Abstract:
A system for and method of determining and compensating for the effect of a field influencing object on a field sensor, preferably a coil, that is within a navigational domain. The navigational domain contains navigational magnetic energy and disturbing magnetic energy, and the field influencing object produces the disturbing magnetic energy in response to the navigational magnetic energy. The correction system includes a first transmitter for projecting into the navigational domain field energy in a first waveform sufficient to induce a first signal value in the sensing coil. The system also includes a second transmitter for projecting into the navigational domain field energy in a second waveform sufficient to induce a second signal value in the sensing coil. The system further includes a signal processor for receiving the first signal value and for receiving the second signal value to determine the effect of the electrically conductive object on the field sensor. In other embodiments, the correction system may utilize a second sensing coil to receive and process an alternate aspect of the field energy from a position that is different from the first field sensor, or the system may utilize a storage device that contains information relating to the effect of the field influencing device within the navigational domain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of United States Provisional Application Number 60/161,991, filed Oct. 28, 1999, the contents of which are incorporated herein by reference in their entirety, and from which priority is claimed. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
         [0002]    Not Applicable  
         REFERENCE TO MICROFICHE APPENDIX  
         [0003]    Not Applicable  
         CONCURRENTLY FILED APPLICATIONS  
         [0004]    The following United States patent applications, which were concurrently filed with this one on Oct. 28, 1999, are fully incorporated herein by reference: Patient-shielding and Coil System, by Michael Martinelli, Paul Kessman and Brad Jascob; Navigation Information Overlay onto Ultrasound Imagery, by Paul Kessman, Troy Holsing and Jason Trobaugh; Coil Structures and Methods for Generating Magnetic Fields, by Brad Jascob, Paul Kessman and Michael Martinelli; Registration of Human Anatomy Integrated for Electromagnetic Localization, by Mark W. Hunter and Paul Kessman; System for Translation of Electromagnetic and Optical Localization Systems, by Mark W. Hunter and Paul Kessman; Surgical Communication and Power System, by Mark W. Hunter, Paul Kessman and Brad Jascob; and Surgical Sensor, by Mark W. Hunter, Sheri McCoid and Paul Kessman.  
         BACKGROUND OF INVENTION  
         [0005]    The present invention relates to a navigation system for medical devices based on the use of magnetic fields. More particularly, this invention relates to a method and system for determining the position and orientation of a catheter probe being used during a surgical procedure in the presence of extraneous objects that may introduce extraneous magnetic fields.  
           [0006]    Systems and methods for determining the position and orientation of surgical probes based on the use of magnetic fields are known. See, for example, U.S. Pat. No. 5,592,939, herein incorporated by reference. Such systems and methods generally rely on the ability to solve a known equation for a known field strength in order to obtain the unknown position and orientation variables. Although the focus here is a rigid catheter probe of known length, width and depth, one skilled in the art will appreciate that the techniques discussed here are equally applicable to other types of probes; for example, the techniques discussed here may be adapted to the use of a flexible probe.  
           [0007]    In general, if the position in three-dimensional space of two points within the rigid probe is known, then the position and orientation of the probe itself is known. Each unknown point P in space corresponds to three unknown variables as shown in FIG. 1. These variables can be what are commonly called x, y, and z in a Cartesian system as is shown in FIG. 1, or they can be the variables r, θ, and φ as are commonly used in spherical coordinates also as shown in FIG. 1. Two unknown points in space correspond to 6 unknown variables. However, when the two points are on a rigid catheter, and when the separation of the two points is known, one unknown variable is removed. Thus, the position and orientation of a rigid catheter probe in three-dimensions is a function of 5 unknown variables. These variables can be expressed in a form that utilizes both Cartesian and spherical coordinates. For example, the position of sensor coil  14  can be defined by three Cartesian coordinates x, y, and z as is shown in FIG. 2, and the orientation of sensor coil  14  along coil axis  21  is defined by the variables θ and φ, also as shown in FIG. 2.  
           [0008]    In order to solve for 5 unknown quantities, one typically requires 5 known linearly independent equations. One can obtain known equations that are linearly independent of each other by exposing a detector located on the catheter in an unknown position and orientation to known independent navigation fields. Thus, to obtain 5 known linearly independent equations requires a sampling of at least 5 known independent navigation fields. Nevertheless, current systems that utilize magnetic fields in order to determine position and orientation frequently sample more than 5 independent fields. See, for example, U.S. Pat. No. 5,592,939, herein incorporated by reference. One of the reasons for sampling more than 5 independent fields is to perform a self-consistency check. Ideally, every sampling above 5 should provide such a system with redundant information regarding position and orientation. However, in an operating room in practice, every sampling of a known navigation field beyond 5 yields slightly different results as to the position or orientation of the catheter probe. One of the reasons for this is the nearby presence extraneous objects that are conductive or ferromagnetic. Such objects respond to the known navigation field and introduce errors that can be large depending on the nature and relative position of the object.  
           [0009]    For example, a conducting object within the area of influence of the known navigation field can develop what is known as an eddy current in response to the known navigation field. These eddy currents can, in turn, generate an extraneous magnetic field of unknown strength and orientation in the vicinity of the catheter. Depending upon the size of the object, this effect can be large.  
           [0010]    In addition, an object with a ferromagnetic core will act to focus magnetic field flux lines through the core and thus distort the known navigation field, again, in an unknown manner. Often, objects with ferromagnetic and conductive cores are used in surgical settings, such as tools used to drill, ream and tap holes in the vertebrae of a patient.  
           [0011]    In light of the foregoing, it is desirable to account for the effects of conducting objects that introduce eddy currents in response to a known navigation field.  
           [0012]    It is further desirable-to account for the effects of objects with ferromagnetic and conductive cores that introduce fluctuations in a known navigation field and that are often moved about near the periphery of the navigation field, such as surgical tools.  
           [0013]    It is further desirable to account for the effects of objects that introduce arbitrary fluctuations in a known navigation field.  
         SUMMARY OF THE INVENTION  
         [0014]    The foregoing and other objects are achieved by the invention which in one aspect comprises a correction system for determining the effect of an interfering object on a field sensor within a navigational domain. The system includes a first transmitter configured to project, into the navigational domain, field energy in a first waveform sufficient to induce a first signal value in the field sensor, where the first signal value is influenced by the interfering object. The system further includes a second transmitter configured to project, into the navigational domain, field energy in a second waveform sufficient to induce a second signal value in the field sensor, where the second signal value is also influenced by the interfering object. The system further includes a signal processor configured to receive the first signal value and the second signal value, and to determine the influences of the interfering object on the field sensor, to thereby permit a substantially precise location of the field sensor to be determined despite the presence of the interfering object.  
           [0015]    In another embodiment of the invention, the field energy is magnetic field energy.  
           [0016]    In another embodiment of the invention, the interfering object is an electrically conductive object.  
           [0017]    In another embodiment of the invention, the field sensor includes an electrically conductive sensing coil.  
           [0018]    In another embodiment of the invention, the first waveform is a sinusoidal waveform at a first frequency, and the second waveform is a sinusoidal waveform at a second frequency.  
           [0019]    In another embodiment of the invention, the first transmitter and the second transmitter include three unidirectional coil sets, where each set is driven by a drive unit capable of driving the unidirectional coil set at the first frequency and at the second frequency. Further, the first and second transmitters include six delta coil sets, where each the set is driven by a drive unit capable of driving the delta coil set at the first frequency and the second frequency. In this embodiment, the three unidirectional coil sets and the six delta coils sets produce the field energy at the first and second frequencies.  
           [0020]    In another embodiment of the invention, the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction. In this embodiment, the x, y and z directions are substantially mutually orthogonal.  
           [0021]    In another embodiment of the invention, the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform. In this embodiment, the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.  
           [0022]    In another embodiment of the invention, the second unidirectional coil set has a first coil element and a second coil element disposed within a platform. The coil elements are spaced apart and substantially parallel to one another.  
           [0023]    In another embodiment of the invention, the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform. In this embodiment, the first lateral wall and the second lateral wall are substantially normal to the major surface, and substantially parallel to one another.  
           [0024]    In another embodiment of the invention, the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements. The coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.  
           [0025]    In another embodiment of the invention, each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface. In this embodiment, for each of the pairs of coils, a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.  
           [0026]    In another embodiment of the invention, each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.  
           [0027]    In another embodiment of the invention, the signal processor includes a first sequencer configured to sequentially activate each of the three unidirectional coil sets and the six delta coil sets at the first frequency, and to measure the first signal value corresponding to each of the unidirectional and delta coil sets at the first frequency. The first sequencer further sequentially activates each of the three unidirectional coil sets and the six delta coil sets at the second frequency, and measures the second signal value corresponding to each of the unidirectional and delta coil sets at the second frequency. The signal processor further includes a processor configured to calculate, for each of the unidirectional and delta coil sets, and adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to field energy from one of the unidirectional coil sets and the delta coil sets.  
           [0028]    Another embodiment of the invention further includes a third transmitter configured to project into the navigational domain a third waveform sufficient to induce a third signal value in the field coil, where the third signal value is influenced by the interfering object. In this embodiment, the signal processor is further configured to receive the third signal value.  
           [0029]    Another embodiment of the invention further includes a fourth transmitter configured to project into the navigational domain a fourth waveform sufficient to induce a fourth signal value ins aid field coil, the fourth signal value being influenced by the interfering object. In this embodiment, the signal processor is further configured to receive the fourth signal value.  
           [0030]    Another embodiment of the invention further includes N-4 transmitters configured to project into the navigational domain N waveforms sufficient to induce N signal values in the field coil. The N signal values are influenced by the interfering object. The signal processor is further configured to receive N signal values.  
           [0031]    In another aspect, the invention comprises a correction system for determining an effect of an interfering object on first and second field sensors in a navigational domain. The system includes a transmitter configured to project into the navigational domain, field energy sufficient to induce a first signal value in the first field sensor, and to induce a second signal value in the second field sensor. The system further includes a signal processor configured to receive the first signal value and the second signal value, and to determine the effect of the interfering object on the first field sensor, to thereby permit a substantially precise location of the first field sensor to be determined despite the presence of the interfering object.  
           [0032]    In another embodiment of the invention, the field energy is magnetic field energy.  
           [0033]    In another embodiment of the invention, the interfering object is a ferromagnetic and electrically conductive object.  
           [0034]    In another embodiment of the invention, the field sensor includes an electrically conductive sensing coil.  
           [0035]    In another embodiment of the invention, the transmitter includes three unidirectional coil sets. Each unidirectional coil set is driven by a unit capable of driving the unidirectional coil set at a first sinusoidal waveform at a first frequency.  
           [0036]    The transmitter further includes six delta coil sets, each of which is driven by a drive unit capable of driving the delta coil set at the first sinusoidal waveform at the first frequency, such that the three unidirectional coil sets and the six delta coil sets produce the field energy at the first frequency.  
           [0037]    In another embodiment of the invention, the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction, such that the x, y and z directions are substantially mutually orthogonal.  
           [0038]    In another embodiment of the invention, the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform. The first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.  
           [0039]    In another embodiment of the invention, the second unidirectional coil set has a first coil element and a second coil element disposed within a platform, and the coil elements are spaced apart and substantially parallel to one another.  
           [0040]    In another embodiment of the invention, the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform. The first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.  
           [0041]    In another embodiment of the invention, the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements. The coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.  
           [0042]    In another embodiment of the invention, each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface. For each of the pairs of coils, a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.  
           [0043]    In another embodiment of the invention, each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.  
           [0044]    In another embodiment of the invention, the signal processor further includes a first sequencer configured to sequentially activate each of three unidirectional coil sets and six delta coil sets at the first frequency, and to measure the first signal value and the second signal value corresponding to each of the unidirectional and delta coil sets at the first frequency. The signal processor also includes a processor configured to calculate, for each of the unidirectional and delta coil sets, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to field energy from one of the unidirectional coil sets and the delta coil sets.  
           [0045]    In another aspect, the invention comprises a correction system for determining an effect of a field influencing shield device on a field sensor in a navigational domain. The correction system includes a transmitter configured to project into the navigational domain field energy sufficient to induce a signal value in the field sensor.  
           [0046]    The correction system further includes a storage device containing information corresponding to the fields energy in the navigational domain at selected locations within the navigational domain. The information includes shield information incorporating the effect of the field influencing shield device at the selected locations.  
           [0047]    The correction system further includes a processor for accessing the storage device and the signal value to determine the effect of the shield device on the field sensor, to thereby permit a substantially precise location of the field sensor to be determined despite the presence of the field influencing shield device.  
           [0048]    In another embodiment of the invention, the field energy is magnetic field energy.  
           [0049]    In another embodiment of the invention, the field sensor includes an electrically conductive sensing coil.  
           [0050]    In another embodiment of the invention, the transmitter includes three unidirectional coil sets, each unidirectional coil set being driven by a unit capable of driving the unidirectional coil set at a first sinusoidal waveform at a first frequency. The transmitter further includes six delta coil sets, each the delta coil set being driven by a drive unit capable of driving the delta coil set at the first sinusoidal waveform at the first frequency, such that the three unidirectional coil sets and the six delta coil sets produce the field energy at the first frequency.  
           [0051]    In another embodiment of the invention, the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction, such that the x, y and z directions are substantially mutually orthogonal.  
           [0052]    In another embodiment of the invention, the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform, wherein the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.  
           [0053]    In another embodiment of the invention, the second unidirectional coil set having a first coil element and a second coil element disposed within a platform, the coil elements being spaced apart and substantially parallel to one another.  
           [0054]    In another embodiment of the invention, the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element. The first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform, wherein the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.  
           [0055]    In another embodiment of the invention, the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements. The coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.  
           [0056]    In another embodiment of the invention, each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface. For each of the pairs of coils, a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.  
           [0057]    In another embodiment of the invention, each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.  
           [0058]    In another embodiment of the invention, the processor further includes a first sequencer for sequentially activating each of the three unidirectional coils and the six delta coils at the first frequency, and measuring the signal value corresponding to each of the unidirectional and delta coils at the first frequency. The processor also includes a data manipulating device for manipulating, for each of the unidirectional and delta coils, the storage means as a predetermined function of the shield device, so as to produce nine sets of manipulated magnetic field values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils.  
           [0059]    Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field interfering object. The method includes inducing within the field sensor a first signal value at a first waveform, the first signal value being influenced by the field interfering object. The method further includes inducing within the field sensor a second signal value at a second waveform, the second signal value being influenced by the field interfering object. The method also includes determining a correction to the first signal value for the effects of the field interfering object.  
           [0060]    In another embodiment of the invention, determining a correction further includes calculating an adjusted signal value as a predetermined function of the first signal value and the second signal value.  
           [0061]    Another aspect of the invention comprises a method of determining a substantially precise location of a first field sensor within a navigational domain influenced by a field interfering object. The method includes inducing within the first field sensor a first signal value, the first signal value being influenced by the field interfering object. The method further includes inducing within a second field sensor a second signal value, the second signal value being influenced by the field interfering object. The method also includes determining a correction to the first signal value for the effects of the field interfering object.  
           [0062]    In another embodiment of the invention, determining a correction further includes calculating an adjusted signal value as a predetermined function of the first signal value and the second signal value.  
           [0063]    Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field influencing shield device. The method includes inducing within the field sensor a first signal value, the first signal value being influenced by the field interfering object. The method further includes accessing information from a storage device, the information including shield information incorporating the effect of the field influencing shield device at selected locations. The method further includes determining a correction to the first signal value for the effects of the field influencing shield device.  
           [0064]    In another embodiment of the invention, determining a correction further including manipulating the storage device as a predetermined function of the shield information, so as to produce a set of manipulated magnetic field values corresponding to the effects of the field influencing shield device.  
           [0065]    In another aspect, the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field interfering object. The method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, navigational energy at a first frequency, and measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values. The method further includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a second frequency, and measuring a second signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the second signal values. The method further includes calculating, for each of the unidirectional and delta coils, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils. The method also includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the sensing coil, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil. The method also includes generating three lines and determining an intersection of the three lines. The intersection corresponds to the compensated position of the sensing coil. Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation.  
           [0066]    In another aspect, the invention comprises a method of determining a substantially precise location of a first field sensor within a navigational domain influenced by a field interfering object. The method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a first frequency, measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values, and measuring a second signal value in a second field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the second signal values. The method further includes calculating, for each of the unidirectional and delta coils, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils. The method also includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the sensing coil, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil. The method also includes generating three lines and determining an intersection of the three lines, the intersection corresponding to the compensated position of the sensing coil. Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation.  
           [0067]    Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field influencing shield device. The method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a first frequency, and measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values. The method further includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to fields from each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the field sensor, the predetermined navigational energy at a first frequency, measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values, and measuring a second signal value in a second field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the second signal values. The method further includes calculating, for each of the unidirectional and delta coils, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils. The method also includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the sensing coil, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil. The method also includes generating three lines and determining an intersection of the three lines, the intersection corresponding to the compensated position of the sensing coil. Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation.  
           [0068]    Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field influencing shield device. The method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a first frequency, and measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values. The method further includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to fields from each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the field sensor, the predetermined field magnitude values being manipulated so as to account for the shield device, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil. The method also includes generating three lines and determining an intersection of the three lines, the intersection corresponding to the compensated position of the sensing coil. Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation, the predetermined field magnitude values being manipulated so as to account for the effect of the shield device. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0069]    The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more filly understood from the following description, when read together with the accompanying drawings in which:  
         [0070]    [0070]FIG. 1 is a graphical view of prior art methods of identifying a general point in three dimensional space;  
         [0071]    [0071]FIG. 2 is a graphical view of a prior art method of identifying the position and orientation of a sensing coil in three dimensional space;  
         [0072]    [0072]FIG. 3 is a perspective view of an examination deck used for implementing the location and compensation methods according to the present invention;  
         [0073]    [0073]FIG. 4 shows a schematic representation of a unidirectional coil set for generating uniform x-directed fields in the navigational domain;  
         [0074]    [0074]FIG. 5 shows a schematic representation of a unidirectional coil set for generating uniform y-directed fields in the navigational domain;  
         [0075]    [0075]FIG. 6 shows a schematic representation of a unidirectional coil set for generating uniform z-directed fields in the navigational domain;  
         [0076]    [0076]FIG. 7 shows a schematic representation of a coil configuration for generating vector gradient fields in the navigational domain;  
         [0077]    [0077]FIG. 8 shows a schematic representation of the coil configuration of FIG. 7, including constant signal surfaces generated by those coils;  
         [0078]    [0078]FIG. 9 shows an upper plan schematic view of a delta coil group from FIG. 7 relative to an inner circular space representing the projection of navigational domain into the plane of the delta coils;  
         [0079]    [0079]FIG. 10 depicts an example of a tool coil attached to a surgical tool used to drill holes in the vertebrae of a patient, as well as an exemplary hysteresis graph of such a ferromagnetic tool.  
         [0080]    [0080]FIG. 11 depicts an example of the use of the surgical tool of FIG. 10 with an attached tool coil.  
         [0081]    [0081]FIG. 12 illustrates a perspective view of an examination deck suitable for implementing the location and compensation methods in the presence of a shield device, according to the present invention;  
         [0082]    [0082]FIG. 13 shows a flow diagram for a method of eddy current compensation in accordance with the present invention;  
         [0083]    [0083]FIG. 14 shows a flow diagram for a method of ferromagnetic and conductor compensation in accordance with the present invention;  
         [0084]    [0084]FIG. 15 shows a flow diagram for a method of shield device compensation in accordance with the present invention;  
         [0085]    [0085]FIG. 16 shows a more detailed flow diagram for the orientation calculation step of FIG. 13;  
         [0086]    [0086]FIG. 17 shows a more detailed flow diagram for the access step of FIG. 13;  
         [0087]    [0087]FIG. 18 shows a more detailed flow diagram for the orientation calculation step of FIG. 15;  
         [0088]    [0088]FIG. 19 shows a more detailed flow diagram for the position calculation step of FIG. 15;  
         [0089]    [0089]FIG. 20A shows a perspective view of the disposition of the nine coil sets of the examination deck;  
         [0090]    [0090]FIG. 20B shows a schematic view of the coil sets of FIG. 20A;  
         [0091]    [0091]FIG. 21A shows a perspective view of an exemplary disturbance which is in the form of a metallic ring, along with the equivalent circuit model;  
         [0092]    [0092]FIG. 21B shows a perspective view of an elliptical slab of metal including an off center circular hole, along with the equivalent circuit model; and,  
         [0093]    [0093]FIG. 21C shows a perspective view of a solid square metal plate, along with the equivalent circuit model. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0094]    The present invention is directed to a system and method for determining the position and orientation of a catheter or other suitable probe inserted into a selected body cavity of a patient undergoing a surgical procedure in the presence of field-influencing objects.  
         [0095]    In one embodiment of the present invention, position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil affixed to the distal end of the catheter probe as a result of the use of multiple waveforms. These voltage amplitudes, as a function of the waveforms, are induced in the sensing coil in response to known independent electromagnetic fields that are projected into the anatomical region of interest. The measurements provide information to compute the angular orientation and the positional coordinates of the sensing coil and account for the distortion of the known field by arbitrary conductors with field-induced eddy currents.  
         [0096]    In another embodiment of the present invention, position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil and a tool coil. The sensing coil is affixed to the distal end of the catheter probe. The tool coil is affixed to a field-influencing object with a ferromagnetic and conducting core. The field-influencing object, in the presence of a known electromagnetic field, distorts that field and influences the measurement of the sensing coil. The voltage amplitudes from both coils are stored and can be mathematically manipulated so as to isolate the effect of the field-influencing object on the sensing coil. Thus, the measurements of the induced voltage amplitudes on the sensing coil and on the tool coil provide information to account for the presence of the field-influencing object with a ferromagnetic and conductor core.  
         [0097]    In another embodiment of the present invention, the position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil affixed to the distal end of the catheter probe in the presence of a shield device. These voltage amplitudes are induced in the sensing coil in response to two fields. One of the fields is the known independent electromagnetic field projected into the anatomical region of interest from field coils. The other field is that of the known field as reflected from the shield device. The measurements of the induced voltage amplitudes and the knowledge of the geometry and effect of the shield device provide sufficient information to compute the angular orientation and the positional coordinates of this sensing coil in the presence of the shield device.  
         [0098]    Definitions As used herein, “sensing coil” refers to an electrically conductive, magnetically sensitive element that is responsive to time-dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time-dependent magnetic field. The sensing coil is adaptable for secure engagement to the distal end of a catheter probe.  
         [0099]    As used herein, “tool coil” refers to an electrically conductive, magnetically sensitive element that is responsive to time-dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time-dependent magnetic field. The tool coil is adaptable for secure engagement to an object with a ferromagnetic and conducting core.  
         [0100]    As used herein, “navigational domain” refers to a fully enclosed spatial region whose internal volume substantially encloses the complete prospective range of movement of the sensing coil. The navigational domain may be defined by any geometrical space, but preferably takes the form of a spherical volume. Under surgical operating conditions, the navigational domain will correspond to an anatomical region of the recumbent patient where surgical viewing or investigation is desired (e.g., a diseased area of tissue or an organ).  
         [0101]    As used herein, “peripheral domain” refers to the spatial region outside of the navigational domain. Under surgical operating conditions, the peripheral domain may include the region that contains the operating table, or the region that encompasses other equipment within the operating room.  
         [0102]    As used herein, “last navigational point” (hereinafter “the LNP”) refers to the most recently determined location of the sensing coil before another iteration of the location algorithm is performed.  
         [0103]    As used herein, “uniform amplitude field” refers to a magnetic field having a large magnetic field amplitude component in a specified direction and relatively smaller magnetic field amplitude components in the other directions. The uniform amplitude field is characterized by substantially uniform field amplitude values, throughout the navigational domain. In the x-y-z coordinate system used herein, where the uniform amplitude fields of interest are the x-directed, y-directed, and z-directed fields, the amplitudes of the induced voltage drops developed by such fields in the sensing coil are designated with superscripts V X , V Y , and V Z , respectively.  
         [0104]    As used herein, “waveform” refers to the temporal shape of a magnetic field, illustrated graphically by a plot of the magnitude of a magnetic field as a function of time. A waveform in general can take on the characteristics of any form. For example, a waveform may be sinusoidal with angular frequency ω, which has the general form exp(−iωt)  
           where                 i     =       (     -   1     )         ,   ω                         
 
