate a sequence of phase-changing excitation pulses to excite at least two passive marker devices having different distances relative to the excitation field generator, the sequence of phase-changing excitation pulses having a first signal having a low amplitude and a second signal having a higher amplitude, the first signal and the second signal having opposite phases.

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