阅读说明：本技术 用于基于磁的体内探头跟踪系统的薄型定位垫 (Low profile location pad for a magnetic based intra-body probe tracking system ) 是由 A·戈瓦里 Y·埃普拉思 A·C·阿尔特曼恩 V·格林尔 于 2014-12-23 设计创作，主要内容包括：本发明的主题是“用于基于磁的体内探头跟踪系统的薄型定位垫”。本发明公开了定位垫,该定位垫包括多个场发生器和具有平坦表面的外壳。多个场发生器固定到外壳并被配置成生成具有与平坦表面垂直的相应轴的相应磁场。(The subject of the present invention is a "low profile location pad for a magnetic-based intrabody probe tracking system". A location pad includes a plurality of field generators and a housing having a planar surface. A plurality of field generators are fixed to the housing and configured to generate respective magnetic fields having respective axes perpendicular to the planar surface.)

1. A kind of locating pad device:

the positioning pad device comprises a positioning pad, a body probe and a console;

wherein the placemat includes a housing that includes at least one planar surface;

the location pad further comprises at least five field generators fixed to the housing, the at least five field generators configured to generate respective magnetic fields having respective axes perpendicular to the planar surface of the location pad;

wherein a plurality of the at least five field generators are each individually surrounded by a respective concave groove, and wherein the grooves are at least partially filled with an elastic material, which thus surrounds the respective field generator and is adapted to suppress frequency resonances;

wherein the at least five field generators are spaced in two different dimensions relative to the planar surface of the location pad such that not all of the at least five field generators are arranged in a straight line;

wherein the placemat comprises a plurality of substantially rigid, planar, parallel rows;

wherein the at least five field generators are distributed in the plurality of rows, wherein at least some of the plurality of rows each hold a plurality of field generators;

wherein the rows are pivotable relative to other adjacent rows such that the plurality of rows of the placemat collectively form a concave surface;

wherein the body probe is a tool for insertion into a patient and comprises a position sensor capable of generating electrical signals in response to magnetic fields generated by the at least five field generators; and

wherein the console is operably linked to the location pad and operably linked to the body probe and adapted to determine the position of the position sensor based on electrical signals received from the position sensor.

2. The placemat device of claim 1, wherein:

the plurality of rows each having an elongated rectangular shape, each row including two longer sides and two shorter sides;

the plurality of rows being positioned side-by-side such that the longer side of each row is adjacent the longer side of each adjacent row; and

each row has a plurality of field generators positioned along its length.

3. The placemat device of claim 1, wherein the plurality of concave grooves are circular, and wherein the respective field generators are in the center of the circular grooves.

4. The placemat device of claim 1, wherein the placemat housing has a thickness of no more than 5 millimeters.

5. The location pad device of claim 1, wherein the at least five field generators comprise coils having windings parallel to the surface.

6. A method for location tracking, the method comprising:

providing a placemat device according to claim 1;

driving a plurality of field generators coupled to a planar surface near a patient's body with a plurality of respective drive signals so as to cause the at least five field generators to generate respective magnetic fields having respective axes perpendicular to the planar surface;

measuring at least one electrical signal induced by the magnetic field in the position sensor of the body probe when the body probe is inserted into the patient's body; and

estimating a position of the probe in the body based on the electrical signals.

7. The method of claim 6, wherein estimating the position of the probe comprises calculating an average magnitude of the electrical signal, and estimating a distance of the probe from the flat surface from the average magnitude.

8. A method according to claim 6, wherein estimating the position of the probe comprises calculating magnitudes of a plurality of components of the electrical signal respectively induced by the magnetic fields generated by the plurality of field generators, and estimating a lateral position of the probe relative to the at least five field generators from the average magnitudes.

9. The method of claim 8, wherein driving the at least five field generators comprises generating the plurality of drive signals having different respective frequencies, and wherein calculating the magnitude comprises distinguishing the components of the electrical signal by discriminating the different frequencies.

10. The method of claim 8, wherein estimating the position of the probe comprises refining the position of the probe by performing an iterative position estimation process that uses at least the lateral position as an initial condition.

11. The placemat device of claim 1,

wherein the location pad field generator comprises at least five coils;

wherein the console includes at least one of a storage medium and a programmable digital hardware component;

wherein the storage medium or programmable digital hardware component holds instructions for controlling the coils; and

wherein the instructions, when executed by the console, cause the at least five coils of the placemat to each generate a signal at a different respective frequency, the at least five coils each being driven by an Alternating Current (AC) signal having a different respective frequency.

