Apparatus, method and software for tracking an object

CROSS-REFERENCES TO RELATED APPLICATION

0001This application is related to European Patent Application No. <patcit id="pcit0001" dnum="JP04253208B"><text>04253208.5</text></patcit> filed on even date, entitled, "Dynamic Metal Immunity by Hysteresis," in the name of Biosense Webster, Inc., and claiming priority from <patcit id="pcit0002" dnum="US10448291B"><text>US Patent Application No. 10/448291 filed on 29th May, 2003</text></patcit> (Attorney's ref: P037724EP).

FIELD OF THE INVENTION

0002The present invention relates generally to non-contact tracking of objects using a magnetic field, and specifically to counteracting the effect of a moving, magnetic field-responsive article in a magnetic field.

BACKGROUND OF THE INVENTION

0003Non-contact electromagnetic locating and tracking systems are well known in the art, with an exceptionally broad spectrum of applications, including such diverse topics as military target sighting, computer animation, and precise medical procedures. For example, electromagnetic locating technology is widely used in the medical field during surgical, diagnostic, therapeutic and prophylactic procedures that entail insertion and movement of objects such as surgical devices, probes, and catheters within the body of the patient. The need exists for providing real-time information for accurately determining the location and orientation of objects within the patient's body, preferably without using X-ray imaging.

0004<patcit id="pcit0003" dnum="US5391199A"><text>US-A-5,391,199</text></patcit> and <patcit id="pcit0004" dnum="US5443489A"><text>US-A-5,443,489</text></patcit> describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as Hall effect devices, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by a plurality of radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.

0005<patcit id="pcit0005" dnum="WO9605768A"><text>WO-A-96/05768</text></patcit> describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates, so that the position and orientation of the catheter are known without the need for imaging the catheter.

0006<patcit id="pcit0006" dnum="US6239724A"><text>US-A-6,239,724</text></patcit> describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.

0007<patcit id="pcit0007" dnum="US5425382A"><text>US-A-5,425,382</text></patcit> describes apparatus and methods for locating a catheter in the body of a patient by sensing the static magnetic field strength gradient generated by a magnet fixed to the catheter.

0008<patcit id="pcit0008" dnum="US5558091A"><text>US-A-5,558,091</text></patcit> describes a magnetic position and orientation determining system which uses uniform fields from Helmholtz coils positioned on opposite sides of a sensing volume and gradient fields generated by the same coils. By monitoring field components detected at a probe during application of these fields, the position and orientation of the probe is deduced. A representation of the probe is superposed on a separately-acquired image of the subject to show the position and orientation of the probe with respect to the subject.

0009Other locating devices using a position sensor attached to a catheter are described in <patcit id="pcit0009" dnum="US4173228A"><text>US-A-4,173,228</text></patcit>, <patcit id="pcit0010" dnum="US5099845A"><text>US-A-5,099,845</text></patcit>, <patcit id="pcit0011" dnum="US5325873A"><text>US-A-5,325,873</text></patcit>, <patcit id="pcit0012" dnum="US5913820A"><text>US-A-5,913,820</text></patcit>, <patcit id="pcit0013" dnum="US4905698A"><text>US-A-4,905,698</text></patcit> and <patcit id="pcit0014" dnum="US5425367A"><text>US-A-5,425,367</text></patcit>.

0010Commercial electrophysiological and physical mapping systems based on detecting the position of a probe inside the body are presently available. Among them, CARTO^™, developed and marketed by Biosense Webster, Inc. (Diamond Bar, California), is a system for automatic association and mapping of local electrical activity with catheter location.

0011Electromagnetic locating and tracking systems are susceptible to inaccuracies when a metal or other magnetically-responsive article is introduced into the vicinity of the object being tracked. Such inaccuracies occur because the magnetic fields generated in this vicinity by the location system's radiator coils are distorted. For example, the radiator coils' magnetic fields may generate eddy currents in such an article, and the eddy currents then cause parasitic magnetic fields that react with the field that gave rise to them. In a surgical environment, for example, there is a substantial amount of conductive and permeable material including basic and ancillary equipment (operating tables, carts, movable lamps, etc.) as well as invasive surgery apparatus (scalpels, catheters, scissors, etc.). The eddy currents generated in these articles and the resultant electromagnetic field distortions can lead to errors in determining the position of the object being tracked.

