WO2011144421A1

WO2011144421A1 - Electromagnetic position measurement with multi-frequency marker

Info

Publication number: WO2011144421A1
Application number: PCT/EP2011/056564
Authority: WO
Inventors: Johannes Reinschke
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-05-21
Filing date: 2011-04-26
Publication date: 2011-11-24
Also published as: DE102010021255B4; DE102010021255A1

Abstract

The invention relates to the locating of a probe, particularly in a medical engineering application. The probe radiates a measurement signal in the form of an electromagnetic alternating field, which induces voltages in a large number of sensor coils of a receiver device, from which it is then possible to infer the position of the probe. However, in an electrically conductive interference body in the environment of the probe, the electromagnetic alternating field emitted by said probe can generate an eddy current, which in turn has the effect that, in each sensor coil, an interference voltage is additionally induced that adversely affects the position measurement. According to the invention, the probe emits a measurement signal consisting of at least two individual measurement signals with different frequencies. The voltage induced in each sensor coil can be divided into the spectral components corresponding to the frequencies. By difference formation of the spectral components, a voltage amplitude free of interference voltage can be determined.

Description

description

Electromagnetic position measurement with multi-frequency marker The present invention relates to the electromagnetic Po ^¬ sitionsmessung of an object, in particular in a ^¬ zinischen in medi applications.

At least one electromagnetic marker whose position is to be measured is attached to an object to be located, for example a catheter tip, a capsule endoscope or a so-called "bonemarker", which is located in the human body in the case of a medical application. A "bonemarker" is, for example, a nail-shaped object that is hammered into a bone and its 5D pose (ie the SD position and the orientation without the angle of rotation about the longitudinal axis) or 6D pose (ie the 3D position An endoscopy capsule, for example, is navigated in the stomach of a patient with the aid of a suitable magnet coil system, whereby the position of the capsule must be known in order to determine the coil currents to be set for capsule navigation. Depending on the application, the position "is to be understood below as meaning the location of the object, for example in Cartesian coordinates (x, y, z), and / or the 2D or 3D orientation of the object in space in the order of +/- 0, 5 ... 10mm for the location and +/- 0.5 ... 10 ° for the Orientie ^¬ tion.

Especially in medical technology, various electromagnetic position measuring methods are known. Essentially, a distinction is made between three different measuring principles:

(a) Sensor marker: Here are one or more small sensor coils in or on the object to be located, eg in a catheter tip. This is, for example, realized in the so-called "Aurora" measuring system of NDI,

(b) A passive marker, such as offered by the company. Calypso for radiotherapy or the company. Olympus for capsule endoscopy, reflects an elekt ^¬ romagnetisches signal that is radiated from the outside of a transceiver. Conclusions as to the position of the marker can be drawn relative to the transceiver based on the reflectors ^¬ formatted signal. (c) Active Marker: Here, the object to be located contains typi ^¬ cally an actively energized marker at a fixed carrier frequency of typically <300 kHz, preferably below 10kHz, radiates an AC dipole field. With a variety of sensor coils whose position and location outside the patient are fixed and known, that will

Measure the dipole field of the active marker in the sense that the voltages induced at the carrier frequency in the sensor coils are measured. US 2004/0254453 A1 describes a position measuring system of the type (a) in which a plurality of transmitters are placed outside the patient at fixed and known locations, each radiating a monofrequency alternating electromagnetic field. The frequencies of the individual stations differ from each other. As a result, each spectral line or monofrequency (voltage) amplitude can be unambiguously assigned to a transmitter from the measurement signal of the object to be located after Fourier transformation. Furthermore, it is described how the influence of electrically conductive disturbing bodies can be calculated from the measuring signal. It is exploited that with monofrequent excitation typically also spurious signals with ei ^¬ nem multiples of the exciting frequency arise. If the harmonic characteristic is known to the interfering element, its influence can be calculated out of the measuring signal.