12. The location pad device of claim 1, wherein the at least five field generators are located in a grid arrangement comprising a plurality of rows and a plurality of columns, wherein each row comprises a plurality of field generators, and wherein each column comprises a plurality of field generators.

13. The placemat device of claim 1,

wherein the location pad arrangement comprises a patient table and an MRI scanner;

wherein the positioning pad is located on a top surface of the patient table and the patient table is shaped to support a human patient thereon;

wherein the patient table is movable into the MRI scanner; and

wherein the MRI scanner and the location pad are adapted to operate simultaneously.

14. The placemat device of claim 1, wherein the position sensor of the body probe is a single-axis sensor that includes a sensor coil.

15. The placemat device of claim 1, wherein the body probe includes a catheter and the position sensor is located at a distal end of the catheter.

16. A kind of location pad:

the placemat includes a housing that includes at least one planar surface;

the location pad further comprising at least five field generators fixed to the housing, the at least five field generators configured to generate respective magnetic fields having respective axes perpendicular to the planar surface of the location pad,

wherein the at least five field generators are arranged in two different dimensions with respect to the planar surface of the location pad such that not all of the at least five field generators are arranged in a straight line;

wherein the location pad comprises a plurality of substantially rigid planar rows that are parallel to each other and collectively form the planar surface;

wherein the at least five field generators are located on a plurality of different rows, wherein at least some of the rows each hold a plurality of field generators; and

wherein the plurality of distinct rows are pivotable relative to other adjacent rows such that the plurality of distinct rows of the placemat collectively form a concavity.

17. The placemat of claim 16,

wherein each of the plurality of different rows is elongate and comprises a plurality of field generators; and

wherein the plurality of field generators in each row are arranged in a row.

18. A kind of location pad:

the placemat includes a housing that includes at least one planar surface;

the location pad further comprising at least five field generators fixed to the housing, the at least five field generators configured to generate respective magnetic fields having respective axes perpendicular to the planar surface of the location pad,

wherein the at least five field generators are arranged in two different dimensions with respect to the planar surface of the location pad such that not all of the at least five field generators are arranged in a straight line;

wherein the location pad comprises a plurality of substantially rigid planar rows that collectively form the planar surface;

wherein the at least five field generators are located on a plurality of different rows, wherein at least some of the plurality of rows each hold a plurality of field generators;

wherein the plurality of rows are pivotable relative to other adjacent rows such that the plurality of rows of the placemat collectively form a concavity; and

wherein a plurality of the at least five field generators are each individually surrounded by a respective concave groove, and wherein each groove is at least partially filled with an elastic material, the elastic material thereby surrounding the respective field generator and being adapted to suppress frequency resonances.

19. A kind of locating pad device:

the placemat device includes the placemat of claim 18, a body probe, and a console.

20. A kind of locating pad device:

the placemat device includes the placemat of claim 18, a body probe, and a console;

wherein the location pad arrangement comprises a patient table and an MRI scanner;

wherein the positioning pad is located on a top surface of the patient table and the patient table is shaped to support a human patient thereon;

wherein the patient table is movable into the MRI scanner; and

wherein the MRI scanner and the location pad are adapted to operate simultaneously.

Technical Field

The present invention relates generally to intra-body position tracking, and in particular to magnetic-based position tracking of intra-body probes.

Background

The position of an intrabody probe, such as a catheter, within a patient's body can be tracked using magnetic position tracking techniques. For example, U.S. patent application 2007/0265526, the disclosure of which is incorporated herein by reference, describes a magnetic position tracking system for performing medical procedures on a patient. The patient rests on the upper surface of a table that includes a location pad that rests on the upper surface of the table beneath the patient. The location pad includes one or more field generators that are operated to generate respective magnetic fields and are arranged such that the thickness dimension of the location pad is no greater than 3 centimeters. The position sensor is fixed to an invasive medical device for insertion into a body of a patient and is arranged to sense a magnetic field in order to measure a position of the medical device within the body.

Magnetic Resonance Imaging (MRI) is an imaging technique for visualizing tissue, particularly soft tissue, of a patient. This technique relies on exciting nuclei from their equilibrium state (usually hydrogen nuclei) and measuring the resonant radio frequency signals emitted by the nuclei as they relax to the equilibrium state. The measured resonant radio frequency signals are used to create high quality tissue images. A medical practitioner may use MRI in conjunction with other medical procedures.