0012It is known to address the problem of the interference of static metal objects by performing an initial calibration, in which the response of the system to a probe placed at a relatively large number of points of interest is measured. This may be acceptable for addressing stationary sources of electromagnetic interference, but it is not satisfactory for solving the interference problems induced by moving magnetically-responsive objects.

0013<patcit id="pcit0015" dnum="US6373240A"><text>US-A-6,373,240</text></patcit> describes an object tracking system comprising one or more sensor coils adjacent to a locatable point on an object being tracked, and one or more radiator coils, which generate alternating magnetic fields in a vicinity of the object when driven by respective alternating electrical currents. For each radiator coil, a frequency of its alternating electrical current is scanned through a plurality of values so that, at any specific time, each of the radiator coils radiates at a frequency which is different from the frequencies at which the other radiator coils are radiating.

0014The sensor coils generate electrical signals responsive to the magnetic fields, which signals are received by signal processing circuitry and analyzed by a computer or other processor. When a metal or other field-responsive article is in the vicinity of the object, the signals typically include position signal components responsive to the magnetic fields generated by the radiator coils at their respective instantaneous driving frequencies, and parasitic signal components responsive to parasitic magnetic fields generated because of the article. The parasitic components are typically equal in frequency to the instantaneous frequency of the driving frequency, but are shifted in phase, so that the effect at each sensor coil is to produce a combined signal having a phase and an amplitude which are shifted relative to the signal when no field-responsive article is present. The phase-shift is a function of the driving frequency, and so will vary as each driving frequency is scanned. The computer processes the combined signal to find which frequency produces a minimum phase-shift, and thus a minimum effect of the parasitic components, and this frequency is used to calculate the position of the object. Varying the driving frequency until the phase shift is a minimum is described as an effective method for reducing the effect of field-responsive articles on the signal.

0015<patcit id="pcit0016" dnum="US6172499A"><text>US-A-6,172,499</text></patcit> describes a device for measuring the location and orientation in the six degrees of freedom of a receiving antenna with respect to a transmitting antenna utilizing multiple-frequency AC magnetic signals. The transmitting component consists of two or more transmitting antennae of known location and orientation relative to one another. The transmitting antennae are driven simultaneously by AC excitation, with each antenna occupying one or more unique positions in the frequency spectrum. The receiving antennae measure the transmitted AC magnetic field plus distortions caused by conductive metals. As described, a computer then extracts the distortion component and removes it from the received signals, providing the correct position and orientation output.

0016<patcit id="pcit0017" dnum="US6246231A"><text>US-A-6,246,231</text></patcit> describes a method of flux containment in which the magnetic fields from transmitting elements are confined and redirected from the areas where conducting objects are commonly found.

0017<patcit id="pcit0018" dnum="US5767669A"><text>US-A-5,767,669</text></patcit> describes a method for subtracting eddy current distortions produced in a magnetic tracking system. The system utilizes pulsed magnetic fields from a plurality of generators, and the presence of eddy currents is detected by measuring rates of change of currents generated in sensor coils used for tracking. The eddy currents are compensated for by adjusting the duration of the magnetic pulses.

0018<patcit id="pcit0019" dnum="US4945305A"><text>US-A-4,945,305</text></patcit> and <patcit id="pcit0020" dnum="US4849692A"><text>US-A-4,849,692</text></patcit> describe tracking systems that circumvent the problems of eddy currents by using pulsed DC magnetic fields. Sensors which are able to detect DC fields are used in the systems, and eddy currents are detected and adjusted for by utilizing the decay characteristics and the amplitudes of the eddy currents.

0019<patcit id="pcit0021" dnum="US5600330A"><text>US-A-5,600,330</text></patcit> describes a non-dipole loop transmitter-based magnetic tracking system. This system is described as showing reduced sensitivity to small metallic objects in the operating volume.

0020<patcit id="pcit0022" dnum="EP0964261A"><text>EP-A-0,964,261</text></patcit> describes systems for compensating for eddy currents in a tracking system using alternating magnetic field generators. In a first system the eddy currents are compensated for by first calibrating the system when it is free from eddy currents, and then modifying the fields generated when the eddy currents are detected. In a second system the eddy currents are nullified by using one or more shielding coils placed near the generators.