In the US 6172499 Bl which starts also from a sensor marker of type (a), the field mehrfrequenter (or min ^¬ least dual more frequent) transmitter outside the patient is measured a receiver in the object to be located. Carrier frequencies in the range of 100 Hz to 1000 Hz are used, and in the disturbance compensation calculation, the phase relationship between the different measured frequency components is included. However, it is problematic that electromagnetic waves with a frequency of 1 kHz or less have a wavelength of 300 km or more, which is why the resulting phase differences are negligibly small at the typical intervals between transmitter, interfering body and receiver in medical applications why a phase compensation method in the low-frequency range is difficult to achieve.

The problem with all electromagnetic position measuring methods is the presence of electrically conductive objects or materials, in particular in the vicinity of the transmitter. Because the source field induces eddy currents in electrically conductive matters, which in turn radiate an electromagnetic field from ^¬ which is measured along with the original transmission field. This leads to a more or less erroneous position determination. However, the presence of electrically conductive objects or materials near the transmitter is not the exception, but rather the rule in medical applications. Electrically conductive are, for example, C-arches for

Fluoroscopy (especially for position

Reference measurements), surgical instruments, implants, etc.

The simplest way of reducing the influence of electrically conductive objects or materials is to lower the carrier frequency, since the interference voltages due to electrically conductive objects or materials increase disproportionately with the carrier frequency ω: neglecting the skin effect, the interference voltages increase with co ^2, taking into the skin effect with sichtigung CO ^3/2. This measure has been taken for example by the company NDI whose first product generation of the Aurora system with a carrier frequency of 12 kHz worked, while the 2nd generation of products a carrier frequency of 800 Hz used. As mentioned above, at such low frequencies, a phase compensation method is difficult to realize. It is an object of the present invention to provide a way to operate a signal emitting object to be located. It is a further object of the dung OF INVENTION ^¬ provide an opportunity for electromagnetic Positionsmes ^¬ solution of an object, wherein the influence of electri- cally conductive objects or materials is reduced to the position measurement.

In the solution according to the invention, use is made of the fact that the disturbance due to eddy-current influences increases disproportionately with the frequency and that this can be used for interference suppression or elimination if the marker transmits with more than one frequency. If the product of marker current amplitude and frequency is set the same for all frequencies used, the interference voltage component can be calculated by subtracting the spectral voltages.

In the method according to the invention for operating a probe to be located, the probe emits a total measurement signal for location, which can be detected by a receiving device. The total measurement signal consists of at least a first and a second single measurement signal, the individual measurement signals having different frequencies ω ₁ , ω ₂ .

Advantageously, the probe transmits the individual measurement signals simultaneously.

The individual measurement signals are generated in the probe by using one of the number of individual measurement signals in the probe

corresponding number of currents I ₁ , I ₂ , in particular

Alternating currents, is generated, wherein the currents

different frequencies ω ₁ , ω ₂ have. The amplitudes A ₁ , A _{2 of} the currents I ₁ , I ₂ and the currents associated frequencies ω ₁ , ω _{2 are} chosen such that the product of the current and the associated frequency is the same for all frequencies used.

The total measurement signal and individual measurement signals are electromagnetic fields.

In a first embodiment, the probe may be an active marker which independently emits the overall measurement signal.

Alternatively, the probe may be formed as a passive marker, wherein the total measurement signal emitted by the marker from a radiated from a transmitting device

electromagnetic signal is generated.

In a method according to the invention for reducing a disturbance voltage component caused by a disturbing body in a sensor coil in one of a probe

emitted total measured signal induced total voltage, it is assumed that the probe according to the above

described method is operated such that the

Overall measurement signal consists of at least a first and a second single measurement signal, wherein the individual measurement signals have different frequencies. In the method, the induced total voltage U _{ges is} measured, the measured total voltage U _tot is in the spectral components corresponding to the frequencies of the individual measurement signals

U _ges ^ _i ) 'U (ω ₂ ) decomposed and the interference voltage component U is excluded by subtraction of the spectral components.