Disclosure of Invention

Embodiments of the present invention provide a location pad that includes a housing having a planar surface and a plurality of field generators. A plurality of field generators are fixed to the housing and configured to generate respective magnetic fields having respective axes perpendicular to the planar surface.

In some embodiments, the planar surface lies in a plane. In other embodiments, the planar surface is curved. In other embodiments, the housing has a thickness of no greater than 5 millimeters. In some embodiments, the field generator comprises a coil having windings parallel to the surface. In other embodiments, the housing comprises a resilient material configured to retain the field generator and suppress resonance in the field generator.

There is also provided, in accordance with an embodiment of the present invention, a method for preparing a placemat, the method including providing a housing having a planar surface. A plurality of field generators are fixed to the housing such that the field generators generate respective magnetic fields having respective axes perpendicular to the planar surface.

There is additionally provided, in accordance with an embodiment of the present invention, a method for position tracking, the method including driving a plurality of field generators coupled to a planar surface proximate a patient's body with a plurality of respective drive signals so as to cause the field generators to generate respective magnetic fields having respective axes perpendicular to the planar surface. At least one electrical signal induced by a magnetic field in a position sensor coupled to an intrabody probe inserted into a patient's body is measured. The position of the probe within the body is estimated based on the electrical signals.

In some embodiments, the position sensor comprises a single axis sensor. In other embodiments, estimating the position of the probe includes calculating an average magnitude of the electrical signal, and estimating a distance of the probe from the flat surface based on the average magnitude.

In some embodiments, estimating the position of the probe includes calculating magnitudes of a plurality of components of electrical signals respectively induced by magnetic fields generated by the plurality of field generators, and estimating a lateral position of the probe relative to the field generators from the averaged magnitudes. In other embodiments, driving the field generator comprises generating a plurality of drive signals having different respective frequencies, and calculating the magnitude comprises distinguishing among the components of the electrical signal by discriminating among the different frequencies. In other embodiments, estimating the position of the probe includes refining the position of the probe by performing an iterative position estimation process that uses at least the lateral position as an initial condition.

Drawings

The invention will be more fully understood from the following detailed description of the embodiments taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a magnetic catheter tracking system collocated with a Magnetic Resonance Imaging (MRI) system in accordance with an embodiment of the present invention;

FIGS. 2A and 2B are schematic illustrations of a placemat according to an embodiment of the present invention; and

FIG. 3 is a flow chart that schematically illustrates a method for estimating the position of a catheter relative to a location pad, in accordance with an embodiment of the present invention.

Detailed Description

Overview

Intracorporeal probes, such as catheters, are used in a variety of therapeutic and diagnostic medical procedures. The probe is inserted into a patient's living body and navigated to a target area within a body cavity to perform a medical procedure. In magnetic field based position tracking systems, an external magnetic field is applied to the patient's body. A sensor mounted near the distal tip of the catheter responds to the magnetic field by generating an electrical signal. The tracking system uses the signals to locate the position and orientation of the catheter within the patient's body. The magnetic field is typically generated by a plurality of field generators, such as field generating coils.

Embodiments of the invention described herein provide a small and flat placemat configuration. The disclosed location pad includes a plurality of magnetic field generators (e.g., planar coils) mounted to a surface. The axes of the field generators are all perpendicular to the surface. When the surface is perfectly flat, the axes of the field generators are parallel to each other.

The resulting location pad has a low profile and can be easily placed under a patient. In some embodiments, the location pad surface is slightly contoured, i.e., slightly out of plane, for example, to fit an MRI scanner.

In some embodiments, the field generators in the location pad are driven with Alternating Current (AC) drive signals having different frequencies so that the signals induced in the sensors at the distal tip of the catheter are distinguishable from each other. The use of a field generator with parallel axes facilitates mathematical modeling of the resulting magnetic field that simplifies the calculation of the position and orientation of the distal tip of the catheter based on the catheter sensor output.

In an example implementation, the probe position is estimated in a two-stage process. In a first phase, the probe height above the plane of the location pad is estimated from the absolute magnitude of the composite signal sensed by the position sensor in the probe. The lateral position of the probe relative to the location pad can then be determined by analyzing the relative magnitudes of the different frequencies in the composite signal. This initial estimate may itself be output, or it may serve as a starting point for a more accurate iterative position estimation process.