0021<patcit id="pcit0023" dnum="US5831260A"><text>US-A-5,831,260</text></patcit> describes a combined electromagnetic and optical hybrid locating system that is intended to reduce the disadvantages of each individual system operating alone.

0022<patcit id="pcit0024" dnum="US6122538A"><text>US-A-6,122,538</text></patcit> describes hybrid position and orientation systems using different types of sensors including ultrasound, magnetic, tilt, gyroscopic, and accelerometer subsystems for tracking medical imaging devices.

0023Article surveillance systems using soft magnetic materials and low frequency detection systems have been known since the Picard patent (Ser. No. <patcit id="pcit0025" dnum="FR763861"><text>763,861</text></patcit>) was issued in France in 1934. Surveillance systems based on this approach generally use a marker consisting of ferromagnetic material having a high magnetic permeability. When the marker is interrogated by a magnetic field generated by the surveillance system, the marker generates harmonics of the interrogating frequency because of the non-linear hysteresis loop of the material of the marker. The surveillance system detects, filters, and analyzes these harmonics in order to determine the presence of the marker. Numerous patents describe systems based on this approach and improvements thereto, including, for example, <patcit id="pcit0026" dnum="US4622542A"><text>US-A-4,622,542</text></patcit>, <patcit id="pcit0027" dnum="US4309697A"><text>US-A-4,309,697</text></patcit>, <patcit id="pcit0028" dnum="US5008649A"><text>US-A-5,008,649</text></patcit> and <patcit id="pcit0029" dnum="US6373387A"><text>US-A-6,373,387</text></patcit>.

0024<patcit id="pcit0030" dnum="US4791412A"><text>US-A-4,791,412</text></patcit> describes an article surveillance system based upon generation and detection of phase shifted harmonic signals from encoded magnetic markers. The system is described as incorporating a signal processing technique for reducing the effects of large metal objects in the surveillance zone.

0025<patcit id="pcit0031" dnum="US6150810A"><text>US-A-6,150,810</text></patcit>, <patcit id="pcit0032" dnum="US6127821A"><text>US-A-6,127,821</text></patcit>, <patcit id="pcit0033" dnum="US5519317A"><text>US-A-5,519,317</text></patcit> and <patcit id="pcit0034" dnum="US5506506A"><text>US-A-5,506,506</text></patcit> describe apparatus for detecting the presence of ferrous objects by generating a magnetic field and detecting the response to the field from the object. A typical application of such an apparatus is detection and discrimination of objects buried in the ground. <patcit id="pcit0035" dnum="US4868504A"><text>US-A-4,868,504</text></patcit> describes a metal detector for locating and distinguishing between different classes of metal objects. This apparatus performs its analysis by using harmonic frequency components of the response from the object.

0026<patcit id="pcit0036" dnum="US5028869A"><text>US-A-5,028,869</text></patcit> describes apparatus for the nondestructive measurement of magnetic properties of a test body by detecting a tangential magnetic field and deriving harmonic components thereof. By analyzing the harmonic components, the apparatus calculates the maximum pitch of the hysteresis curve of the test body.

0027An article by <nplcit id="ncit0001" npl-type="s" url="http://www.ndt.net/article/ecndt98/nuclear/245/245.htm"><text>Feiste KL et al. entitled, "Characterization of Nodular Cast Iron Properties by Harmonic Analysis of Eddy Current Signals," NDT.net, Vol. 3, No. 10 (1998), available as of May 2002 at http://www.ndt.net/article/ecndt98/nuclear/245/245.htm</text></nplcit> describes applying harmonic analysis to nodular cast iron samples to evaluate the technique's performance in predicting metallurgical and mechanical properties of the samples.

0028There is a need for a straightforward, accurate, real-time method that addresses the problem of interference induced in electromagnetic locating and tracking systems caused by the introduction of non-stationary metallic or other magnetically-responsive articles into the measurement environment.

SUMMARY OF THE INVENTION

0029It is an object of some aspects of the present invention to provide apparatus and methods for improving the accuracy of electromagnetic locating and-tracking systems.

0030It is also an object of some aspects of the present invention to provide apparatus and methods for increasing accuracy of electromagnetic location and tracking systems without concern for the presence of moving electrically- and/or magnetically-responsive materials in the space wherein measurements are being taken.