         [0105]    is the angular frequency, t is the time, exp(n) denotes the natural base of logarithms e raised to the power n. A waveform can also be sawtooth in nature, or square in nature.  
         [0106]    As used herein, “unidirectional coils” refer to a magnetic assembly that is operative to generate a uniform amplitude field (as defined above) within the navigational domain. A distinct magnetic assembly is employed for each uniform amplitude field. Although the unidirectional coils described herein are preferably implemented with a collection of appropriately designed magnetic coils, this implementation should not be construed as a limitation of the present invention. Rather, the unidirectional coils may be constructed from any magnetic configuration that is sufficient to generate the uniform amplitude fields.  
         [0107]    As used herein, “vector gradient field” refers to a time-dependent magnetic field having nonzero vector gradient field components (i.e., magnetic field vector components with a high spatial gradient) in two of the three magnetic field components, and a substantially zero vector gradient field component in the remaining magnetic field component in an appropriately chosen coordinate system. For example, if the appropriately chosen coordinate system is an x-y-z coordinate system at a position R in the navigational domain, then the magnetic field amplitude H n  (R)(a vector field) can be written as:  
           H   n ( R )=( H   x   n ( R ),  H   y   n(   R ),  H   z   n ( R ))  
         [0108]    where the components H x   n (R), H y   n (R), and H z   n (R) represent the magnetic field amplitude strengths of the nth coil (designated by the superscript “n”) in the x-direction, y-direction, and z-direction, respectively and are individually scalar quantities. The value of a vector gradient of such a magnetic field amplitude H n (R) where the magnetic field has a substantially zero vector gradient component in the z-direction can be written as the following vector gradient (or tensor) field:  
         {right arrow over (Δ)} H   n ( R )=({right arrow over (Δ)} H   x   n ( R ), {right arrow over ()} H   y   n ( R ), 0), where  
         {right arrow over (Δ)} H   x   n ( R )≠0, and  
         {right arrow over (Δ)} H   y   n ( R )≠0  
         [0109]    and where the gradient operator {right arrow over (Δ)} has the usual representation in x-y-z coordinates:  
         ∇   →          =     (       ∂     ∂   x       ,     ∂     ∂   y       ,     ∂     ∂   z         )                             
 
