Abstract:
A method of localizing a device, medical or otherwise, within a three dimensional environment, the method comprising: (a) transmitting time varying magnetic fields from at least three transmitters, (b) receiving the transmitted electromagnetic radiation as induced voltage signals from at least one receiver mounted on or within said device, and (c) a processing scheme for processing said received voltage signals in order to extract position and orientation localization information for said device, said processing scheme including correction for conducting materials in the vicinity by the use of information gathered from at least three distinct transmission frequencies for each of said transmitters.

Description:
FIELD OF THE INVENTION 
   This invention relates to localization, (the determination of position and/or orientation) of an instrument within a three dimensional region in the presence of external scatterers of electromagnetic fields, and in particular to the localization of a medical device in a patient&#39;s body. 
   BACKGROUND OF THE INVENTION 
   In the context of medical procedures, it is often necessary to determine the position and/or orientation of a medical device within a patient&#39;s body. For example to safely and accurately navigate the distal tip of a catheter through the body, it is desirable to know the position and orientation of the catheter. Standard imaging methods such as x-ray, CT, or MRI may not provide adequate information for use in navigation. Similarly, methods such as image reconstruction and processing, electrical potential measurements, ultrasound and electromagnetic measurements have been used for this purpose. They typically suffer from a lack of sufficient accuracy due to inherent methodological problems, inhomogeneities in the environment, or interference from the environment. 
   A convenient method for determining position and orientation of a medical device employs the transmission of time-dependent electromagnetic signals between transmitters at a known location and receivers, or vice versa. In the typical situation, a plurality of transmitters are disposed in fixed, known locations, each transmitting at a difference frequency and one or more sensors on the medical device inside the body. From an analysis of the signals received by the receiver coils, sufficient information may be obtained to determine the desired position and/or orientation of the sensor, and thus the medical device relative to known position and orientation of the transmitters. Various examples of such magnetic localization techniques include U.S. Pat. Nos. 5,694,945, 5,846,198, 5,738,096, 5,713,946, 5,833,608, 5,568,809, 5,840,025, 5,729,129, 5,718,241, 5,727,553, 5,391,199, 5,443,489, 5,558,091, 5,480,422, 5,546,951, 5,752,513, 6,092,928, 5,391,199, 5,840,025, U.S. patent application Ser. No. 09/809523, filed Mar. 15, 2001, and published Nov. 29, 2001, as No. 20010045826, and PCT Application No. PC/US01/08389, filed Mar. 16, 2001, and published Sep. 20, 2001, as WO 01/69594 A1, and PCT/GB/01429, published Nov. 16, 2000, as WO 00/68637, the disclosures of all of which are incorporated herein by reference. 
   The use of time-dependent electromagnetic fields for localization purposes suffers from a significant drawback—in the presence of external electromagnetic scatterers, such as metals, in the environment that can support induced currents, backscatter from these metals can alter the signals received by the receiver coils and lead to significant loss of accuracy in determination of the position and/or orientation. In cases where the external environment is changing, such as moving metals, these changes in signal are not predictable in advance. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of magnetically localizing a device in the presence of external scatterers such as metals. Generally the method of localization of this invention involves transmitting, receiving, and processing magnetic signals so that changing external metallic environments can be accounted for in a refinably accurate manner. In the preferred embodiment, the method employs a plurality of transmitters (e.g. coils) generating electromagnetic signals with at least three distinct frequencies placed at known fixed locations outside a patient&#39;s body, and a plurality of sensors (e.g. coils) placed on or within a device or instrument, such as a medical device inserted into a patient&#39;s body. 
   Furthermore, it is necessary that each transmit coil be able to transmit at at least three distinct frequencies. By having each coil transmit at a larger number of distinct frequencies, the accuracy of the localization can be refined successively to the extent desired by suitable processing. The sensors may be any magnetic field sensors such as coils, Hall probes and other devices known to those skilled in the art. The signal processing discards pieces of the signal that arise from external scattering, leaving behind a “true” or desired signal that is, by the use of an increasing number of transmit frequencies, an increasingly accurate approximation to the signal that would be received in a scatter-free environment. This process is convergent and fast, even in a “worst case” situation where the receiver coil and the scatterer are geometrically maximally coupled. Furthermore, the entire process can be carried out in real time, meaning that as long as angular velocities associated with movements of conductors in the environment are significantly smaller than the circular frequencies of the electromagnetic radiation used, changes in environment such as moving metallic equipment are always automatically accounted for. The analysis also provides design methods whereby the accuracy can be further improved. 
   Thus, the method of this invention provides for accurate and fast localization by electromagnetic means of devices, including but not limited to medical devices, within a three dimensional region such as within a patient&#39;s body, which is not significantly affected by the presence of metal near the region of interest. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of the general geometry for the case of a medical device (whose position and orientation at the tip is desired) inserted into a patient; 
       FIG. 2  is a schematic diagram showing the geometry of the “worst case” analysis of the scattering problem; 
       FIG. 3  is a process flow diagram for the preferred embodiment of this invention; and 
       FIG. 4  is a system diagram of a system that implements the method of the invention. 
   

   Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The localization method described herein is based on the use of multiple transmitters (e.g., coils), at least three in number, each fixed at a known spatial location and capable of transmitting electromagnetic radiation at at least three distinct frequencies. Preferably the transmission method uses driving currents in each coil with the time dependence of the currents being sinusoidal at each of the distinct frequencies. 
   One or more receivers (such as coils or other magnetic field sensor such as Hall probes) is located within or on the device whose localization is desired. While receiver coils are described in this preferred embodiment, it should be understood that this invention includes the use of other magnetic field sensors. An example of a medical device where such coils could be used is a catheter with receiving coils mounted at or near the catheter tip. Due to the time dependence of the transmitted magnetic field, the changing magnetic flux in the receiver coils produces a measurable induced voltage. In the ideal case where external scatterers are absent, localization information may be obtained from an analysis or suitable processing of the induced voltages in the receiver coils. In general external conductors cause interfering signals due to induced currents which produce significant deviations from the ideal case. The general geometry is as shown in  FIG. 1 , where the transmitter coils  119 ,  120  and  121  produce changing magnetic fields received by receiver coils  211 ,  212  and  213 , mounted within a catheter  311  introduced into a patient&#39;s body  314 . The presence of an external conductor such as  320  produces interfering signals in the receiver coils  211 ,  212  and  213 . 
     FIG. 2  illustrates the “worst case” geometry for the receiver coil and external scatterer where the respective magnetic fluxes are maximally linked. As shown in  FIG. 2 , external scatterer  401  and receiver coil  402  both have circular geometries and share a common axis  403  (referred to herein as the z-axis) so that the planes defined by the respective circles are parallel. The coils are separated by a distance d. The external scatterer  401  has radius r 1  and the receiver coil  402  has radius r 2 . A spatially uniform time-dependent magnetic field along the z-direction, B(t)=B 0  e jωt , is incident upon the entire geometry. Because other relative orientations and positions of the receiver coil and external scatterer with respect to each other and with respect to the incident field lead to suppression of flux linkage by various trigonometric factors smaller than unity in magnitude, the geometry considered in  FIG. 2  is indeed a “worst case” geometry. 
   As a result of the time-dependent magnetic field, voltages and currents are induced in both receiver coil and external scatterer. The presence of receiver coil and external scatterer in the incident magnetic field scatters the incident field due to induced currents. One way to account for the effect of scattering is as follows. The (time-dependent) induced currents in receiver coil and external scatterer further produce time-dependent secondary magnetic fields which may be considered to act upon each other in an iterative manner. This iterative process is referred to as the Born expansion in the literature (see, for example, J. D. Jackson, Classical Electrodynamics, incorporated herein by reference). For the worst-case geometry shown in  FIG. 2 , this iterative process can be actually followed through and will be shown to have useful consequences. If A 1 =πr 1   2  and A 2 =πr 2   2  are the cross-sectional areas of scatterer and receiver coil respectively, the primary induced voltage in the receiver coil is determined by Faraday&#39;s law to be (apart from the complex time-dependent exponential factor)
 
 V   2   0   =−B   0   A   2   jω   (1)
 
Likewise, the induced voltage in the scatterer is V 1   0 =−B 0 A 1 jω. If R 1  and R 2  are the resistances of scatterer and receiver coil respectively, the induced current in the scatterer is I 1   0 =V 1   0 /R 1 . For the present analysis we will assume that inductive effects are small relative to resistive effects; later this assumption will be relaxed. The magnetic field at the center of the receiver coil produced by the current I 1   0  is
 
 B   1   1 =μ 0   I   1   0   r   1   2 /2( d   2   +r   1   2 ) 3/2   (2)
 
where μ 0  is the standard free space magnetic permeability. The field elsewhere in the area defined by the receiver coil is smaller, and so the induced voltage V 2   1  in the receiver coil due to the current I 1   0  in the scatterer is
 
 V   2   1   =−B   1   A   2   jω   (3)
 
with B 1  bounded in magnitude by |B 1   1 |. Correspondingly there is a current I 2   1 =V 2   1 /R 2  in the receiver coil. Likewise there is a similar correction I 1   1  to the current in the scatterer, and so on.
 