0031It is a further object of some aspects of the present invention to provide apparatus and methods for enabling electromagnetic location and tracking systems to function accurately in the presence of moving electrically- and/or magnetically-responsive materials in the space wherein the measurements are being taken, substantially without regard to the quantity of such materials, their conductive characteristics, velocities, orientation, direction and the length of time that such materials are within the space.

0032It is yet a further object of some aspects of the present invention to provide apparatus and methods for operating electromagnetic location and tracking systems without the necessity of employing means for reducing or circumventing the effects caused by eddy currents induced in moving electrically- and/or magnetically-responsive objects in the space wherein measurements arc being taken.

0033According to a first aspect of the invention, there is provided an assessment apparatus of the type defined in the accompanying claim 1.

0034According to a second aspect of the invention, there is provided a method of assessing of the type defined in the accompanying claim 7.

0035According to a third aspect of the invention, there is provided a computer software product for assessing of the type defined in the accompanying claim 12.

0036Further preferred aspects are set out in the accompanying dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0037The present invention will be more fully understood from the following detailed description of preferred embodiments thereof, taken together with the drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a schematic, pictorial illustration of an electromagnetic locating and tracking system used during a medical procedure, in accordance with a preferred embodiment of the present invention;</li><li><figref idref="f0002">Fig. 2</figref> is a schematic, pictorial illustration of an assessment system, in accordance with a preferred embodiment of the present invention; and</li><li><figref idref="f0003">Figs. 3A, 3B, and 3C</figref> are simplified frequency response graphs illustrating an example assessment of an interfering element, in accordance with a preferred embodiment of the present invention.</li></ul>

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

0038<figref idref="f0001">Fig. 1</figref> is a schematic, pictorial illustration of an electromagnetic locating and tracking system 18 utilized to track an object, such as a probe 20, in the body of a patient 24 while providing immunity to the introduction, movement (dx), or removal of an interfering element 40, such as a ferromagnetic element, in or near a space 60 around the patient, in accordance with a preferred embodiment of the present invention. System 18 comprises a set of radiators 34, which are driven by a control unit 50 to track probe 20, preferably but not necessarily using methods and apparatus which are described in the above-cited US Patents and PCT Patent Publication to Ben-Haim and Ben-Haim et al. Thus, probe 20 comprises a position sensor (not shown), which preferably comprises field sensors, such as Hall effect devices, coils, or other antennae, for use in position determination. Alternatively or additionally, methods and apparatus known in the art are used to facilitate the tracking of probe 20. Control unit 50 comprises circuitry for processing signals received from probe 20, for detecting element 40, and for calculating the absolute position of probe 20 using a harmonic correction algorithm, as described hereinbelow.

0039Element 40 typically comprises an article made completely or partially of magnetically permeable material, such as ferromagnetic material. Examples of such articles include surgical tools, movable lamps, and carts. Element 40 generates parasitic fields, the phases and amplitudes of which generally depend on properties of element 40, including its dielectric constant, magnetic permeability, geometrical shape and orientation relative to probe 20. It will be appreciated that although element 40 is shown in <figref idref="f0001">Fig. 1</figref> as a single element, element 40 could comprise a number of separate elements, which are often brought in and out of the area of a medical procedure.

0040<figref idref="f0002">Fig. 2</figref> is a schematic, pictorial illustration of an assessment system 16, in accordance with a preferred embodiment of the present invention. In this preferred embodiment, prior to system 18 being used on a patient, each element 40 which may interfere with measurements of the position of probe 20 is preferably assessed by assessment system 16. To assess each element 40, the element is initially not present in space 60. One or more radiators 34 radiate a fundamental frequency for which assessment is desired. Alternatively, radiators 34 radiate a plurality of fundamental frequencies for which assessment is desired, one frequency at a time. Suitable frequencies are typically between about 200 Hz and about 12 kHz. The one or more fundamental frequencies radiated are preferably those used by radiators 34 for position sensing of probe 20 during a procedure. A receiving coil 22, fixed at any point in space 60, receives the radiated signals and conveys them to control unit 50. In a preferred embodiment of the present invention, receiving coil 22 comprises a sensor, such as a coil, Hall effect device or other antenna, dedicated to this function. In this case, the sensor in receiving coil 22 preferably is substantially identical to the sensor in probe 20. Alternatively, probe 20, temporarily fixed at any point in space 60, functions as receiving coil 22. Further alternatively, one of radiators 34, which is not being used for radiating during assessment, functions as receiving coil 22. For some applications, receiving coil 22 is oriented so that at least one of its sensors (such as a coil) is oriented to increase or maximize the strength of the signal received.