         [0110]    In practical settings, a substantially zero vector gradient component is generated when the magnitude of the substantially zero vector gradient component value is small compared to the magnitude of the net vector resulting from the other two magnetic field components.  
         [0111]    As used herein, “fixed orientation” with respect to a catheter probe refers to a catheter probe with constant values of orientation variables θ and φ, over a selected range of x, y, and z positional values.  
         [0112]    As used herein, “constant signal surface” or “constant voltage surface” refers to a surface contour along which at every possible point of location for the sensing coil, the same induced voltage is developed in the sensing coil. In practice, the constant signal surface will be a small planar region located near the LNP with the sensing coil at a fixed orientation (as defined above).  
         [0113]    As used herein, “delta coil” refers to a magnetic assembly for generating a vector gradient field (as defined above) within the navigational domain. As will become more apparent hereinafter, the delta coil will typically be described in the context of delta coil pairs including a long delta coil and a short delta coil, each pair generating vector gradient fields with the substantially zero component in the same-axial dimension but whose magnetic field patterns in the remaining components are independent of each other. Each of the long and short delta coils may be considered to generate a family of constant signal or constant voltage surfaces for the sensing coil within the navigational domain. Although the delta coils are preferably implemented with an array of appropriately designed magnetic coils (discussed below), this preferred implementation should not serve as a limitation of the present invention as it should be apparent to those skilled in the art that other magnetic configurations may be used to adequately generate the vector gradient fields.  
         [0114]    As used herein, “magnetic look-up-table” (alternatively referenced as “the LUT”) refers to a database including the magnetic field amplitude values at every x-y-z coordinate position within the navigational domain for the unidirectional coils and for each delta coil used by the present invention. Accordingly, input data consisting of an x-y-z coordinate and a magnetic field amplitude identifier, which designates a selected magnetic coil assembly, is indexed within the database to a corresponding set of magnetic field amplitude values constituting the output data. For the x-y-z coordinate system, the output data is represented by the magnetic field amplitude variables H x   n H y   n H z   n  where the subscript x-y-z indicates the axial dimension along which the magnetic field amplitude value is being reported and the superscript is the identifier for a selected magnetic coil assembly acting as the source. The database is created through a computational analysis of the magnetic field amplitude patterns generated by the magnetic coil configurations used herein. The mathematical model to develop the necessary formulae defining the field patterns may be developed, for example, from near field electromagnetic theory. An instructive text for facilitating such an analysis is “Field and Wave Electromagnetics” 2nd edition Addison Wesley (1989) by D. K. Cheng, herein incorporated by reference. The database may be stored in any type of facility including, inter alia, read-only memory, firmware, optical storage, or other types of computer storage. Additionally, the database information may be organized into any type of format such as a spreadsheet. It should be apparent to those skilled in the art that any suitable technique may be used to ascertain or record the magnetic field values for the magnetic coil assemblies used herein.  
         [0115]    In the coordinate system for describing the present invention, the z-axis coincides with the longitudinal dimension extending from the patient&#39;s head to foot. The x-axis coincides with a lateral dimension across the patient&#39;s body, and the y-axis is perpendicular to the planar top of the pallet or examination deck. These dimensions are identified as the patient is disposed in the recumbent position on the pallet.  
         [0116]    Assembly for Implementing Location and Compensation Methods  
         [0117]    Magnetic Assembly for Determining Angular Orientation of Sensing Coil  
         [0118]    [0118]FIG. 3 schematically illustrates a perspective view of examination deck  17  that facilitates implementation of the location and compensation methods in accordance with all preferred embodiments of the present invention. Examination deck  17  employs a magnetic coil assembly arranged in a flat configuration. Examination deck  17  includes planar top platform  10  suitable for accommodating a recumbent patient disposed lengthwise. Navigational domain is illustratively depicted as the spherical volume enclosing sensing coil  14 . Tool coil  19  is located in peripheral domain  15 . Sensing coil  14  is attached via connection means  16  to an external signal detection apparatus (not shown). Tool coil  19  is likewise attached to an external signal detection apparatus. Although sensing coil  14  functions optimally within navigational domain  12 , tool coil  19  may lie within peripheral domain  15  or navigational domain  12 . The coil sets embedded in platform  10  (and described in connection with FIGS.  4 - 7 ) are activated by a signal drive unit (not shown) connected via line  18 . Examination deck  17  is preferably constructed from a suitable magnetically-permeable material to facilitate magnetic coupling between the embedded coil sets and the overlying sensing coil.  
         [0119]    First conducting body  23  and second conducting body  31  are shown in FIG. 3 in peripheral domain  15 . However, both first conducting body  23  and second conducting body  31  can lie within navigational domain  12  as well in accordance with a preferred embodiment of the present invention. First conducting body  23  and second conducting body  31  respond to the fields generated by the field coils by developing eddy currents. These eddy currents, in turn, generate new fields that can influence the measured voltage across sensor coil  14  as is discussed in more detail below.  
         [0120]    Ferromagnetic body  29  is shown in peripheral domain  15  and is shown enveloped within tool coil  19 . Again, ferromagnetic body  29  and tool coil  19  can lie within navigational domain  12  as well in accordance with a preferred embodiment of the present invention. Ferromagnetic body  29  responds to the fields generated by the field coils by both focusing the magnetic flux lines and by introducing a phase shifted field. The focusing and phase shifting effect of ferromagnetic body  29  can influence the voltage drop as measured across sensor coil  14  as is discussed in more detail below.  
         [0121]    Coil Sets for Generating x-directed. y-directed, and z-directed Fields  
         [0122]    FIGS.  4 - 6  schematically illustrate unidirectional coil sets for generating substantially uniform amplitude x-directed, y-directed and z-directed fields, respectively, throughout navigational domain  12 . The uniform amplitude field in one embodiment of the present invention can be operated at multiple waveforms, for example, a sinusoidal waveform with angular frequency ω 1  and a sinusoidal waveform with angular frequency ω 2  are two different waveforms.  
         [0123]    Unidirectional coil set  25  of FIG. 4 includes a first coil pair with elements  20  and  24  and a second coil pair with elements  22  and  26 , where the current flow as supplied by drive unit  28  is indicated by the arrow symbol. Coil elements  20  and  22  are disposed in the major surface of platform  10 , while elements  24  and  26  are disposed in the lateral walls of platform  10 . Elements  24  and  26  are preferably used as compensation coils to substantially cancel undesirable field components generated by elements  20  and  22  in the y and z directions. The coils cumulatively generate a substantially uniform amplitude x-directed field as indicated by representative field line  27 .  
         [0124]    Unidirectional coil set  35  of FIG. 5 schematically illustrates a coil set for generating a substantially uniform amplitude y-directed field throughout navigational domain  12  as indicated by representative field line  33 . The coil set includes a coil pair with elements  30  and  32  disposed in spaced-apart and parallel relationship within platform  10 , with the indicated current flow as supplied by drive unit  34 .  
         [0125]    Unidirectional coil set  45  of FIG. 6 generates a substantially uniform amplitude z-directed field as indicated by representative field line  43 . Coil set  45  includes a first coil pair with elements  38  and  42  and a second coil pair with elements  36  and  40 , where the current flow as supplied by drive unit  44  is indicated by the arrow symbol. Coil elements  36  and  38  are disposed in the major surface of platform  10 , while elements  40  and  42  are disposed in the lateral walls of platform  10 . Elements  40  and  42  are preferably used as compensation coils to substantially cancel undesirable field components generated by elements  36  and  38  in they direction.  
         [0126]    Unidirectional coil sets  25 ,  35 , and  45  are illustrative only and should not be construed as a limitation of the present invention. It should be apparent to those skilled in the art that other coil configurations are possible within the scope of the present invention provided such other configurations produce the desired magnetic field patterns in navigational domain  12  at the desired frequency. A first connection means (not shown) couples sensing coil  14  to a signal measuring device, and a second connection means (also not shown) couples tool coil  19  to a signal measuring device.  
         [0127]    Coil Sets for Generating Vector Gradient Fields  
         [0128]    [0128]FIGS. 7 and 8 show a coil configuration that can be used to determine the positional coordinates of sensing coil  14  in accordance with all of the preferred embodiments of the present invention. The configuration includes six coils grouped into three pairs of long and short delta coils ( 50  and  52 ,  54  and  56 ,  58  and  60 ). The delta coils are mutually coplanar and are disposed in the planar top of the examination deck immediately beneath the recumbent patient. Interconnection means between delta coil group  51 , delta coil group  55 , delta coil group  59  and a signal drive unit (not shown) is indicated.  
         [0129]    The coils are preferably arranged in a circular orientation about the y-axis such that there is an axis perpendicular to the direction of elongation of the coils, proceeding from the long coil set to the short coil set, where that axis is at 0°, 120° and 240° to the z-axis. By way of reference, the x-axis is considered to be oriented at 270° with respect to the z-axis in the x-z plane. The magnetic field generated by delta coil group  51  is shown representatively by the field lines extending from the upper region of the coils. In one embodiment of the present invention, the resulting vector gradient field can be operated at multiple waveforms. For example, a sinusoidal waveform with angular frequency ω 1  and a sinusoidal waveform with angular frequency ω 2  are two different waveforms. The field lines from delta coil group  51  form the family of constant signal surfaces shown in FIG. 8 within the navigational domain  12 . Superposition of the constant signal surfaces generated by long coil set  50  and short coil set  52  produce the bounded regions indicated in FIG. 8. The intersection of two such constant signal surfaces generated by short coil set  52  and long coil set  50  is line  70 .  
         [0130]    [0130]FIG. 9 shows an upper plan schematic view of delta coil group  51  relative to an inner circular space representing the projection of navigational domain  12  into the plane of the delta coils. This design creates a high spatial gradience in two of the axis dimensions and a substantially zero field value in the remaining axial dimension. This particular design is accomplished by modifying the termination points of the coils with compensation coils such that the modified coil is effectively operative as an infinitely long coil. Long coil sets  50 ,  54 , and  58  are further compensated by central “sucker” coils, indicated in FIG. 9 for long coil set  50  as central “sucker” coil  88 . Accordingly, each of the long coils and short coils is modified by representative compensation coils  80  and  82 ,  84  and  86 ,  88 ,  90  and  94 , and  92  and  96  respectively, disposed at the indicated endpoints and center of the corresponding delta coil. The long coil and short coil configurations are shown schematically for only delta coil group  51 , but similar configurations likewise exist for delta coil group  55  and delta coil group  59 , shown representatively as the indicated lines.  
         [0131]    Parameters related to the quality of the coils, such as (i) the degree of uniformity of the uniform amplitude field coils and (ii) how close to zero the vector gradient field is in the non-gradient direction for the delta coils, determine the size of navigational domain  12  and peripheral domain  15 .  
         [0132]    Tool Coil System for Ferromagnetic and Conductor Compensation  
         [0133]    [0133]FIG. 10 indicates tool coil  19  affixed to surgical tool  108 , where surgical tool  108  is a ferromagnetic and conducting object. Also shown is exemplary hysteresis graph  100  of surgical tool  108 . Hysteresis graph  100  illustrates the nonlinear behavior of a magnetic field H associated with surgical tool  108  in response to an applied magnetic field B. Such a response is typical of ferromagnetic objects.  
         [0134]    [0134]FIG. 11 indicates how surgical tool  108  and tool coil  19  of FIG. 10 may be used in practice. Surgical tool  108  is used to drill holes into vertebrae  110 .  
         [0135]    Shield Device  
         [0136]    [0136]FIG. 12 schematically illustrates a perspective view of examination deck  17  that facilitates implementation of the location and compensation algorithms in accordance with shield device  120 . Examination deck  17  includes planar top platform  10  suitable for accommodating a recumbent patient disposed lengthwise. Examination deck  17  rests on base  122 . Navigational domain  12  is illustratively depicted as the spherical volume enclosing a sensing coil  14  attached via connection means  16  to an external signal detection apparatus (not shown). The coil sets embedded in platform  10  (and described in connection with FIGS.  4 - 7 ) are activated by a signal drive unit (not shown) connected via line  18 . The examination deck is preferably constructed from a suitable magnetically permeable material to facilitate magnetic coupling between the embedded coil sets and the overlying sensing coil. Shield device  120  can be made from aluminum, copper or virtually any other conductive material. It is also possible to use materials other than a conductive sheet such as a mesh or strips of material. A further possibility is to use a plastic of polymer film with a conductive coating.  
         [0137]    Overview of Compensation Methods  
         [0138]    System and Method for Extraneous Eddy Current Compensation  
         [0139]    [0139]FIG. 13 indicates a schematic of a method for eddy current compensation consistent with the present invention. Measuring steps  132  and  134  involve activating each of nine coil sets and measuring the field at sensing coil  14  at different waveforms. For example, a sinusoidal waveform with angular frequency ω 1  and a sinusoidal waveform with angular frequency ω 2  correspond to two different waveforms.  
         [0140]    The nine coil sets correspond to three unidirectional coil sets and three delta coil groups, where each delta coil group contains a long coil set and a short coil set. The unidirectional coil sets-generate uniform amplitude fields in the x, y, and z-directions and are depicted in FIGS. 4, 5, and  6 . From FIG. 7, delta coil group  25  includes short coil set  52  at 0° (denoted by n=1(s)) and a long coil set  54  at 0° (denoted by n=2(L)); delta coil group  55  includes short coil set  56  at 120° denoted by n=2(s)) and long coil set  54  at 120° (denoted by n=2(L)); and delta coil group  59  includes—short coil set  60  at 240° (denoted by n=3(s)) and long coil set  58  at 240° (denoted by n=3(L)). The angular designations associated with the delta coil groups indicate the angle with respect to the z-axis of the coil dimension that is perpendicular to the direction of elongation of the delta coils as in FIG. 7. Accordingly, the three delta coil groups are arranged pair-wise in a circular orientation about the y-axis at angles of 0°, 120°, and 240°.  
         [0141]    As described above, a series of fields are generated and measured in sensing coil  14  at a first waveform (measuring step  132 ) and at an mth waveform (measuring step  134 ). For example, considering two waveforms where the waveforms correspond to substantially uniform amplitude fields with sinusoidal waveforms and angular frequencies ω 1  and ω 2  and considering x-directed fields of coil set  25  of FIG. 4 with measured values of  
           V   x          (     ω   1     )       ,       V   x          (     ω   2     )                             
 