   By a careful examination of this iterative process, it can be shown that the induced voltage in the receiver coil can be expressed as a power series in the frequency:
 
 V   2   =V   2   0   +V   2   1   +V   2   2   +V   2   3   +. . . =−jωB   0   A   2 [ P − Q ](1− jβ )  (4)
 
where
 
β=πωμ 0   r   1   4 /2 R   1 ( d   2   +r   1   2 ) 3/2   (5)
 
and P and Q can both be expanded as power series in frequency. In fact, P and Q are bounded from above by P 1 =1+(α 2 +α 4 +α 6 + . . . ) and Q 1 =(α+α 3 +α 5 + . . . ) with
 
α=(πωμ 0 ) 2    r   1   4   r   2   4 /[4 R   1   R   2 ( d   2   +r   1   2 ) 3/2 ( d   2   +r   2   2 ) 3/2 ]  (6)
 
Note that α and β are dimensionless parameters. If α&lt;1, P and Q are bounded and finite and so is the series expansion, equation (4), for the induced voltage. The induced voltage in the receiver in the presence of external scattering effects can be expanded in a power series in the frequency as given by equation (4). The coefficients in such an expansion in general depend on the details of the scattering environment.
 
   It is important to note that in the absence of external scatterers, only the first term V 2   0  in the expansion (4) above survives, and it is linear in the frequency. This is the primary signal, and non-linear (in frequency) modifications to it arise from scattering effects. 
   In medical device applications where precise localization is essential, d˜10 cm, r 1 ˜1 m, r 2 ˜2 cm, R 1 ˜1 ohm, R 2 ˜0.1 ohm are typical representative values. At frequencies of the order of 10 kHz, it can be seen from equation (6) above that α˜0.015. The series approximation converges rapidly in such cases. At such kilohertz frequencies, low order polynomials can provide excellent approximations to the scattered signal. The linear term (in frequency) in such an approximation then is directly the primary signal and is free of external scattering effects. It must be noted also that the primary voltage signal from the receivers is out of phase with the currents in the transmitter generating the incident radiation. 
   Low order polynomial (in frequency) fits to the signal may be obtained by a standard method such as least square fitting to the signal obtained at as many distinct frequencies as the order of the desired polynomial fit. The linear term that the polynomial fitting yields is the filtered primary signal. Furthermore, the fit can be refined and improved by collecting the signal at a larger number of distinct frequencies and correspondingly using successively higher order polynomials as required. 
   While the analysis above applied to the case of thin loops and neglected inductive effects, a more accurate analysis including inductive effects may be performed in a more general case to obtain the induced currents in continuous metal due to time-varying magnetic fields. In this case the induced eddy current density J on the metal may be computed in idealized but representative cases and may be shown to have a convergent expansion in powers of frequency. 
   Consider the case of a circular metal sheet of radius R and thickness δ, with the metal resistivity being ρ. We will suppose for convenience that the thickness δ is smaller than the skin depth of the metal over the range of frequencies being considered; if not the skin depth (proportional to the square-root of the frequency) should be used in place of δ. If the metal sheet is placed in a sinusoidally time varying and spatially uniform magnetic field perpendicular to the sheet, with amplitude B 0  and frequency ω, the induced eddy current density in the sheet depends only on radial distance r from the center of the sheet. The induced eddy currents are circumferential and it can be shown that the current density has a magnitude J(r) given by
 
 J ( r )=(ε B   0   r/ 2)[1+(εμ 0 /2)( R/ 2− r (⅓+ 1/24+ . . . ))+ O ((εμ 0 /2) 2 )+ . . . ]  (7)
 
where ε=−jωδ/ρ. In particular, it may be shown that this expansion in powers of ε (and thus frequency ω) is convergent. The general scattering signal picked up by the receiver coil then involves sums of products of convergent expansions in powers of frequency and is itself convergent for cases of interest such as those arising in medical applications. The primary scattered signal picked up by the receiver coil is determined by the real part of J in equation (7) and has an expansion in integral powers of frequency.
 
