Metal immunity in a reverse magnetic system

1. A method for tracking a metal tool, for use during a medical procedure, comprising:

providing a map of distortion of a position-indicative magnetic field caused by a metal tool in a frame of reference of the tool by applying a calibration procedure to generate a map of the distortion associated with the metal tool, wherein the calibration procedure comprises measuring the distorted field, measuring an undistorted field, and subtracting the distorted field from the undistorted field so as to derive the map of the distortion;

providing a metal tool having a transmitter for transmitting a position-indicative magnetic field;

fixing at least one field sensor within a frame of reference of a medical procedure;

tracking the metal tool as the metal tool moves within the frame of reference of the medical procedure;

sensing a first distorted magnetic field transmitted from the metal tool, the first distorted magnetic field comprising the position-indicative magnetic field subject to the distortion caused by the metal tool within the frame of reference of the medical procedure;

determining estimated coordinates of the metal tool based on the first distorted magnetic field;

estimating the distortion component present in the magnetic field caused by the metal tool being tracked using the map and the estimated coordinates;

determining a second distorted magnetic field by subtracting the estimated distortion component from the first distorted magnetic field; and

computing corrected coordinates of the metal tool based on the second distorted magnetic field.

2. The method of claim 1 , wherein computing the corrected coordinates comprises replacing the estimated coordinates with the corrected coordinates and performing an iterative calculation to determine an accurate value of the corrected coordinates.

3. The method of claim 1 , wherein the calibration procedure comprises:

placing the metal tool in a jig and measuring the distorted field; and

placing the transmitter in the jig, without the metal tool, and measuring an undistorted field.

4. The method of claim 1 , wherein providing the map of distortion comprises providing a table of data points representing the distortion at given spatial coordinates.

5. The method of claim 1 , wherein providing the map of distortion comprises providing a table of parameters of a mathematical model describing the distortion.

6. The method of claim 1 , wherein sensing the distorted magnetic field comprises measuring an amplitude and a direction of the field so as to determine location and orientation coordinates of the metal tool.

7. The method of claim 1 further comprising generating a graphical output indicative of the position of the metal tool, based on the corrected coordinates.

8. The method of claim 1 , wherein the metal tool is adapted for insertion into a body of a patient.

9. Apparatus for tracking a metal tool for use during a medical procedure, comprising:

a first processor configured to provide a map of distortion of a position-indicative magnetic field caused by a metal tool in a frame of reference of the tool by applying a calibration procedure to generate a map of the distortion associated with the metal tool, wherein the calibration procedure comprises measuring the distorted field, measuring an undistorted field, and subtracting the distorted field from the undistorted field so as to derive the map of the distortion;

a transmitter, adapted to be fixed to a metal tool, the transmitter having a transmit antennae operative to transmit a position-indicative magnetic field and a microcontroller adapted to drive the transmit antennae;

at least one field sensor, adapted to sense a distorted magnetic field comprising the position-indicative magnetic field subject to distortion caused by the metal tool within a frame of reference; and

a second processor configured to estimate coordinates of the metal tool as the metal tool moves within the frame of reference, based on a first distorted magnetic field and to compute corrected coordinates of the metal tool responsively to the estimated coordinates and to a map of the distortion caused by the metal tool;

wherein the second processor is adapted to estimate the distortion component present in the magnetic field caused by the metal tool being tracked using the map and the estimated coordinates, and to determine a second distorted magnetic field by subtracting the estimated distortion component from the first distorted magnetic field, wherein the corrected coordinates are computed based on the second distorted magnetic field.

10. The apparatus of claim 9 , wherein the processor is adapted to replace the estimated coordinates with the corrected coordinates and to perform an iterative calculation to determine an accurate value of the corrected coordinates.

11. The apparatus of claim 9 , wherein the map of distortion is a table of data points representing the distortion at given spatial coordinates.

12. The apparatus of claim 9 , wherein the map of distortion is a table of parameters of a mathematical model describing the distortion.

13. The apparatus of claim 9 , wherein one of the microcontroller and the processor is adapted to store the map of distortion.

14. The apparatus of claim 9 , wherein the field sensor is adapted to sense the distorted magnetic field by measuring an amplitude and a direction of the field so as to determine location and orientation coordinates of the metal tool.

15. The apparatus of claim 9 , wherein the processor is adapted to generate a graphical output indicative of the position of the metal tool.

16. The method of claim 1 , wherein sensing the distorted magnetic field transmitted from the metal tool comprises sensing a parasitic field in the distorted magnetic field caused by eddy currents induced in the metal tool.

17. The method of claim 16 , further comprising fixing an antenna, a power coil, and a communication coil to a flexible printed circuit board, and a processor to the tool.

18. The method of claim 1 , wherein sensing the distorted magnetic field transmitted from the metal tool comprises sensing a parasitic field in the distorted magnetic field caused by eddy currents induced in the metal tool and sensing a parasitic field in the distorted magnetic field caused by eddy currents induced in at least a second metal object different than the metal tool.

19. The method of claim 18 , wherein the step of sensing the parasitic field in the distorted magnetic field caused by eddy currents induced in at least the second metal object different than the metal tool is sensing the parasitic field in the distorted magnetic field caused by eddy currents induced in at least an implant or a second metal tool.