         [0142]    in sensing coil  14 , the value of a constant τ eddy  defined by:  
         τ   eddy     =       Re        {           V   x          (     ω   2     )         ω   2       -         V   x          (     ω   1     )         ω   1         }         Im        {         V   x          (     ω   1     )       -       V   x          (     ω   2     )         }                               
 
         [0143]    is calculated where “Re” indicates the real part (0 radians phase shifted part), and “Im” indicates the imaginary part (τ/2 radians phase shifted part) of the terms enclosed in the brackets. This corresponds to a portion of calculation step  138  of FIG.  13  in one embodiment of the present invention and will be discussed in more detail below.  
         [0144]    Finally, an adjusted value for the potential drop  
       V   adjst   x                         
 
         [0145]    is calculated using τ eddy . Again, using the above example of substantially uniform amplitude fields with sinusoidal waveforms and angular frequencies ω 1  and ω 2 , the value of  
       V   adjst   x                         
 
         [0146]    is:  
         V   adjst   x     =       Re        {     V   x     }       +     ω                   τ   eddy        I                 m          {     V   x     }     .                               
 
         [0147]    This completes calculation step  138  of FIG. 13 in one embodiment of the present invention. Again, this will be discussed in more detail below.  
         [0148]    In another embodiment of the present invention, measurements are performed at four different waveforms (measuring steps  132  and  134  in FIG. 13). For example, considering a magnetic field waveform that is sinusoidal in nature with angular frequency ω, measurements at four frequencies ω 1 , ω 2 , ω 3 , and ω 4  correspond to measuring steps  132  and  134  at four different waveforms.  
         [0149]    Using the measured values of the real and imaginary portions of the potential drop across sensing coil  14  yields the following four equations:  
         Re        {     V   n     }       =       V   adjst   n     -     ω                 Im        {     V   n     }          (       τ   *     +     ω                   τ   **         )                               
 
         [0150]    with three unknowns: τ*, τ**, and  
         V   adjust   n     .                         
 
         [0151]    Thus, the value of  
       V   adjust   n                         
 
         [0152]    can be determined using singular value decomposition as is described in more detail below. Again, this corresponds to calculation step  138  in FIG. 13 in one embodiment of the present invention.  
         [0153]    At query step  140 , it is determined whether all unidirectional field coils have been activated. In the example here, only the x-directed coils have been activated (coil set  25  of FIG. 4). Thus, a corresponding set of substantially uniform amplitude fields with different waveforms are generated and measured at sensing coil  14  in the y-direction by coil set  35  of FIG. 5, and then likewise in the z-direction by coil set  45  of FIG. 6, with the appropriate calculations.  
         [0154]    Next, at query step  142 , it is determined whether all of the delta coil sets have been activated. As above, each delta coil set is activated in succession the induced voltage is measured in sensing coil  14  at different waveforms. Again as above, the measurement of the voltage drops at the two waveforms allow for the calculation of an adjusted voltage drop across sensing coil  14 . V adjst  is the signal that would have been picked up by sensor coil  14  if the conductive body disturbance had not been present.  
         [0155]    Following the calculation of the adjusted voltage drop for all nine coils, an orientation calculation is performed in step  144  to determine the values of the sensing coil  14  orientation variables φ and θ, independent of the unknown sensing coil  14  positional variables (x, y, z). A more detailed breakdown of orientation calculation  144  is shown in FIG. 16 and is discussed in more detail below.  
         [0156]    Finally, in step  146  of FIG. 13, a position calculation is performed to obtain the positional variables (x, y, z) of sensing coil  14 . A more detailed breakdown of position calculation  146  is shown in FIG. 17. In FIG. 17, access step  176  indicates that orientation calculation  144  is preferably performed before positional calculation  146 . Next, in access step  178 , the LUT is accessed to obtain the magnetic field values at the LNP for a long delta coil set and a short delta coil set. These magnetic field values and the as-computed values for the orientation angles θ and φ are substituted into the appropriate induced voltage equations to calculate for each delta coil the value of the voltage amplitude signal induced in the sensing coil at the LNP. Based on the difference between the measured and the LNP values for the induced voltage signals, a calculation is performed in step  180  that permits identification of a line ℑ n  on which sensing coil  14  lies, as described in more detail below. In query step  182 , it is determined whether such a line ℑ n  has been identified for each delta coil group. Following the identification of three lines ℑ 1 , ℑ 2 , ℑ 3  calculation step  184  is performed to determine where in space the lines ℑ 1 , ℑ 2 , ℑ 3  intersect. That position in space is the location (x, y, z) of sensing coil  14 .  
         [0157]    System and Method for Ferromagnetic and Conductive Object Compensation  
         [0158]    [0158]FIG. 14 indicates a schematic for ferromagnetic and conductor compensation consistent with the present invention. Measuring step  152  is similar to measuring step  132  as described above for FIG. 13. Measuring step  154  is different, however, in that a measurement is performed across tool coil  19 .  
         [0159]    As above, the nine coil sets correspond to three unidirectional coil sets and three delta coil groups, where each delta coil group contains a long coil set and a short coil set. The unidirectional coil sets generate uniform amplitude fields in the x, y, and z-directions and are depicted in FIGS. 4, 5, and  6 . The delta coil sets are depicted in FIG. 7 and described above.  
         [0160]    In particular, a series of substantially uniform amplitude fields are detected by sensing coil  14  (measuring step  152 ) and tool coil  19  (measuring step  154 ) originating from unidirectional coil set  25  of FIG. 4, unidirectional coil set  35  of FIG. 5, and unidirectional coil set  45  of FIG. 6. Also, as described in conjunction with FIG. 13, each delta coil is activated in succession and a corresponding induced voltage is measured in sensing coil  14  (measuring step  152 ) and tool coil  19  (measuring step  154 ).  
         [0161]    Again, as discussed in more detail below, the measured voltage drops allow for the calculation of an adjusted voltage drop across sensing coil  14 , the voltage that would be present if there were no disturbance. The first step, indicated in calculation step  156 , is to calculate the value of a constant K where  
         Im        {       V   n     -     KV   ferro   n       }       =   0                         
 