In some embodiments, the field generator comprises a coil embedded in silicone within the housing so as to suppress audio resonances that may be generated by the MRI scanner. In other embodiments, a transformer is used for impedance matching between a low impedance amplifier used to drive a signal into a high impedance magnetic coil.

In general terms, the improved placemat configuration described herein allows the magnetic probe tracking system to be operated while the patient is within the second magnetic environment of the MRI scanner. The disclosed location pad is suitable for use with a single axis position sensor in a probe so that a simpler and thinner probe may be used in a medical procedure.

Description of the System

Fig. 1 is a schematic illustration of a system 20 for magnetic catheter tracking in conjunction with Magnetic Resonance Imaging (MRI) according to an embodiment of the present invention. The system 20 includes an MRI scanner 22, an intrabody probe 24, such as a catheter, and a console 26. The probe 24 includes a sensor (as will be shown later in fig. 2A) at the distal tip 34 of the catheter 24 for tracking the position of the catheter 24 within the body of the patient 32.

For example, catheter 24 may be used to map electrical potentials in a chamber of heart 28 of patient 32, where a plurality of electrodes are disposed near distal tip 34 of catheter 24, the plurality of electrodes contacting tissue of the heart cavity at multiple points. In alternative embodiments, catheter 24 may be used for other therapeutic and/or diagnostic functions in the heart or other body organs, mutatis mutandis.

An operator 30, such as a cardiologist, percutaneously inserts the probe 24 through the vascular system of a patient 32 such that the distal tip 34 of the probe enters a body cavity, which is assumed herein to be the heart cavity. Distal tip 34 is shown and more particularly described with respect to fig. 2A.

Console 26 determines the orientation and position coordinates of distal tip 34 of catheter 24 inside heart 28 using magnetic position sensing. For sensing, the console 26 operates a drive circuit 36, which drive circuit 36 drives one or more magnetic field generators 39 in a location pad 38 as shown in the inset and in cross-section below the torso of the patient on an underlying table 37. In response to the magnetic field generated by location pad 38, a position sensor mounted in distal tip 34 generates an electrical signal, thereby causing console 26 to determine the position and orientation of distal tip 34 relative to location pad 38, and thus within the heart of patient 32.

The MRI scanner 22 includes magnetic field coils 29, including field gradient coils, which together generate a spatially shifted magnetic field. The spatially shifted magnetic field provides spatial localization for Radio Frequency (RF) signals generated by the scanner. Further, the scanner comprises a transmit/receive coil 31. In the transmission mode, the coil 31 radiates radio frequency energy to the patient 32, which interacts with the nuclear spins of the patient tissue and thereby re-adjusts the magnetic moments of the nuclei away from their equilibrium position. In the receive mode, the coil 31 detects radio frequency signals received from the patient tissue as the tissue nuclei relax to their equilibrium state.

In the embodiment shown in fig. 1, the processor 40 has dual functionality. First, in response to the magnetic field generated by location pad 38, processor 40 has interface circuitry (not shown) to receive electrical signals induced in the sensor at catheter distal tip 34 and use the received electrical signals to locate the catheter within the patient's body.

Second, the processor 40 operates the MRI scanner 22 by using circuitry to control the MRI coils 29, including forming the required magnetic field gradients, and other circuitry to operate the transmit/receive coils 31 around the patient 32. The processor 40 uses the signals received by the coils 31 to obtain MRI data of the heart 28 of the patient 32, or at least of the heart chamber to be imaged. Processor 40 uses this data to display an image 44 of heart 28 to operator 30 on display 42. Alternatively, the functions of the processor 40 may be divided between two processors, one to manage the magnetic position tracking system and one to manage the MRI scanner.

In some embodiments, the position of the catheter obtained by the magnetic tracking system may be superimposed on an image 44 of the heart 28 obtained by the MRI scanner 22 on the display 42. In other embodiments, the operator 30 may manipulate the image 44 using one or more input devices 46.

The processor 40 may also be configured to reduce any magnetic interference or coexistence effects of the respective MRI system and magnetic catheter tracking system, which may, for example, reduce system performance. In other words, the processor 40 is configured to compensate for any coupling effects, for example, between the magnetic fields generated by the MRI coils 29 and 31 used in the MRI scanner 22 and the magnetic generators 39 in the location pads 38 for the magnetic catheter tracking system.

The processor 40 typically comprises a general purpose computer that is programmed with software to perform the functions described herein. For example, the software may be downloaded to processor 40 in electronic form, over a network, or it may be disposed on non-transitory tangible media, such as optical, magnetic, or electronic memory media. Alternatively, some or all of the functions of the processor 40 may be performed by dedicated or programmable digital hardware components, or by using a combination of hardware and software elements.