In this case, for at least one of the frequencies ω ₁ , ω ₂ of

Single measurement signals the interference voltage free voltage amplitude U _M calculated according to

where k = 2, ..., F and where F is the number of different frequencies.

A probe according to the invention is characterized in that it can be operated according to the method described above.

An arrangement according to the invention for determining the position of a probe, which is operable in accordance with the method described above, has a receiving device with at least one sensor coil and a signal processing means, said signal processing means is formed to a determine störspannungsfreie clamping ^¬ voltage amplitude as described above. In summary, the invention thus relates to the location of a probe, in particular in a medical application, wherein the probe emits a measurement signal in the form of an electromagnetic alternating field ^{¬ which} induces voltages in a plurality of sensor coils of a receiving device, from which ultimately closed to the position of the probe can be. However, the light emitted from the probe electromagnetic Wech ^¬ selfeld can generate an eddy current in an electrically conductive interfering body in the vicinity of the probe, which in turn causes that in each sensor coil in addition egg ne disturbing voltage is induced that affects the position measurement. According to the invention, the probe sends a measuring signal consisting of at least two individual measurement signals with different union under ^¬ frequencies. The voltage induced in each sensor coil can be divided into the spectral components corresponding to the frequencies. By subtraction of the spectral components of a störspannungsfreie Spannungsamp ^¬ litude can be determined.

Further advantages, features and details of the invention will become apparent from the embodiment described below and with reference to the drawings.

Showing: 1 shows a measuring arrangement for implementing the method according to the invention, FIG. 2 shows an active marker,

FIG. 3 shows a passive marker.

In the figures, identical or corresponding areas, components, component groups or method steps are identified by the same reference numerals.

FIG. 1 shows a measuring arrangement for position measurement which operates according to the measurement principle (c) mentioned in the introduction, ie with an active marker. FIG. 1 shows the active marker 100 to be located, which emits an electromagnetic field EM, a disruptive body 200 in the form of an electrically conductive plate 200 and a sensor coil 310, which here represents a whole array of sensor coils. The active marker 100 has a coil 110, which is connected to a multi-frequency AC voltage source 120 and is energized by it. The sensor coil 310 or the array of sensor coils (not shown in detail) is part of a receiving device 300, which also has a signal processing device 320. A signal received by the sense coil signal is supplied and the Signalverarbeitungsein ^¬ device 320 further processed as described below. When the active marker 100 is in a transmit state, a current I _M = I _I + I ₂ flows in the marker coil 110 with I _j = A _j sin ((O _j t) and I ₂ = A ₂ sin (co ₂ t). the marker 100 operates according to the invention with two different (circular) frequencies ω ₁ and ω _2, which is not preferred multiples of einan- the are. When applies _ω ι <ω _2, so further preferred ω applies ₂ <2ω _ι. One possible choice for the to be used Kreisfre ^¬ quences would eg. ω _ι = 2π · 7 kHz and ω ₂ = 2π × 10 kHz. The amplitudes Ai and A ₂ are signed, ie they may in principle be arbitrary real-valued numbers. A limitation on the choice of amplitudes is given below.

The current I _M directly induces a voltage U _M in the sensor coil 310 or in each sensor coil of the array. The following applies where G is a ^¬ Geomet

Rie factor which depends on the design and number of turns of marker coil 110, the design and number of turns of the sensor coil 310 and the geometric position of the marker coil 110 and the sensor coil 310 relative to each other.

Furthermore, the electromagnetic field EM, which is generated by the marker coil 110 energized by the current I _M, generates an eddy current ϊ in the electrically conductive interfering body 200, which in turn induces an interference voltage U in the sensor coil 310. Overall, therefore, a voltage U _ges = U _M + U is induced in the sensor coil 310 due to the measurement signal emitted by the marker 100. The Störspan ^¬ voltage U has the effect that an error occurs in the context of position determination in an evaluation of the induced voltage in the sensor coil 310, which depends on the value of the interference voltage.