         [0162]    where “Im” indicates the imaginary part (τ/2 radians phase shifted part) as above. In calculation step  160 , the value of  
       V   adjst   n                         
 
         [0163]    is calculated using K  
         V   adjst   n     =       V   n     -     KV   ferro   n                             
 
         [0164]    This is step  160  of FIG. 14 and is described in more detail below.  
         [0165]    The remaining steps indicated in FIG. 14 have already been discussed above in the context of FIG. 13 with the same reference numbers.  
         [0166]    System and Method for Shield Device Compensation  
         [0167]    [0167]FIG. 15 indicates a schematic of a method for shield device compensation consistent with the present invention. Again, a magnetic assembly of nine individual coil sets and shield device  120  are used to generate magnetic fields sufficient to develop a corresponding set of nine induced voltage signals at sensing coil  14 . Measurement step  162  indicates such a measurement. The major difference associated with shield device compensation is observed in calculation steps  164  and  166 . Rather than the standard orientation calculation step  144  of FIGS. 13 and 14, and presented in more detail in FIG. 16, orientation calculation step  164  of FIG. 15 is shown in more detail in FIG. 18. Additionally, rather than the standard position calculation step  146  of FIGS. 13 and 14, and presented in more detail in FIG. 17, position calculation step  166  of FIG. 15 is shown in more detail in FIG. 19. In FIG. 18, an additional manipulation step  188  is incorporated as compared to the schematic of FIG. 16. Likewise, in FIG. 19, an additional manipulation step  192  is incorporated, as compared to the schematic of FIG. 17. The additional manipulation step associated with the shield device compensation method involves the manipulation of the values of the LUT. Specifically, knowledge of the geometry of the coil sets and shield device  120  is sufficient to allow manipulation of the LUT in order to account for the effect of shield device  120  within navigational domain  12 . The effects of arbitrary field-influencing objects that lie in peripheral domain  15  and anterior to shield device  120  are thereby cancelled.  
         [0168]    In particular, the LUT consists of a database containing the magnetic field amplitude values H x   n (R), H y   n (R), and H z   n (R) at every x-y-z coordinate location within the navigational domain for five coil sets: the unidirectional coil sets for generating the uniform amplitude fields in the x, y, and z-directions denoted by n=x, y, or z; the short coil (SC) set at 0° denoted by n=1(s); and the long coil (LC) set at 0° denoted by n=1(L). The magnetic field value data for the short and long coil sets at 120° and 240° may be obtained from the LUT by rotating the field vectors for the long and short coil sets at 0° (n=(n=1(s)and n=1(L)) by the angle (i.e., ±120°) appropriate for the given coil set. The input data for the LUT consists of the x-y-z coordinates and a designation of which coil set is being used to generate the magnetic fields (the superscript “n”). In response to this input data, the LUT supplies the magnetic field amplitude values H x   n (R), H y   n (R), and H z   n (R) at the selected x-y-z coordinates for the designated coil set. Note that in the previously discussed preferred embodiments of the present invention, the LUT can only be successfully utilized after compensation for field-influencing objects has occurred. However, in the preferred embodiment of the present invention, compensation for shield device  120  may be incorporated into the LUT.  
         [0169]    The LUT is present to speed up the operational sequence of the location algorithm. Otherwise, an undesirable computational delay exists if the required magnetic fields from the nine coil sets must be individually calculated during each iteration of the algorithm. By predetermining the magnetic field values and storing them in LUT, the location algorithm need only access the LUT to retrieve the appropriate field value without endeavoring into any complex field analysis. This is especially true when shield device compensation is an issue. At x-y-z coordinates other than those for which magnetic field values are determined in the LUT, an interpolation procedure is employed to calculate the field value.  
         [0170]    Detailed Description of Method for Determining Unknown Position Coordinates by Sampling a Known Navigation Field  
         [0171]    The system and method used herein is directed to the development of a series of measurements and calculations able to account for the effects of a field-influencing object that would otherwise introduce error into a position and orientation determination. The relationships defined by these systems and methods are such that the unknown effects of the field-influencing object are separable.  
         [0172]    Determining the Angular Orientation of a Probe in the Absence of Extraneous Fields  
         [0173]    This section describes in more detail the orientation calculation as indicated by the schematic of FIG. 16. The angular orientation of sensor coil  14  is represented by an angle θ corresponding to the angle of departure from the z-axis and an angle φ corresponding to the angle between the x-axis and the projection onto the x-y plane of the vector coincident with the longitudinal axis of sensing coil  14  as shown in FIG. 2.  
         [0174]    The unidirectional coils are activated in succession, each generating a substantially uniform amplitude field that projects into navigational domain  12  and induces a corresponding voltage signal in sensing coil  14 .  
         [0175]    In access step  170  of FIG. 16, the LUT is then accessed three times to acquire the magnetic field values at the LNP for each of the three unidirectional coils. In calculation step  172 , these values and the measured voltage signals are then substituted into the appropriate equations set forth below to solve for the unknown variables θ and φ that define the orientation of sensing coil  14 .  
         [0176]    By way of background, the time-dependent magnetic fields projected into the navigational domain induce voltages in sensor coil  14  that are representative of the orientation of coil axis a  21  relative to the lines of magnetic flux. The development of an induced voltage in sensing coil  14  in response to a changing magnetic field is defined by Faraday&#39;s law. For a closed stationary path in space which is linked by a changing magnetic field, the induced voltage as a function of time V(t) around this path is equal to the negative time rate of change of the total magnetic flux through the closed path (one turn). For an open surface S bounded by a closed path C, the magnetic flux ψ through S is given by, 
         [0177]    [0177]         ψ   n     =     N          ∫   S            μ   0              H   n          (   t   )       ·     a   ^               a                                   
         [0178]    where μ 0  is the magnetic permeability of free space and is a constant, the superscript “n” simply associates a given flux ψ n  with a given field H n (t), â is a vector normal to surface S, and da is a differential unit of area, and N is the number of turns in the coil  14 . The mathematical statement of Faraday&#39;s law is, thus,  
           V   n          (   t   )       =       -             t                         ψ   n                             
 
         [0179]    Within the time period of a measurement of sensing coil  14 , neither the surface S, or the closed path C determined by the position of sensing coil  14  are a function of time. Thus,  
           V   n          (   t   )       =       -   N            ∫   S            μ   0            ∂       H   n          (   t   )           ∂   t          •                   a   ^                        a                                 
 
         [0180]    For an N-turn coil of wire of radius r located in a uniform amplitude magnetic field generated by coil n in a time varying harmonic field of angular frequency ω, with a sinusoidal waveform H n (t)=H n exp(−iωt), where coil axis  21  is displaced at an angle ρ with respect to the lines of magnetic flux, i={square root}{square root over ((−1))}, t is the time, and exp(n) denotes the natural base of logarithms e raised to the power n, the induced voltage measured between the two open ends of the sensing coil  14  is expressed as:  
           V   n          (   t   )       =         -   N            ∫   S            μ   0            ∂       H   n          (   t   )           ∂   t          •                   a   ^                        a           =       -   ω                   N                 π                   r   2          μ   0                     H   n                     cos        (   ρ   )                       exp        (         -                      ω                 t     -                        π   /   2         )                                 
 
         [0181]    where H n =|H n |. This assumes that each turn of the coil is separately and equally linked by the magnetic flux (e.g., in tightly wound coils), the induced voltage within sensing coil  14  may be approximated as the summation of the induced voltages developed in each turn. 
           V   n ( t )=− Nωπr   2 μ 0   H   n  cos(ρ)exp(− iωt−iπ/ 2) 
         [0182]    The time dependence and phase of V n (t) is given by exp(−iωt−iπ/2). Notice that the phase factor of π/2 indicates that the time variation of the induced voltage V n (t)=−Nωπr 2 μ 0 H n  cos(ρ)exp(−iω−iπ/2) lags behind the time varation of the driving magnetic field H n (t)=H n exp(−iωt) by π/2 radians, or 90°. This time dependence and phase can be factored out to provide a solution for the induced voltage amplitude V n  only, where 
           V   n ( t )= V   n exp(− iωt−iπ/ 2). 
         [0183]    Thus 
           V   n ( t )=− Nωπr   2 μ 0   H   n  cos(ρ) 
         [0184]    The induced voltage amplitude V n  in sensing coil  14  will vary with changes in the angular orientation between the coil axis and the direction of the magnetic field lines ρ.  
         [0185]    A useful reference frame for spatially conceptualizing the interaction between sensing coil  14  and the magnetic fields in navigational domain  12  is the Cartesian coordinate system defined by mutually perpendicular axes x-y-z. As above, coil axis  21  is through sensing coil  14 .  
         [0186]    The angles α,β,γ that the coil axis  21  makes with the unit coordinate vectors x, y, and z respectively, are called the direction angles of coil axis  21 ; the trigonometric terms cos(α).cos(β) and cos(γ)represent direction cosine values. Employing vector product notation, the following expressions are provided: {circumflex over (α)}·x=cos(α), {circumflex over (α)}·y=cos(β), and {circumflex over (α)}·z=cos(γ). Referencing the induced voltage equations set forth above, these angles α, β, and γ correspond to the angular displacement of coil axis  21  with respect to uniform fields generated along the x-axis, y-axis, and z-axis directions, respectively. Thus, the correspondence between direction cosine expressions is as follows:  
           â·x =cos(α)=sin(φ)cos(θ)  
           â·y =cos(β)=sin(φ)sin(θ), and  
           â·z =cos(γ)=cos(φ)  
         [0187]    Accordingly, the following relationships illustrate the dependence of induced voltage on the orientation parameters θ, and φ.  
           V   x   =κωH   x   x  sin(φ)cos(θ)+κω H   y   x  sin(θ)sin(φ)+κω H   z   x  cos(φ)  
         [0188]    where κ=−Nπr 2 μ 0  and is independent of frequency. The subscripts in the field intensity amplitudes indicate the axial dimension along which the magnetic field amplitude value was determined by accessing the LUT for the given coil set at the LNP, while the superscript in the voltage amplitude and the field intensity terms indicates the field-generating coil set: in this case the x-directed uniform amplitude field. For an x-directed substantially uniform field, the terms H y   x  and H z   x  are small compared to H x   x . Similar equations are developed below for the induced voltages produced by the unidirectional coils successively generating a y-directed and z-directed substantially uniform field:  
           V   y   =κωH   x   y  sin(φ)cos(θ)+κω H   y   y  sin(φ)sin(θ)+κω H   z   y  cos(φ), and  
         V z   =κωH   x   z  sin(φ)cos(θ)+κω H   y   z  sin(φ)sin(θ)+κω H   z   z  cos(φ)  
         [0189]    As above for the x-directed field, the terms H x   y  and H z   y  in the equation for V y  and the terms H x   z  and H y   z  in the equation for V Z  are small compared to H y   y  and H z   z  respectively. After substituting the measured values for the induced voltage amplitude signals, the linearly independent equations are simultaneously solved to determine the unknown variables θ and φ defining the orientation of coil axis  21 . This corresponds to calculation step  172  of FIG. 16.  
         [0190]    Determining the Positional Coordinates of a Probe in the Absence of Extraneous Fields.  
         [0191]    This section describes in more detail the position calculation as indicated by the schematic of FIG. 17. To begin with, a sequence of vector gradient fields are generated by successive delta coils. As above, consider a magnetic field waveform that is sinusoidal in nature with angular frequency ω. Each vector gradient field generates a family of constant signal surfaces. The voltage amplitude V o   n  is calculated at the LNP, R 0 =(x 0 , y 0 , z 0 ), from the LUT at the fixed orientation  
         (θ, φ), or  v   0   n ( x   0   , y   0   , z   0 , θ, φ)exp(− iωt−iπ/ 2)=− Nωr   2 μ 0   H   0   n   ·â exp(− iωt−iπ/ 2)  
         [0192]    where coil axis {circumflex over (α)}  21  is a unit vector that is normal to the plane determined by sensing coil  14  and is uniquely determined by the orientation variables (θ, φ), and H o   n  is the vector magnetic field produced by the “n” coil set and is evaluated at the LNP which is R 0 . This corresponds to step  176  of FIG. 17. As noted above, the last navigation point (LNP) refers to the x-y-z positional coordinates of sensing coil  14  as determined by the immediately previous computation cycle. For the first cycle, the LNP is the center of the viewing field. The voltage amplitude V 0   n  calculated above is subtracted from the measured value of the voltage amplitude V n  at the unknown location R=(x, y, z) at the known fixed orientation (θ, φ) or V n (x, y, z, θ, φ,). The difference in these amplitude values ΔV n V 0   n −V n  is related to the dot product of the vector gradient field at the LNP {right arrow over (∇)}H 0   n (R 0 ) and a displacement vector ΔR=R−R 0 =(Δx, Δy, Δz), where the displacement vector ΔR is assumed to be small, in the following manner:  
         Δ                   V   n          exp        (         -                      ω                 t     -     π   /   2       )         =       -   N                   ω                   r   2          μ   0        Δ                   R   ·       ∇   →          (       H   0   n     ·     a   ^       )              exp        (         -                      ω                 t     -                        π   /   2         )                               
 