0041In the next step of the assessment process, element 40 is introduced into space 60, preferably near receiving coil 22. Each of the one or more fundamental frequencies radiated before the introduction of element 40 is again radiated by the same radiators 34. Receiving coil 22 receives the radiated signals and conveys them to control unit 50. In the case of a ferromagnetic element, the signal received is distorted by interference caused in part by phase shifting caused by the non-linearity of the element's hysteresis loop. This non-linearity of the hysteresis loop also induces higher harmonics of the radiated fundamental frequency. Each type of material generally has a unique hysteresis curve and therefore generates different interference and a corresponding different pattern of higher harmonics.

0042Reference is now made to <figref idref="f0003">Figs. 3A, 3B, and 3C</figref>, which show, for a single radiated fundamental frequency, a simplified example of received signals and a calculated "fingerprint," in accordance with a preferred embodiment of the present invention. <figref idref="f0003">Fig. 3A</figref> shows the signal received by receiving coil 22 prior to the introduction of element 40 in space 60 (the "clean received signal"), the amplitudes of which at F₀, 3F₀, and 5F₀ are 4.0, 0.0, and 0.0, respectively. <figref idref="f0003">Fig. 3B</figref> shows the signal received by receiving coil 22 after the introduction of element 40 in space 60 (the "distorted received signal"), the amplitudes of which at F₀, 3F₀, and 5F₀ are 4.3, 2.0, and 1.0, respectively. For each radiated fundamental frequency, control unit 40 analyzes the received signals, preferably by removing the clean received signal from the distorted received signal, such as by subtraction. The resulting signal, shown in <figref idref="f0003">Fig. 3C</figref>, represents the effect of electromagnetic interference of element 40 on the clean received signal. In this example, this "fingerprint" signal has amplitudes at F₀, 3F₀, and 5F₀ of 0.3, 2.0, and 1.0, respectively. Each type of material generally causes a unique resulting subtracted signal, which allows these signals to serve as fingerprint signals. It will be understood that the harmonic frequencies shown in <figref idref="f0003">Figs. 3B and 3C</figref> are illustrative only; in practice, among other differences, higher harmonics generally are present. As described hereinbelow, the ratios of the amplitudes at the different frequencies, rather than the absolute values of the amplitudes, are typically stored and used for correction during position determination.

0043Alternatively, in a preferred embodiments of the present invention, to assess each element, radiators 34 generate fundamental frequencies which are measured by a receiving coil 22 twice: first, with element 40 at a first location in space 60, and second, with element 40 at a second location in space 60. The distortion of a first signal received when the element is at the first location differs from the distortion of a second signal received when the element is at the second location. For each radiated fundamental frequency, control unit 50 preferably calculates the difference between the first and second signals, such as by subtraction. The resulting signal (or ratios of its amplitudes at different frequencies, as described hereinbelow), representing the effect of the interference of element 40 on a signal that would have been received in the absence of element 40, is generally unique for each element 40 and therefore serves as a fingerprint of the element. To reduce the effect of noise and other random variations in measurement, measurements may be made when the element is in more than two locations, and the results of the calculation averaged in order to generate the fingerprint.

0044For example, assume that the signal shown in <figref idref="f0003">Fig. 3B</figref> represents the first signal generated when element 40 is at the first location (as mentioned above, the amplitudes of this signal at F₀, 3F₀, and 5F₀ are 4.3, 2.0, and 1.0, respectively). Further assume that the amplitudes of the second signal (not shown) at F₀, 3F₀, and 5F₀, generated when element 40 is at the second location, are 4.6, 4.0, and 2.0, respectively. Subtracting the first signal from the second signal results in a fingerprint signal with amplitudes at F₀, 3F₀, and 5F₀ of 0.3, 2.0, and 1.0, respectively. Ratios of these values are stored and used for correction during a procedure, as described hereinbelow. Substantially the same ratios typically result even when measurements are made at multiple first and second locations.