The magnetic catheter tracking system may be implemented as a CARTO XP EP navigation and ablation system available from Biosense Webster, inc (diamond bar, california), with appropriate modifications to perform the procedures described herein.

The embodiment shown in fig. 1 is only for conceptual clarity purposes and in no way limits the embodiments of the invention. The MRI scanner 22 and magnetic catheter tracking system may have separate processors for each system rather than being shared as in the embodiment shown in system 20. A single or separate display may be used for both the MRI scanner and the catheter tracking system.

MRI compatible location pad

Fig. 2A is a schematic illustration of a placemat 38 in accordance with an embodiment of the present invention. Location pad 38 includes a plurality of magnetic field generators 39 arranged in an array as shown in the transverse XY plane of FIG. 2A. Twelve generators 39 of the same size are shown in the embodiment of fig. 2A. The array is held in a housing which may be made of any suitable material, such as various plastics. The X-Y-Z coordinate axis is shown on the lower left side of the housing with the location pad 39 having a thickness t.

Each generator 39 comprises a planar coil 100 with windings parallel to the X-Y plane. In some embodiments, the coil 100 is surrounded by a trench 105. The coil may be formed of any suitable material, such as copper. When a signal, typically a current, is applied to the coil 100, the coil 100 generates a magnetic field B oriented along the Z-axis and perpendicular to the coil plane (the X-Y plane) in response to the applied signal. In this example, the axes of all magnetic fields are parallel to each other and perpendicular to the surface of the location pad. The composite magnetic field in the region above the location pads comprises a superposition of the magnetic fields B from the plurality of field generators.

As shown in the inset of fig. 1, when the patient 32 is lying on the location pad 38 and the catheter 24 is navigated to a target region within the patient's body above the location pad, the magnetic sensor coil 120 proximate the distal tip 34 of the catheter generates an electrical signal, typically a voltage, in response to the composite magnetic field. The sensor coil 120 is assumed herein to be a single axis sensor at the distal tip 34 of the catheter 24. (alternatively, catheter 24 may include a multi-axis position sensor, such as a sensor including three mutually orthogonal coils.)

In the embodiments presented herein, the positioning pad is configured to be placed between the patient and the top surface of table 37, e.g., with the patient lying on top of the positioning pad. The lateral dimension of the location pad is typically limited to the lateral dimension of the patient table 37 that is moved into the MRI scanner. The thickness t of the location pad is often configured to not exceed 5 mm. In this way, the MRI scanner does not interfere with or interfere with the location pad 38 of the magnetic tracking system, or vice versa.

Processor 40 in system 20 is configured to use the electrical signals sensed by sensor 120 to calculate a position vector of sensor 120 relative to the X-Y-Z origin of axesAnd orientation vector. Position vectorIs a vector from the origin to the sensor 120. Orientation vectorIs an axial vector through the conduit 24. The location of the origin of the X-Y-Z coordinate system shown in FIG. 2A is for conceptual clarity purposes only and is in no way limiting embodiments of the invention. The origin may be defined in any suitable position relative to the location pad.

Fig. 2B is a schematic illustration of an alternative embodiment of a placemat according to an embodiment of the present invention. In this embodiment, although each row 140 of coils 100 is planar, all rows lie on a slightly curved surface. Also in this configuration, the axis of the magnetic field generated by the coil 100 is perpendicular to the surface of the location pad. For example, the curved configuration of fig. 2B may be used to conform to the bore of the MRI scanner 22.

On the rightmost row shown in fig. 2B, the magnetic field generator 39 has a cover 150 covering the coil 100, the cover 150 being formed of any suitable material, such as plastic, so as to cover the entire array.

As will be described below, in the configuration of fig. 2B, the magnetic fields B generated by the coils 100 are nearly parallel to each other. It was found that any small parallel deviation of the magnetic field B due to the bending shown in fig. 2B has negligible impact on the accuracy of the catheter position tracking system.