         [0193]    As before, the time dependence and phase exp(−iωt−iπ/2) can be factored out and using κ=−Nπr 2 μ 100  , we are left with:  
         Δ V   n   =κωΔR ·{right arrow over (∇)}( H   0   n·   â )  
         [0194]    In the above equation, ΔV n  is known, the variables κ and ω are known, as is coil axis a  21 . The gradient {right arrow over (∇)}(H 0   n ·â) can be calculated from the LUT using standard techniques from numerical methods. Thus, the only unknown that remains is the small displacement vector ΔR which is expressed merely as the difference between the LNP R 0 =(x 0 , y 0 , z 0 ) and the unknown location vector R=(x, y, z). Thus, a measurement of the amplitude of the voltage across sensor coil  14 , provided that {right arrow over (∇)}(H 0   n ·â)≠0, provides one with a non-singular equation of a function of  
         [0195]    R=(x, y, z):  
         R   ·       ∇   →          (       H   0   n     ·     a   ^       )         =         Δ                   V   n       κω     +       R   0     ·       ∇   →          (       H   0   n     ·     a   ^       )                                 
 
         [0196]    which has the general form  
         [0197]    where the variables A j   n  and B n  are constants. Such an equation in general determines a plane in three dimensions. Since it is assumed that the displacement vector ΔR is small, then, for a small region around the LNP R 0 =(x 0 , y 0 , z 0 ), a small planar portion of the constant signal surface associated with a delta coil and a given sensing coil  14  orientation can be identified.  
         [0198]    In particular, if the nth delta coil pair is activated and voltage measurements are taken, where the nth short coil is denoted by n(s) and the nth long coil is denoted by n(L), then the following linearly independent equations can be solved for  
         R   ·       ∇   →          (       H   0     n        (   S   )         ·     a   ^       )         =         Δ                   V     n        (   S   )           κω     +       R   0     ·       ∇   →          (       H   0     n        (   S   )         ·     a   ^       )                     R   ·       ∇   →          (       H   0     n        (   L   )         ·     a   ^       )         =         Δ                   V     n        (   L   )           κω     +       R   0     ·       ∇   →          (       H   0     n        (   L   )         ·     a   ^       )                                 
 
         [0199]    Each equation independently determines a small planar region of a constant signal surface, and the simultaneous solution of both of the equations determines a line ℑ n  in three dimensional space on which the unknown point R=(x, y, z) lies. This corresponds to calculation step  180  of FIG. 17. In practice, the delta coil pairs are constructed so that the substantially zero gradient component of both the short coil and the long coil field are in the same direction. Denoting such a direction by c n , this implies by definition that  
             ∇   →          (       H   0     n        (   s   )         ·     c   n       )       =   0     ,   and               ∇   →          (       H   0     n        (   L   )         ·     c   n       )       =   0                         
 
         [0200]    Thus, the line ℑ n  in three dimensional space determined by the above equations is parallel to the vector c n    
         [0201]    Based on the above analysis, each delta coil pair provides information for determining a line ℑ n  in three dimensional space upon which the point R=(x, y, z) lies. Notice that, in theory, the point R=(x, y, z) could be determined using only two coil pairs where the orientation of the corresponding vectors c 1  and c 2  are at 90° to each other. However, in practice, three coils are desirable because of the possibility that the orientation of the sensing coil  14  {circumflex over (α)} may be such that  
             ∇   →          (       H   0     n        (   s   )         ·     a   ^       )       =   0     ,   and               ∇   →          (       H   0     n        (   L   )         ·     a   ^       )       =   0                         
 
         [0202]    In such a case, the equations generated by such a measurement are not linearly independent of each other, and there is no solution that determines a unique line in three dimensional space. Thus, in practice, three delta coil pairs are used where the orientation of the corresponding vectors c 1 , c 2 , and c 3  are at 120° to each other.  
         [0203]    Detailed Description of Extraneous Eddy Current Compensation  
         [0204]    This section describes in more detail the eddy current compensation method as indicated by the schematic of FIG. 13.  
         [0205]    The measurements of induced voltage amplitudes in sensing coil  14  form the basis for determining position and orientation of sensing coil  14 . In practice, extraneous conducting object  23  may be present either in navigational domain  12  or peripheral domain  15  that responds to the fields generated for measurement purposes.  
         [0206]    [0206]FIG. 20A shows the disposition of the nine coil sets of the examination deck  17 ; the coil sets are also shown in FIG. 20B where they are schematically represented as  1 - 9 . Also shown in  20 B is first conducting object  23 , it is subjected to a field H 1  (R d ) which is the field produced by coil set #1 at the location of first conducting object  23 . This field will induce eddy currents ID that flow in first conducting object  23  and these currents will produce disturbing magnetic fields of their own, these are designated H D   1  (x, y, z). Thus the field present at X,Y,Z, the location of sensing coil  14  has two parts, H 1 (X, Y, Z) the desired field upon which navigation is calculated and H D   1 (X, Y, Z) the disturbance. the signal produced in sensing coil  14  also has two components corresponding to the two fields. In order to isolate and separate these, one must understand the characteristics of eddy currents produced in first conducting object  23  of arbitrary shape and characteristics.  
         [0207]    [0207]FIG. 21A depicts an exemplary disturbance in the form of a metallic ring. This ring will have an inductance L and a resistance R that depend on the dimensions and materials of the ring. The circuit on the right describes the eddy current equivalent, this circuit will have a characteristic time constant τ=L/R and have a level of excitation dependent on the orientation of the incoming field to the axis. FIG. 21B shows an elliptical slab of metal with an off center circular hole cut in it. The eddy current performance of this shape is largely described by three current loops or modes of excitation. These are numbered  1 ,  2 ,  3 , each of these modes has its&#39; own L, R and τ and its own degree of interaction to an incoming field. the eddy current equivalent is therefore a summation of three circuits each similar to that for the case shown in FIG. 21A. A similar analysis prevails in the case shown in FIG. 21C, which corresponds to a solid square plate. In each of these cases there is a dominant mode (#1) which corresponds to the current circulation around the largest dimensions of the disturbing shape, this gives the largest interaction with the incoming field and also produce the largest field generation. In general, any shape disturbance can always be described as an infinite summation of simple LR circuits.  
         [0208]    The first objective in the following description is to show how the dominant mode of any disturbance  23  can be eliminated in its effect of the signal V N (t) and therefore eliminate its effect on navigation. The second objective is to show how higher order modes can be eliminated.  
         [0209]    Dominant Mode Only:  
         [0210]    The induced voltage at the sensing coil  14  has two components, the direct coupling from the transmitter coils and the indirect coupling from the first conducting object  23 , which gives  
           V   n          (   t   )       =       ω                   exp        (         -                      ω                 t     -                        π   /   2         )                       η   0   n       +         ω   2          exp        (         -                      ω                 t     -                      π       )                       η   eddy   n          L   eddy           R   eddy     -                      ω                   L   eddy                                   
 
         [0211]    where the coupling variables  
         η   0   n                   and                   η   eddy   n                           
 
         [0212]    are incorporated to indicate the coupling with the source field and the resulting eddy field respectively. Factoring out the time dependence and the phase factor π/2 as before, where, 
           V   n ( t )= V   n exp(− iωt−iπ/ 2)  
         [0213]    the following result is obtained:  
         V   n     =       ω                   η   0   n       -                          ω   2          η   eddy   n          L   eddy           R   eddy     -                      ω                   L   eddy                                   
 
         [0214]    If the variable τ eddy i s defined as:  
         τ   eddy     =       L   eddy       R   eddy                             
 
         [0215]    Then the above equation can be rewritten as:  
           V   n     ω     =       η   0   n     -       η   eddy   n                                        ω                   τ   eddy         1   -                      ω                   τ   eddy                                     
 
         [0216]    This equation can be rewritten in terms of real and imaginary parts:  
             V   n     ω     =       η   0   n     +       η   eddy   n                         ω   2          τ   eddy   2         1   +       ω   2          τ   eddy   2             -                        η   eddy   n                       ω                   τ   eddy         1   +       ω   2          τ   eddy   2                 ,              or               V   n     ω     =       η   0   n     +       η   eddy   n                         ω   2          τ   eddy   2         1   +       ω   2          τ   eddy   2             +       exp        (       -                        π   /   2       )                       η   eddy   n                       ω                   τ   eddy         1   +       ω   2          τ   eddy   2                                     
 
         [0217]    Thus, first conducting object  23  has altered the magnitude of the measured voltage amplitude due to the source coils, at the expected phase shifted point exp(−ωt−iπ/2), by an amount ΔV n-eddy  equal to  
         Δ                   V     n   -   eddy         =       η   eddy   n                         ω   3          τ   eddy   2         1   +       ω   2          τ   eddy   2                                   
 
         [0218]    ΔV n-eddy  contains the two unknowns τ eddy  and  
         η     n   eddy       .                         
 
         [0219]    An additional voltage signal is phase shifted by a further or π/2 radians with an amplitude  
         V     π   /   2       n   -   eddy       :                         
 
         V     π   /   2       n   -   eddy       =       η   eddy   n                         ω   2          τ   eddy         1   +       ω   2          τ   eddy   2                                   
 
         [0220]    The measured voltage drop V n  can be rewritten as:  
         V   n     =       ωη   0   n     +     ω                   τ   eddy                     V     π   /   2       n   -   eddy         -     iV     π   /   2       n   -   eddy                               
 
         [0221]    Thus, if  
       V     π   /   2       n   -   eddy                           
 
         [0222]    and τ eddy  are known, the measured signal V n  can be adjusted for the presence of first conducting object  23  to obtain the adjusted potential drop  
         V   adjst   n     =     ω                     η   0   n     .                             
 