   We now describe a preferred scheme for generating, acquiring and processing electromagnetic signals so as to eliminate scattering effects based on the discussion above. A programmable signal generator is used to generate sinusoidal currents at as many distinct frequencies as the order of the polynomial fit desired. If there are n transmitters, and m (m greater than or equal to n) frequencies, the m frequencies are ordered in magnitude and applied to the transmitters n at a time in circularly permuted fashion, so that each of the m frequencies has been transmitted by each of the n transmitters exactly once. Ordering of frequencies by magnitude is used here only for purposes of discussion. As an example, if there are 3 transmitters A, B and C, and four ordered frequencies (f 1 , f 2 , f 3 , f 4 ), the transmission signal is applied to (A,B,C) in the sequence (f 1 ,f 2 ,f 3 )-(f 2 ,f 3 ,f 4 )-(f 3 ,f 4 ,f 1 )-(f 4 ,f 1 ,f 2 ). The frequencies are chosen to be integrally related such that they are all multiples of a base frequency f 0  so that the signal can be frequency multiplexed and easily demodulated upon reception. With such a scheme the signal generator simultaneously uses a superposition or batch of n frequencies at a time, and the m batches are time multiplexed in an entire signal generation cycle. Although this is a preferred scheme for reasons of optimality and efficiency, other signal generation schemes employing signal generation at multiple frequencies would also work, and it is possible to use non-optimal schemes of signal generation and acquisition as well. As an example of a variation on our basic scheme, in the presence of a larger number of transmitters it may be sometimes desirable to use a further level of time multiplexing within each batch of n frequencies. 
   The signals from the receiver from each batch of frequency multiplexed signals can be easily demodulated given the choice of integral relationships among the frequencies mentioned above. A microprocessor-based controller is used to keep track of the frequency and signal obtained from each of the n transmitters in each of the m batches of multiplexed frequencies. At the end of the m batches, the received signal for each of the m frequencies is available for each of the n transmitters.  FIG. 3  shows a process flow chart illustrating the method. 
   Referring to  FIG. 3 , in step  501  the receiver gain is calibrated as a function of frequency to account for any frequency-dependent gain changes in the signal reception circuitry. In this step the scaling between received voltage signal and measured flux amplitude is also calibrated at fixed frequencies using known fluxes. Additionally the transmitter drive currents are calibrated as a function of frequency. Although it is most convenient to hold the transmitter currents fixed as frequency varies, in some cases it may be advantageous to allow the transmitter currents to vary with frequency, and in this case it is necessary to calibrate this variation. The signal generation step  502  produces in time multiplexed fashion m frequency multiplexed batches of n frequencies as sinusoidal superpositions. For each of the m batches of transmitted signal, the received signal is phase shifted by π/2 (since the primary signal we are interested in is out of phase with the transmitted signal) and demodulated in step  503  in order to separate the signals received from distinct transmitters. This information is labeled and stored in step  504 . In step  505 , the calibration performed in step  501  is used to rescale the signals at various frequencies stored in step  504 , if appropriate and the rescaled signals are stored. In step  506 , an m-th order polynomial fit is used to determine the coefficients in a polynomial expansion (in powers of frequency) of the rescaled signals obtained in step  505 . In step  507 , the coefficient c 1  of the linear term obtained from step  506  is used (so c 1 ω is the free-space or metal-free signal) together with the scaling between signal and flux to obtain the flux from each transmitter. It is to be noted that steps  504  through  507  may also be performed on a computer connected to the microprocessor. 
   If there is more than one receiver, steps  501  through  507  are performed for each receiver. It is optimal to perform this series of steps as a parallel process for the various received signals from each receiver although this could be performed in serial fashion. From the various fluxes it is possible to determine the required position and orientation of the device by a variety of computational methods not addressed herein. 
     FIG. 4  shows a system diagram for a preferred embodiment of the invention. A programmable interface preferably on a computer  601  is used to program the order of the polynomial fit desired. A microprocessor-based controller  602  uses this information to pick a sequence of transmission frequencies. The signal generator  603  applies the desired currents in frequency multiplexed batches to the transmitters  604  in a time multiplexed fashion as outlined in  FIG. 3  and explained in the description above. The signals from the receivers  605  are phase shifted by a phase shifter  606  and demodulated by the signal demodulator  607 . The signals are stored in the microprocessor-based controller  602  and buffered to the computer  601  together with associated frequency information and receiver labels. Previously stored calibrations in the computer  601  account for variations in signal gain with frequency and a suitable correction is applied to the buffered signals to obtain rescaled signals. The computer  601  then performs, for each receiver, a polynomial fit at the chosen order of fit and extracts the coefficient of the linear term in the polynomial expansion corresponding to the transmission from each transmissitter. This set of coefficients is rescaled based on information previously stored in the computer  601  in order to obtain a corresponding set of fluxes. The set of fluxes is suitably processed in the computer  601  to obtain position and orientation information which is available as system output. 
   The above system description is a preferred embodiment and variations depending for instance on convenience of application are possible as may be familiar to those skilled in the art. For example, it may sometimes be desirable to perform more of the information processing within the microprocessor-based controller. 
   The range of frequencies employed may range from 100 Hz to 40,000 Hz and more preferably from 500 Hz to 15,000 Hz. From equation (6), it may be seen that a method to improve the accuracy of the fit and lower the order of polynomial required is provided by increasing receiver resistance (R 2 ). The use of higher resistance metal elements or alloys (as compared to commonly used copper conductors) in the receiver construction is particularly indicated. So also is receiver size or total area of flux-capturing cross section, although this trades off with lower reception voltage levels.