Error compensation for device tracking systems employing electromagnetic fields

• US 6,201,987 B1
• Filed: 05/26/1998
• Issued: 03/13/2001
• Est. Priority Date: 05/26/1998
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Dumoulin, Charles Lucian
• Primary Examiner(s): Lateef, Marvin M.
• Assistant Examiner(s): SHAW, SHAWNA JEANNINE
• Application Number:
  US09/084,336
• Time in Patent Office: 1,022 Days
• Field of Search: 600/424, 600/423, 600/410, 128/899, 324/207-1-2, 324/207.22, 324/260, 324/309, 324/318
• US Class Current: 600/424
• CPC Class Codes: G01R 33/285 Invasive instruments, e.g. catheters or biopsy needles, specially adapted for tracking, guiding or visualization by NMR
• CPC Class Codes: G01R 33/56518 due to eddy currents, e.g. caused by switching of the gradient magnetic field

Abstract

The present invention is capable of more accurately tracking an invasive device in a subject'"'"'s body in the presence of eddy current inducing structures. Current patterns applied to the tracking system'"'"'s transmit coils are modified to compensate for the effect of the eddy currents. The current supplied to the coils is a linear combination of the current needed to create the desired electromagnetic field in the region of interest; and one or more error terms. These terms are determined experimentally during system calibration and are mathematically modeled as a series of exponential functions having a given amplitude and time constant. The error terms in the current applied to the transmit coils cancel the magnetic fields created by eddy currents within the tracking region and result in an actual electromagnetic field which is close to the desired ideal electromagnetic field. The fidelity of the electromagnetic field is further increased by reducing eddy currents within the eddy current inducing structures. This is done by constructing shield coils which are placed between the transmit coil and the eddy current inducing structures. These shield coils create canceling magnetic fields within the eddy current inducing structures without substantially altering the electromagnetic fields in the region over which the invasive device is tracked.

Claims

1. A system for determining a position of a device within a selected tracking volume, in the presence of at least one eddy current inducing structure, the system comprising:

a) at least one transmit coil at a known location capable of creating a magnetic field within the selected tracking volume;

b) at least one receive coil incorporated into the device being tracked for detecting changes in the magnetic field within the selected tracking volume;

c) system electronics operating to supply a current of a selected amplitude, frequency and duration to the at least one transmit coil, and capable of receiving signals detected by the at least one receive coil; and

d) a current correction means capable of modifying the current supplied to the at least one transmit coil by the electronics subsystem, said current correction means modifying said current by combining an ideal current with a compensation current, said compensation current being determined so that the resulting magnetic field within the selected tracking volume is substantially free of deviations from an ideal magnetic field caused by eddy currents in the at least one eddy current inducing structure.

2. The system for determining a position of a device within a selected tracking volume of claim 1 wherein the current correction means defines the compensation current mathematically by at least one exponential function having a selected amplitude and time constant.

3. The system for determining a position of a device within a selected tracking volume of claim 2 wherein current correction means operates to determine the amplitude and time constant of the compensation current iteratively by providing a plurality of different current waveforms measuring results and choosing the waveform with the best result.

4. The system for determining a position of a device within a selected tracking volume of claim 1 wherein the eddy current inducing structure is part of a Magnetic Resonance Imaging system.

5. A system for determining a position of a device within a selected tracking volume, in the presence of at least one eddy current inducing structure, the system comprising:

a) at least one transmit coil at a known location capable of creating a magnetic field within the selected tracking volume;

b) at least one receive coil incorporated into the device being tracked for detecting changes in the magnetic field within the selected tracking volume;

c) system electronics operating to supply a current of a selected amplitude, frequency and duration to the at least one transmit coil, and capable of receiving signals detected by the at least one receive coil; and

d) at least one respective shield coil placed in series with at least one corresponding transmit coil for conducting current to null the magnetic fields created by the at least one transmit coil in the at least one eddy current inducing structure.

6. The system for determining a position of a device within a selected tracking volume of claim 5 wherein the eddy current inducing structure is part of a Magnetic Resonance Imaging system.

7. The system for determining a position of a device within a selected tracking volume of claim 5 wherein the shield coil is driven independently of the corresponding transmit coil.

8. The system for determining a position of a device within a selected tracking volume of claim 5 wherein the shield coil has a planar geometry.

9. The system for determining a position of a device within a selected tracking volume of claim 5 wherein the shield coil has a complex geometry matched to the geometry of the eddy current inducing structure(s).

10. A system for determining a position of a device within a selected tracking volume, in the presence of at least one eddy current inducing structure, the system comprising:

a) at least one transmit coil capable of creating a magnetic field within the selected tracking volume;

b) at least one receive coil incorporated into the device being tracked for detecting changes in the magnetic field within the selected tracking volume;

c) a system electronics subsystem capable of supplying a current of a selected amplitude, frequency and duration to said at least one transmit coil, and capable of receiving signals detected by the at least one receive coil;

d) a current modification means capable of modifying the current supplied to the at least one transmit coil by the electronics subsystem so that the resulting magnetic field within the selected tracking volume is substantially free of deviations from an ideal magnetic field caused by eddy currents within the eddy current inducing structure; and

e) at least one respective shield coil placed near at least one corresponding transmit coil for conducting current to null the magnetic fields created by the at least one transmit coil in the eddy current inducing structure.