         [0223]    Once  
       V   adjst   n                         
 
         [0224]    is obtained, the orientation and position calculations discussed previously can be applied using the LUT in order to obtain the proper orientation and position of sensing coil  14 .  
         [0225]    The value of the amplitude  
       V     π   /   2       n   -   eddy                           
 
         [0226]    can be obtained directly by measurement, since it lags the expected voltage drop signal by the additional phase of π/2 radians, or 90°. Thus, only τ eddy  need be determined in order to compensate for an extraneous eddy current.  
         [0227]    As before, at least one more linearly independent equation is needed in order to solve for eddy. A second linearly independent equation is obtained by taking a voltage amplitude measurement using a second magnetic field waveform, for example, taking a voltage amplitude measurement at a second angular frequency. Thus, denoting our two angular frequencies by ω 1  and ω 2 , two linearly independent measurements are obtained:  
           V   n          (     ω   2     )       =         ω   1          η   0   n       +       ω   1          τ   eddy            V     π   /   2       n   -   eddy            (     ω   1     )         -       iV     π   /   2       n   -   eddy            (     ω   1     )                     V   n          (     ω   2     )       =         ω   2          η   0   n       +       ω   2          τ   eddy            V     π   /   2       n   -   eddy            (     ω   2     )         -       iV     π   /   2       n   -   eddy            (     ω   2     )                               
 
         [0228]    The difference of the amplitudes between the two frequencies with the time dependence and phase that goes as exp(−iωτ−iπ/2) can be written as:  
         Re                   {           V   n          (     ω   1     )         ω   1       -         V   n          (     ω   2     )         ω   2         }       =       τ   eddy                     {         V     π   /   2       n   -   eddy            (     ω   1     )       -       V     π   /   2       n   -   eddy            (     ω   2     )         }                             
 
         [0229]    where “Re” denotes the real part of the enclosed quantity. Meanwhile, the difference of the amplitudes between the two frequencies with the time dependence and phase that goes as exp(−iωτ−iπ) can be written as:  
         Im        {         V   n          (     ω   1     )       -       V   n          (     ω   2     )         }       =     {         V     π   /   2       n   -   eddy            (     ω   2     )       -       V     π   /   2       n   -   eddy            (     ω   1     )         }                           
 
         [0230]    where “Im” denotes the imaginary part of the enclosed quantity. Thus:  
         τ   eddy     =       Re                   {           V   n          (     ω   2     )         ω   2       -         V   n          (     ω   1     )         ω   1         }         Im                   {         V   n          (     ω   1     )       -       V   n          (     ω   2     )         }                               
 
         [0231]    This corresponds to a portion of calculation step  138  of FIG. 13 in one embodiment of the present invention. Therefore, since τ eddy  can be determined according to voltage measurements at two angular frequencies, and since the magnitude of the phase lagged amplitude  
       V     π   /   2       n   -   eddy                           
 
         [0232]    can also be measured, then the adjusted voltage drop to be used with the LUT,  
           V   adjst   n     =     ω                   η   0   n         ,                         
 
         [0233]    can be determined as a function of the measured voltage drop V n :  
         V   adjst   n     =       ωη   0   n     =       Re        {     V   n     }       +       ωτ   eddy        Im        {     V   n     }                                 
 
         [0234]    This, again, corresponds to calculation step  138  of FIG. 13 in one embodiment of the present invention.  
         [0235]    Eddy Current Compensation in the Case of Two Extraneous Conducting Objects  
         [0236]    The analysis in the previous section applied to the case a disturbing magnetic field introduced by first conducting object  23 . the analysis in this section applies to the case where there is first conducting object  23  and second conducting object  31  where each has, respectively, independent values of τ 1  and τ 2 .  
         [0237]    For the case of first conducting object  23  and second conducting object  31 , each with a different value of τ 1  and τ 2 , respectively, and with different coupling constants: η 1   n  and η 2   n , the above equation determining  
         V   adjst   n     =     ω                   η   0   n                             
 
         [0238]    changes. In this case, the potential drop measured across sensing coil  14  based on a model consistent with the present invention becomes:  
                 V   n     ω     =                  η   0   n     +       η   1   n                         ω   2          τ   1   2         1   +       ω   2          τ   1   2             -                        η   1   n            ωτ   1       1   +       ω   2          τ   1   2             +                                  η   2   n                         ω   2          τ   2   2         1   +       ω   2          τ   2   2             -                        η   2   n            ωτ   2       1   +       ω   2          τ   2   2                                           
 
         [0239]    where η 1   n  and τ 1  correspond to first conducting object  23  and and η 2   n  and τ 2  correspond to second conducting object  31 . Thus, for a given real and imaginary measurement of V n  across sensing coil  14 , there are 5 unknowns η 0   n , η 1   n , η 2   n , η 1   n , τ 1 , and τ 2 . By equating the real and imaginary portions of the measurements across sensing coil  14  with the real and imaginary portions of the above equations, there are two linearly independent equations. thus, there are an insufficient number of equations to determine all five of the unknowns η 0   n , η 1   n , η 2   n , η 1   n , τ 1 , and τ 2 .  
         [0240]    A measurement at two frequencies ω 1  and ω 2  as was outlined above, yields four equations. There are still five unknowns, however. Thus, there are still an insufficient total number of equations to solve for all of the terms and determine  
         V   adjst   n     =       ωη   0   n     .                           
 
         [0241]    A measurement at three frequencies ω 1 , ω 2 , and ω 3 , however, yields a sufficient number of equations in order to determine all of the variables, and allows for the calculation of  
         V   adjst   n     =       ωη   0   n     .                           
 
         [0242]    Specifically, by taking voltage drop measurements across sensing coil  14  at three frequencies, the following equations are obtained:  
                   V   n          (   ω   )       ω     =                  η   0   n     +       η   1   n                         ω   2          τ   1   2         1   +       ω   2          τ   1   2             -                        η   1   n            ωτ   1       1   +       ω   2          τ   1   2             +                                  η   2   n                         ω   2          τ   2   2         1   +       ω   2          τ   2   2             -                        η   2   n            ωτ   2       1   +       ω   2          τ   2   2                                           
 
         [0243]    where ω is selected from the set ω=(ω 1 , ω 2 , ω 3 ). In the above equations, the unknowns η 1   n  and η 2   n  appear as linear coefficiients. Therefore, by subtracting a measurement of  
         V   n       ω   1                           
 
         [0244]    taken by sensing coil  14  from a measurement of  
           V   n       ω   2       ,                         
 
         [0245]    the above equations can be rewritten in the general form  
           η   1   n     =       f   1          (       ω   1     ,     ω   2     ,     τ   1     ,     τ   2       )         ,   and             η   2   n     =       f   2          (       ω   1     ,     ω   2     ,     τ   1     ,     τ   2       )                             
 
         [0246]    where ƒ 1 (ω 1 , ω 2 , τ 1 , τ 2 ) and ƒ 2 (ω 1 , ω 2 , τ 1 , τ 2 ) are polynomial functions of ω 1 , ω 2 , τ 1 , and τ 2 .  
         [0247]    In addition, by subtracting a measurement of  
         V   n       ω   1                           
 
         [0248]    taken by sensing coil  14  from a measurement of  
           V   n       ω   3       ,                         
 
         [0249]    the above equations can also be rewritten in the general form  
           η   1   n     =       g   1          (       ω   1     ,     ω   3     ,     τ   1     ,     τ   2       )         ,   and             η   2   n     =       g   2          (       ω   1     ,     ω   3     ,     τ   1     ,     τ   2       )                             
 
         [0250]    where again g 1 (ω 1 , ω 3 , τ 1 τ 2 ) and g 2 (ω 1 , ω 3 , τ 1 , τ 2 ) are polynomial functions of ω 1 , ω 3 , τ 1 , and τ 2 . Therefore, the following polymial equations provide two equations with two unknowns: 
           g   1 (ω 1 ,ω 3 ,τ 1 ,τ 2 )=ƒ 1 (ω 1 ,ω 3 ,τ 1 ,τ 2 ), 
         [0251]    and 
           g   2 (ω 1 ,ω 3 ,τ 1 ,τ 2 )=ƒ 2 (ω 1 ,ω 3 ,τ 1 , τ 2 ). 
         [0252]    The above two equations can be solved to determine τ 1 , and τ 2 , and, thus, the value of  
         V   adjst   n     =     ω                   η   0   n                             
 
         [0253]    can be determined in the presence of first conducting object  23  and second conducting object  31 . The solution to the above equations yields the adjusted value of the potential drop measurement and corresponds to calculation step  138  of FIG. 13 in one embodiment of the present invention.  
         [0254]    Eddy Current Compensation in the Case of Two or More Extraneous Conducting Objects  
         [0255]    The general result, consistent with the present invention, is that resolving m values of τ m , requires m measurements of real and imaginary parts of the potential drop across sensing coil  14 , using m different waveforms. For example, measurements of a sinusoidal waveform at the set of frequencies (ω 1 , ω 2 , . . . ω m ) are sufficient to resolve m values of τ m . Since each additional conducting body introduces a new variable τ m  and an associated coupling constant η m   n , each additional conducting body introduces two new unknowns. However, a measurement of a potential drop across sensing coil  14  at a new waveform m, including both the real and imaginary portions, yields the required number of two new equations necessary to resolve τ m  and η m   n .  
         [0256]    As above, with first conducting object  23  and second conducting object  31 , higher order polynomial expressions can be derived and the value of  
         V   adjst   n     =     ωη   0   n                           
 
         [0257]    can be solved for, consistent with the present invention.  
         [0258]    Alternatively, one embodiment of the present invention employs a numerical method to solve for  
         V   adjst   n     =       ωη   0   n     .                           
 
         [0259]    In one embodiment of the present invention, a Taylor series about ω=ω 1  can be derived as follows:  
           V   n     ω     =         V   n       ω   1       +       ∂     ∂   ω            (       V   n       ω   1       )          (     ω   -     ω   1       )       +       1   2            ∂   2       ∂     ω   2              (       V   n       ω   1       )            (     ω   -     ω   1       )     2       +     R   3                             
 
         [0260]    where R 3  indicates that the neglected terms are of the third order in the derivative of  
           V   n     ω     ,       ∂     ∂   ω            (       V   n       ω   1       )                             
 
         [0261]    denotes a first derivative of the function  
         V   n     ω                         
 
         [0262]    with respect to ω evaluated at ω=ω 1 , and  
           ∂   2       ∂     ω   2              (       V   n       ω   1       )                           
 
         [0263]    denotes a second derivative of the function  
         V   n     ω                         
 
         [0264]    with respect to ω evaluated at ω=ω 1 .  
         [0265]    Considering the relationship derived above for the effect of first conducting object  23  and second conducting object  31   
           V   n     ω     =       η   0   n     +       η   1   n              ω   2          τ   1   2         1   +       ω   2          τ   1   2             -     i                   η   1   n            ωτ   1       1   +       ω   2          τ   1   2             +       η   2   n              ω   2          τ   2   2         1   +       ω   2          τ   2   2             -     i                   η   2   n            ωτ   2       1   +       ω   2          τ   2   2                                     
 
         [0266]    the term  
       η   0   n                         
 
         [0267]    is the only variable that appears without any associated factors of ω. Thus, collecting similar powers of co yields the relationship:  
         η   0   n     =         V   n       ω   1       -       ∂     ∂   ω            (       V   n       ω   1       )          ω   1       -       1   2            ∂   2       ∂     ω   2              (       V   n       ω   1       )          ω   1   2       +       R   3                   o                 r                 V   adjst   n     =       V   n     -       ∂     ∂   ω            (       V   n       ω   1       )          ω   1   2       +       1   2            ∂   2       ∂     ω   2              (       V   n       ω   1       )          ω   1   2       +     R   3                             
 
         [0268]    The quantity V n  is measured at sensing coil  14  at a frequency ω 1  while the quantity  
         ∂     ∂   ω            (       V   n       ω   1       )                           
 
         [0269]    can be determined from measurements at sensing coil  14  at any two of three frequencies ω 1 , ω 2 , and ω 3 , or using all three frequencies ω 1 , ω 2 , and ω 3 . In addition, the quantity  
         1   2            ∂   2       ∂     ω   2              (       V   n       ω   1       )                           
 
         [0270]    can be determined from measurements at sensing coil  14  at three frequencies ω 1 , ω 2 , and ω 3 . Thus, using measurements at sensing coil  14  at three frequencies ω 1 , ω 2 , and ω 3 , the adjusted value for the potential drop across sensing coil  14  can be determined according to:  
         V   adjst   n     =       V   n     -       ∂     ∂   ω            (       V   n       ω   1       )          ω   1   2       +       1   2            ∂   2       ∂     ω   2              (       V   n       ω   1       )          ω   1   2       +     R   3                             
 