0045The table below illustrates examples of three fingerprint signals (with arbitrary example values), in accordance with a preferred embodiment of the present invention. Element #1 represents the example element reflected in <figref idref="f0003">Figs. 3A, 3B, and 3C</figref>, and elements #2 and #3 represent two other example elements. Reference is made to the left portion of the table, labeled "Frequency." f_o represents the transmitted fundamental frequency, the multiples of f_o represent harmonic frequencies thereof, and the values represent relative amplitudes. <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="8"><colspec colnum="1" colname="col1" colwidth="21mm" /><colspec colnum="2" colname="col2" colwidth="10mm" /><colspec colnum="3" colname="col3" colwidth="10mm" /><colspec colnum="4" colname="col4" colwidth="10mm" /><colspec colnum="5" colname="col5" colwidth="10mm" /><colspec colnum="6" colname="col6" colwidth="10mm" /><colspec colnum="7" colname="col7" colwidth="14mm" /><colspec colnum="8" colname="col8" colwidth="13mm" /><thead><row><entry morerows="1" valign="top" /><entry namest="col2" nameend="col6" align="left" valign="top">Frequency</entry><entry namest="col7" nameend="col8" align="left" valign="top">Ratio</entry></row><row><entry valign="top">f_o</entry><entry valign="top">2f_o</entry><entry valign="top">3f_o</entry><entry valign="top">4f_o</entry><entry valign="top">5f_o</entry><entry valign="top">5f_o/3f_o</entry><entry valign="top">3f_o/f_o</entry></row></thead><tbody><row><entry>Element #1</entry><entry>0.2</entry><entry>--</entry><entry>2.0</entry><entry>--</entry><entry>1.0</entry><entry>0.5</entry><entry>10.0</entry></row><row><entry>Element #2</entry><entry>0.3</entry><entry>--</entry><entry>1.5</entry><entry>--</entry><entry>1.2</entry><entry>0.8</entry><entry>5.0</entry></row><row><entry>Element #3</entry><entry>0.1</entry><entry>--</entry><entry>1.0</entry><entry>--</entry><entry>0.6</entry><entry>0.6</entry><entry>10.0</entry></row></tbody></tgroup></table></tables>

0046Reference is now made to the right portion of the table, labeled "Ratio." In a preferred embodiment of the present invention, for each element for which assessment is performed, ratios between two or more harmonic frequencies, and between one or more harmonic frequencies and the transmitted fundamental frequency (f_o) are calculated, preferably by control unit 50 or, alternatively, by an external computer system (not shown). Each individual element assessed is uniquely characterized by its calculated ratio and/or ratios. Other possible algorithms, e.g., using combinations of two or more harmonic frequencies with the transmitted fundamental frequency and/or with each other, will be apparent to those skilled in the art having read the disclosure of the present patent application. Such other algorithms may be performed in order to produce alternative or additional values that uniquely identify different types of elements. Alternatively or additionally, assessment ratios and/or other calculated results are obtained for specific types of materials rather than specific types of elements. (Identifying the specific interfering material, without necessarily identifying the object comprising the material, is generally sufficient to perform the correction techniques described herein) These ratios and/or results of other calculations are preferably stored in a database to which control unit 50 has access, and are used during a procedure for position compensation calculations, as described hereinbelow. ("Database," as used in the specification and in the claims, is to be understood as including substantially any suitable repository, memory device or data structure that may be used for storing this information.)

0047In a preferred embodiment, the assessment procedure is performed in a location other than an operating room environment. For example, the assessment procedure is performed in a different location in the medical facility in which the procedure is to be performed. In this embodiment, preferably one or more radiators substantially identical to radiators 34 are provided. After assessment, the resulting assessment data and calculations are transferred to control unit 50 using methods obvious to those skilled in the art.

0048Alternatively or additionally, assessment is performed offsite, preferably by a third party. In this case, preferably a large number of elements and/or materials commonly used in performing medical procedures are assessed. These assessments are stored as a library in a repository, such as a database. (It is to be understood that substantially any suitable memory device and data structure may be used for storing the library.) This library is transferred to control unit 50, either before or after control unit 50 is delivered to its user, using methods known in the art. Alternatively, the library is transferred to a computer system or network to which control unit 50 has access during a procedure. It will be appreciated that onsite and offsite assessment can easily be combined, giving the user of system 18 the ability to add elements 40 not included in the available library or libraries. Other details of implementing such a library system will be evident to those skilled in the art, having read the disclosure of the present patent application.