For the interference voltage U applies (without skin effect in the interfering body 200) where also here

G is a geometry factor that depends on the design and number of turns of the marker coil 110, the design and number of turns of the sensor coil 310, the design and material properties of the bluff body 200 and the relative geometric position between marker and bluff body 200 on the one hand and bluff body 200 and sensor coil 310 on the other hand depends. As already indicated, a real system has not only a single sensor coil, but a multiplicity of such coils. To select the number N of sensor coils 310 (in Fig. 1, n = l) is that N must be at least as large as the number to be measured of the Freiheitsgra ^¬ de markers of the marker 100. In a typical application of a Bone-, a catheter tip or an endoscopy capsule to be considered is a 5D Position of marker 100, ie three Cartesian coordinates and two angle coordinates. To measure the 5D position of such a marker N> 5 must be selected. Typically, however, N is chosen to be significantly larger, for example N = 24 or N = 32.

The N total measurable ^¬ voltage in the sensor coil i with i = 1, 2, ..., is given by. For the voltage ^¬

amplitudes of the two spectral lines at the two frequencies (O _j with j = 1, 2 applies accordingly.

The amplitudes A _1, A ₂ and the frequencies ω _1, ω ₂ are coordinated such that: A ₁ ^■ ω ₁ = A ₂ · ω _2: = c, where c represents only an auxiliary variable for explaining the method represents ^¬ that is not needed for the practical implementation. This condition also applies

, Accordingly, can be from the

Determining the difference between the two measured spectral voltages for each sensor coil i The geometry factor of the interference voltage U ^(l) :

The interference voltage-free voltage amplitudes thus result for the two frequencies ω ₁ and co ₂

One of these two relationships or a balancing bill between both relationships can be used to get out of the total voltages measured at the two frequencies

and to generate interference-free measuring voltages

and then calculate from these in a known manner the right Markerpo ^¬ sition and orientation. This occurs, for example, in the signal processing device 320 of the receiving device 300.

The invention can be realized with both an active and a passive marker. Figures 2 and 3 show two possible alternative forms of realization of the marker 100, where ^¬ at in Figure 2 as in Figure 1, an active marker 100 is shown, while the figure 3 is at the marker 100 to a passive marker having a from a transmitting device 400 irradiated electromagnetic signal S _paS s reflected.

The active marker of figure 2, in contrast to the acti ^¬ ven marker of Figure 1 on two separate circuits for the generation ^¬ supply two signals with different frequencies. The circuits are identically constructed and each have a monofrequent AC voltage source 120/1, 120/2 and a coil 110/1, 110/2, wherein the respective coil is energized by the corresponding AC voltage source. The coils 110/1 and 110/2 are in particular arranged close to one another or in a known distance and a known orientation to each other. When the active marker 100 of FIG. 2 is in a transmitting state, a current Ii flows in the marker coil 110/1 and a current ± 2 flows in the marker coil 110/2, and they are described as in connection with FIG generates two individual measuring signals with frequencies ω ₁ and co ₂ .

The active marker 100 in the embodiment of FIG. 2 operates analogously to the active marker of FIG. 1, ie the markers differ only in the way in which the individual measurement signals are generated, while those introduced above Conditions with respect to the frequencies ω ₁ and ω ₂ and the amplitudes A ₁ , A _{2 of} the currents I ₁ , I _{2 are} maintained.