When location pad 38 is used in an MRI environment, a large magnetic MRI coil generates a very large magnetic field, such as a magnetic field in the range of 0.5-3 Tesla (Tesla). Magnetic catheter tracking systems such as the CARTO system use magnets with AC frequencies in the audio frequency range. Thus, when the magnetic coil 100 is driven with audio frequencies in the presence of a large MRI magnet, the small magnetic coil 100 may resonate at audio frequencies, for example, from 19kHz to 22 kHz. Thus, in some embodiments, the coil is potted in an elastomeric material, such as silicone or any other suitable material, in order to suppress or otherwise prevent resonance. For example, the trench 105 and any other areas surrounding the coil 100 may be filled with silicone or any other suitable material that may suppress audio resonance of the coil 100 in the location pad 38 in an MRI environment.

For example, small magnetic coils 100 may also exhibit large impedances of about 600 ohms due to skin effects at these frequencies and at small coil sizes 100. These coils are driven by a driver amplifier 36, the driver amplifier 36 typically having an output characteristic impedance of about 6 ohms. In some embodiments, to drive these high impedance coils, a transformer with an impedance to voltage ratio may be used in the driver amplifier 36 to overcome the impedance mismatch from 6 ohms to 600 ohms.

The array configuration of fig. 2A and 2B is shown for visual clarity purposes only and is in no way limiting of embodiments of the present invention. Any suitable number of magnetic coils 100 in any suitable configuration may be used. The coil 100 is not limited to a flat circular shape, but may be any suitable shape.

Calculating catheter position and orientation with MRI compatible location pads

As described above, in the disclosed embodiment, the magnetic fields generated by the coils 100 of the location pad 38 are parallel to each other and perpendicular to the surface of the location pad. Thus, the magnitude of the composite magnetic field varies with the Z coordinate, but is substantially constant as a function of X and Y. Thus, when using a single axis sensor (e.g., sensor 120 in fig. 2A), the magnitude of the composite signal sensed by the sensor strongly indicates the height of distal tip 34 above location pad 38, but the magnitude of the composite signal is insensitive to the lateral position of the distal tip relative to the location pad. This insensitivity may lead to inaccuracies or even lack of translation of the position and orientation estimation process performed by the processor 40.

One possible solution to this problem is to use more complex position sensors in the catheter, such as a three-axis sensor. Such an arrangement is described in the above-cited US patent application US 2007/0265526. However, this solution is complex and increases the catheter diameter.

In some embodiments, processor 40 estimates the position and orientation of catheter 24 is a two-stage process. This process, described below, allows a single axis sensor to be used in conjunction with a thin location pad. The disclosed process is computationally simple and converges quickly and efficiently. Generally, while a total of five coils 100 is sufficient to provide accurate positioning, a greater number of coils (e.g., twelve coils as shown in fig. 2A and 2B) is preferred for greater accuracy and robustness.

In some embodiments, coils 100 are driven with AC signals having different respective frequencies such that the signals induced in the single-axis sensor coils can be distinguished from one another.

Fig. 3 is a flow chart that schematically illustrates a method for estimating the position of distal tip 34 of catheter 24 relative to location pad 38, in accordance with an embodiment of the present invention. At positioning step 200, a location pad 38 is placed under the patient 32. At an insertion step 210, catheter 24 is inserted into patient 32. In a generating step 220, the coils 100 are driven with respective AC drive signals having different frequencies.

In a measurement step 230, processor 40 measures the voltage signal induced in catheter sensor 120 in response to the magnetic field. In a first estimation step 240, the processor 40 estimates the initial Z-distance of the sensor from the location pad by calculating the average (e.g., RMS) strength of the voltage signal, which is proportional to the average magnitude of the composite magnetic field produced by the coil 100.

Processor 40 estimates the initial X-Y position of the sensor relative to location pad 38 by analyzing the relative amplitudes of the various frequency components in the induced voltage signal, at a second estimation step 250. Because each signal component has a different frequency, the processor 40 is able to distinguish among the signal components induced by the different coils 100. To this end, the processor 40 may filter the signals sensed by the sensors 120 using suitable digital filtering.

At an iterative estimation step 260, processor 40 refines the initial X-Y position estimate from the sensor of step 250 and the Z position estimate from the sensor of step 240. In general, the processor 40 implements an iterative position estimation process that uses the initial X-Y-Z coordinates (the output of steps 240 and 250) as an initial condition. Due to the relatively accurate initial conditions, the iterative process converges rapidly and reliably to the accurate X-Y-Z coordinates of the distal tip of the catheter.

It is to be understood, therefore, that the foregoing examples are given by way of illustration and that the invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference into this patent application are to be considered an integral part of the patent application, but the definitions in this specification should only be considered if the manner in which any term is defined in these incorporated documents conflicts with the definitions explicitly or implicitly set forth in this specification.