0049Reference is again made to <figref idref="f0001">Fig. 1</figref>. During a procedure being performed on a patient, when element 40 is introduced into the vicinity of space 60, the measured position of probe 20 differs from its actual position because of the interference generated by element 40. In a preferred embodiment of the present invention, to compensate for this interference, the harmonics induced in the signal received by probe 20 are analyzed by control unit 50. Calculations are performed on the amplitudes of the harmonics, such as the determination of ratios between the amplitudes of two or more harmonics. The results of these calculations are compared with those stored in the memory of control unit 50 in order to identify the type of previously-assessed element and/or material that element 40 is or includes, respectively. Once the element and/or material is known, the distorting effect of element 40 on the amplitude of the fundamental signal of interest received by probe 20 is calculated, for example by using the ratio of the amplitude of one or more of the harmonics to the amplitude of the fundamental signal, as calculated during the assessment and stored in a database to which control unit 50 has real-time access. The amplitude of this distorting effect is subtracted from the measured amplitude of the received fundamental signal of interest. The remaining signal, no longer distorted by the presence of element 40, is used as an input by control unit 50 for calculating the absolute position of probe 20.

0050Reference is again made to the right portion of the table above, labeled "Ratio," in order to provide an example of the calculation of an interference correction, using simple ratios of two harmonics, in accordance with a preferred embodiment of the present invention. For example, assume that, in the signal received by probe 20, the relative amplitude of f_o is 4.1, the relative amplitude of 3f_o is 1.0, and the relative amplitude of 5f_o is 0.5. The ratio of 5f_o to 3f_o is therefore 0.5. By comparing this ratio to the stored values reflected in the table, control unit 50 determines that, in this case, element 40 is the same type as element #1. To determine the distorting effect of element 40 on the received signal, the value of 3f_o (1.0) is divided by the stored ratio of 3f_o/f_o (10.0), resulting in 0.1. This result is subtracted from the measured amplitude of f_o (4.1), resulting in a corrected amplitude of 4.0, which is no longer distorted by the presence of element #1. This amplitude is used as an input for calculating the accurate position of probe 20.

0051In a preferred embodiment of the present invention, only one harmonic frequency is used to determine the identity of element 40. This is possible, for example, when one or more elements 40 being used during a procedure produce very different relative amplitudes at a certain harmonic frequency.

0052In a preferred embodiment of the present invention, during procedures in which multiple elements 40 are introduced into space 60 during a procedure, control unit 50 identifies each element 40 individually by performing suitable calculations. In some cases, these calculations use a number of higher harmonics, which provide additional distinguishing characteristics for elements 40, in order to aid in distinguishing multiple elements 40.

0053In a preferred embodiment of the present invention, when two or more elements 40 are made of the same ferromagnetic material or combination of ferromagnetic materials, there is generally no need to distinguish between these elements during a procedure. The interfering effect of such elements is combined and uniquely identifiable by the fingerprint of their material. Detection of and corrections for such elements is therefore preferably performed as a group by system 18, substantially without modification to the procedures described hereinabove.

0054Preferred embodiments of the present invention have been described with respect to a location system 18 wherein radiators 34 transmit electromagnetic signals and probe 20 receives these signals. It is to be understood that the scope of the present invention includes application of the techniques described herein to location systems wherein the probe transmits electromagnetic signals and radiators receive these signals.

0055It is to be understood that preferred embodiments of the present invention are described herein with respect to invasive medical techniques by way of example only. The scope of the present invention includes application of the techniques described herein to electromagnetic locating and tracking systems used for any purpose whatsoever.

0056It is to be further understood that the techniques described herein are applicable to assessment, identification and compensation for particular elements, e.g., a particular tool of known composition, as well as to assessment, identification and compensation for particular materials, e.g., a common ferromagnetic material. An "interfering article," as used in the specification and the claims, is thus to be understood as including both a particular discrete element (such as a tool) or a particular material (such as steel). Techniques described specifically with respect to element 40, element 41, or any other interfering article may be interchanged, as appropriate.

0057It is still further to be understood that control unit 50 may comprise a general-purpose computer, which is programmed in software to carry out the functions described herein. The software may be downloaded to the computer in electronic form, over a network, for example, or it may alternatively be supplied to the computer on tangible media, such as a CD-ROM. Further alternatively, control unit 50 may be implemented in dedicated hardware logic, or using a combination of hardware and software elements.

0058It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes is limited by the appended claims.