The same applies for the marker of Figure 3. This differs from the active markers 100 of Figures 1 and 2, characterized in that the individual measuring signals generated not active, son ^¬ countries passively from an irradiated by the transmitting device 400 electromagnetic signal S _pass to be generated. For this purpose, the passive marker 100p has two circuits, each having a capacitor 130/1, 130/2 and a coil

110/1, 110/2 include. Again, the coils are 110/1 and 110/2 in particular arranged close to each other or in a known distance and a known orientation to each other. The resulting from this circuit frequencies ω ₁ and ω _{2 of} the individual measurement signals are calculated according to

Of course, the described method can be extended for both the active and the passive marker to the effect that more than two marker frequencies are used. The voltages at the additional frequencies are used in the same way as described above, and the results are combined in compensatory calculations. On the one hand, this leads to improved accuracy. On the other hand, an increased computational effort, a higher complexity of the marker hardware and more complex analog pre-filtering of the sensor coil signals in purchase to neh ^¬ men. In the present invention, frequencies in a range of 1 ... 100 kHz, preferably around 10 kHz are used. The phasing plays - with the exception of the sign - no Rol ^¬ le, that is, only (signed) considered amplitude values.

Claims

claims

1. A method for operating a probe to be located (100), wherein the probe (100) for locating a total measurement signal emits, which is detectable by a receiving device (200),

characterized in that

the total measuring signal consists of at least one first and one second individual measuring signal, the individual measuring signals having different frequencies (ω ₁ , ω ₂ ).

2. The method according to claim 1, characterized in that the probe (100) the individual measuring signals simultaneously

sends out.

3. The method according to any one of the preceding claims, characterized in that the individual measurement signals in the probe (100) are generated by a number of the individual measurement signals in the probe (100) corresponding number of

Currents (I ₁ , I ₂ ), in particular alternating currents, is generated, the currents having the different frequencies (ω ₁ , ω ₂ ).

4. The method according to claim 3, characterized in that the amplitudes (A ₁ , A ₂ ) of the currents (I ₁ , I ₂ ) and the currents (I ₁ , I ₂ ) associated frequencies (ω ₁ , ω ₂ ) in such a way are chosen such that the product of current (I ₁ , I ₂ ) and associated frequency (ω ₁ , ω ₂ ) for all frequencies used (ω ₁ , ω ₂ ) is the same.

5. The method according to any one of the preceding claims, characterized in that the total measurement signal and individual measurement signals are electromagnetic fields.

6. The method according to any one of the preceding claims, characterized in that the probe is an active marker, which automatically emits the total measurement signal.

7. The method according to any one of claims 1 to 5, characterized in that the probe (100) is a passive marker (100p), wherein the from the marker (100p) emitted total measurement signal from one of a transmitting device (400) irradiated electromagnetic signal (S _pass ) is generated.

8. A method for reducing a disturbing body (200) caused interference voltage component (U) in one in a

Sensor coil (310) due to a total voltage signal (U _ges = U _M + U) induced by a probe (100), the probe (100) being operated in such a way according to a method according to one of Claims 1 to 7, in that the total measuring signal consists of at least one first and one second individual measuring signal, the individual measuring signals having different frequencies (ω ₁ , ω ₂ ), in which

the induced total voltage (U _tot ) is measured,

- The measured total voltage (U _ges ) in the spectral components corresponding to the frequencies of the individual measurement signals

( ^u ges ( ^ω ι) ' ^u ges ( ^ω ₂ )) is decomposed and

- The interference voltage component (U) is subtracted by subtraction of the spectral components.

9. The method according to claim 8, characterized in that for at least one (ω ₁ ) of the frequencies (ω ₁ , ω ₂ ) of the individual measurement signals, the interference voltage free voltage amplitude (U _M ) is calculated according to

where k = 2, ..., F and where F is the number of different frequencies.

10. probe (100) operable according to an operating method according to one of claims 1 to 7.

11. Arrangement for determining the position of a probe (100), which according to a method according to one of claims 1 to 7 is operated, comprising a receiving device (300) with at least one sensor coil (310) and a Signalverarbei ^¬ processing device (320), wherein the signal processing means (320) is designed to carry out a method according to one of claims 8 or 9.