         [0271]    for the situation where there are multiple conducting objects that induce eddy currents and when there are multiple values of τ m .  
         [0272]    The above example utilized magnetic field waveforms that were sinusoidal in nature at a set of frequencies ω. However, a Taylor series expansion about any suitable waveform variable can be variably derived and is consistent with an embodiment of the present invention.  
         [0273]    Eddy Current Compensation Using Singular Value Decomposition  
         [0274]    Another embodiment of the present invention utilizes the relationship derived above for the case of first conducting object  23  only:  
         V   adjst   n     =       ωη   0   n     =       Re        {     V   n     }       +       ωτ   eddy        Im        {     V   n     }                                 
 
         [0275]    this equation can be rewritten:  
         Re        {     V   n     }       =       V   adjst   n     -       ωτ   *        Im        {     V   n     }                               
 
         [0276]    where  
       Re        {     V   n     }                           
 
         [0277]    is determined from a measurement across sensing coil  14 ,  
       Im        {     V   n     }                           
 
         [0278]    is also determined from a measurement across sensing coil  14 , ω 1  is known and both τ* and  
       V   adjst   n                         
 
         [0279]    are unknown. The notation τ* is used here merely to indicate a composite τ eddy  that may be a result from several conducting objects. from the general relationship derived above  
           V   n     ω     =       η   0   n     +       η     e                 d                 d                 y     n              ω   2          τ     e                 d                 dy     2         1   +       ω   2          τ     e                 d                 dy     2             -     i                   η     e                 d                 d                 y     n            ωτ     e                 d                 dy         1   +       ω   2          τ     e                 d                 dy     2                                     
 
         [0280]    the next higher order correction appears as to a composite τ* can be written as:  
         Re        {     V   n     }       =       V   adjst   n     -     ω                 Im        {     V   n     }          (       τ   *     +     ωτ   **       )                               
 
         [0281]    where the notation τ* * indicates that it is a correction to the term τ* appearing as a coefficient to ω. the units of τ* * correspond to the units of (τ*) 2 .  
         [0282]    In the above equation, there are three unknowns τ*, τ* *, and  
         V   adjst   n     .                         
 
         [0283]    In one embodiment of the present invention, it is found that measurements at sensing coil  14  of  
       Re        {     V   n     }                   and                 Im        {     V   n     }                           
 
         [0284]    at four frequencies ω 1 , ω 2 , ω 3 , and ω 4 , and which yields four equations  
         Re        {     V   n     }       =       V   adjst   n     -     ω                 Im        {     V   n     }          (       τ   *     +     ωτ   **       )                               
 
         [0285]    with three unknowns τ*, τ* *, and  
         V   adjst   n     ,                         
 
         [0286]    is sufficient to determine  
       V   adjst   n                         
 
         [0287]    with the desired precision. Specifically, the above four equations with three unknowns is solved using singular value decomposition, as described, for example, in Numerical Recipes, Cambridge (1986), by W. H. press, B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling. This, again, corresponds to calculation step 138 of FIG. 13.  
         [0288]    Again, the above example utilized magnetic field waveforms that were sinusoidal in nature at a set of frequencies **. however, a similarly derived relationship about any suitable waveform variable can be utilized and is consistent with an embodiment of the present invention.  
         [0289]    Detailed Description of Ferromagnetic and Conductive Object Compensation  
         [0290]    This section describes in more detail the ferromagnetic and conductive object compensation method as indicated by the schematic of FIG. 14.  
         [0291]    A pure magnetic core is a source of magnetic flux. The primary quality of a pure magnetic core is that it can enhance, or focus, magnetic field flux lines along a preferred direction. With respect ,to Faraday&#39;s Law  
           V   n          (   t   )       =     -       ∫   S          μ            ∂       H   n          (   t   )           ∂   τ       ·   a             a                                 
 
         [0292]    where the quantity on the right is proportional to the time rate of change of magnetic flux, it can be seen that, as a source of flux lines, a pure magnetic object will act to enhance the voltage drop. Thus, for a pure magnetic object with tool coil  19  surrounding the magnetic core, tool coil  19  will produce a voltage drop as a function of a source field H n (R, t)  
           V     m                 a                 g                 n     n          (   t   )       =       -   T            ∫     S     m                 a                 g                 n              μ            ∂       H   n          (   t   )           ∂   τ       ·     a     m                 a                 g                 n                   a     m                 a                 g                 n                                     
 
         [0293]    where T is a multiplication factor that represents the enhancement of the magnetic flux through tool coil  19 .  
         [0294]    Surgical tool  108  of FIG. 10 has some qualities of a pure magnetic object, as well as additional properties. Like a pure magnetic object, it can act to enhance the detection of flux lines through the center of tool coil  19  as above. However, it also responds to an applied field in a nonlinear manner, as is indicated on hysteresis graph  100 . Furthermore, the additional presence of conductive elements introduce additional fields that are out of phase by π/2 with any enhanced field in the vicinity. Thus, a general form for the voltage drop across tool coil  19  affixed around surgical tool  108 :  
           V     f                 e                 r                 r                 o     n          (   t   )       =       -     (       T     R                 e       -     i                   T     I                 m           )              ∫     S     f                 e                 r                 r                 o              μ            ∂       H   n          (   t   )           ∂   τ       ·     a     f                 e                 r                 r                 o                   a     f                 e                 r                 r                 o                                     
 
         [0295]    where the multiplicative factor now has both a real T Re  and imaginary T Im  component. As before, with the discussion of eddy currents, the voltage drop across tool coil  19  located on surgical tool  108  can be written as:  
           V   ferro   n          (   t   )       =     ω                   exp        (         -                      ω                 t     -                        π   /   2         )            (       T   Re     -                        T   Im         )          T   ferro   n                             
 
         [0296]    To understand how the field that affects tool coil  19  located at surgical tool  108  affects sensing coil  14  located at the catheter, consider the above equation for tool coil  19  at surgical tool  108 :  
           V     f                 e                 r                 r                 o     n          (   t   )       =       -     (       T     R                 e       -     i                   T     l                 m           )              ∫     S     f                 e                 r                 r                 o              μ            ∂       H   n          (   t   )           ∂   τ       ·     a     f                 e                 r                 r                 o                   a     f                 e                 r                 r                 o                                     
 
         [0297]    If both the surface area contained by tool coil  19  S ferro  and the normal vector α ferro  were slowly changed such that they ultimately coincided with the surface area S of sensing coil  14  at the catheter probe sensor and the unit vector associated with sensing coil axis a  21 , but all other quantities were kept fixed, including the position—surgical tool  108 , the source field that is being integrated over H n (R, t) would no longer be associated with the multiplicative factor (T Re −iT Im ). This is the usual source field that can provide orientation and location information. However, ferromagnetic and conductive object  29  still acts as a multiplier of flux lines in this source field along a preferred axis with the factor (T Re −iT Im ). And so there are two sources of flux to be considered. Thus, the following general equation is obtained for the two sources of flux in this case at sensing coil  14 :  
           V   n          (   t   )       =       ω                   exp        (         -                      ω                 t     -                        π   /   2         )            η   0   n       +     ω                   exp        (         -                      ω                 t     -                        π   /   2         )            (       T   Re     -                        T   Im         )          η   ferro   n                               
 
         [0298]    where  
       η   ferro   n                         
 
         [0299]    is related to  
       Γ   ferro   n                         
 
         [0300]    by a proportionality constant. As before, V n (t)is the measured voltage drop across sensing coil  14  located at the catheter.  
         [0301]    Again, factoring out the time dependence and the phase factor π/2 as before, where  
             v   n          (   t   )       =       V   n          exp        (         -                      ωτ     -                        π   /   2         )           ,       and                     V   ferro   n          (   t   )         =       V   ferro   n          exp        (         -                      ωτ     -                        π   /   2         )                                 
 
         [0302]    the following is obtained:  
           V   n     =       ωη   0   n     +       ωη   ferro   n          (       T   Re     -     iT   Im       )           ,       and                   V   ferro   n       =     ω                     Γ   ferro   n          (       T   Re     -     iT   Im       )                                 
 
         [0303]    For a suitable choice of K that satisfies the equation:  
         Im        {       V   n     -     KV   ferro   n       }       =   0                         
 
         [0304]    the precise proportionality constant between  
         η   ferro   n                   and                   Γ   ferro   n                           
 
         [0305]    is obtained. This corresponds to calculation step  156  of FIG. 14. Therefore, the effect of surgical tool  108  can be subtracted out from the voltage measurement induced at the catheter:  
         V   adjst   n     =       ωη   0   n     =       V   n     -     K                   V   ferro   n                                 
 
         [0306]    This corresponds to calculation step  160  of FIG. 14. As before, with first conductive object  23  and an eddy current, the adjusted fields have been obtained to be used with the LUT in order to determine the proper orientation and position.  
         [0307]    Detailed Description of Shield Device Compensation  
         [0308]    This section describes in more detail the shield device compensation method as indicated by the schematics of FIGS. 15, 18, and  19 .  
         [0309]    As is evident from the discussion above, small distortions may have negligible results on the navigated position. However, large distortions can cause the system to report larger errors or even fail to compute a position. For a medical application, disturbances can include the operating table, a head-holder or other any number of other metallic items.  
         [0310]    With respect to an operating room table, the field coils will necessarily be adjacent to the operating room table. Thus, any field-influencing effect of an operating room table has the potential to create a larger than typical distortion in the fields located in the navigational domain.  
         [0311]    In particular, shield device  120  is provided to restrict the propagation of magnetic fields through it, as, for example, a sheet of conductive material. It may be arranged in any suitable geometry that has a fixed relationship to the transmission coils. There are many possible materials that the plate can be made from, such as aluminum, copper or virtually any other conductive material. It is also possible to use materials other than a conductive sheet such as a mesh or strips of material. A further possibility is a plastic of polymer film with a conductive coating.  
         [0312]    Shield device  120  should preferably be placed between the transmitter coil array and the disturbance. In an operating room, if the patient were to lay on the transmitter coil array, then shield device  120  could be placed under the array to block effects of the operating table. An additional enhancement could be made by bending the sides of shield device  120  up at an angle to negate the effects of the length of the operating table. Other arrangements for shield device  120  are possible, such as placing it to block the effects of microscopes or C-arms that may be present in the field.  
         [0313]    The device alters the fields produced by the transmitter coils. The effect of the device can either be computed from electromagnetic theory, as for example, from “Static and Dynamic Electricity” third edition, Taylor &amp; Francis (1989) by William R. Smythe. The effect could also be measured in a calibration process. Both of these techniques correspond to steps  188  and  192  of FIGS. 18 and 19 respectively. A fixed geometry can be characterized by a fixed alteration of the transmitted field. If the device is to be moved, a dynamic correction of the effect should be performed.  
         [0314]    Conclusion  
         [0315]    Methods and apparatus consistent with the present invention correct for the effects of field-influencing objects on a position and orientation determinations using magnetic fields. The foregoing description of an implementation of the invention has been presented for purposes of illustration and description. It is not exhaustive and does not limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practicing the invention. For example, step  144  in FIGS. 13 and 14 was indicated as occurring after all of the coil sets were activated and measured. Step  144 , however, can be placed after step  140  in both FIGS. 13 and 14 and is consistent with the present invention. Also, there may be a more efficient manner of processing measurements and calculations in parallel, rather than the linear schematic presented. In addition, the magnetic field was considered to have a sinusoidal waveform at an angular frequency ω, but other examples of waveforms are possible consistent with an embodiment of the present invention, including sawtooth waves, or square waves. Also, in the discussion above with regard to the analysis of voltage signals resulting from multiple waveforms, reference was made to a technique known as singular value decomposition. However, the use of any numerical technique for solving an overspecified equation is consistent with the present invention.