Calibration and compensation for errors in position measurement

1. A method for sensing a position of an object in a body, comprising the steps of:

positioning a probe in the body;

making measurements of mapping electrical currents passing between at least one first electrode on the object and a plurality of second electrodes on a surface of the body;

calibrating the measurements so as to compensate for one or more non-ideal features of the measurements including effects of system-dependent electrical coupling to one or more medical devices other than the first electrode and the second electrodes; and

computing the position of the probe in the body using the calibrated measurements.

2. The method according to claim 1 , wherein calibrating the measurements comprises calculating the effects of system-dependent electrical coupling, and calculating mapping generator-induced crosstalk.

3. The method according to claim 2 , wherein calculating the effects of system-dependent electrical coupling is performed prior to positioning the probe in the body, and comprises the steps of:

providing respective patch measurement circuits to determine respective portions of the mapping electrical currents passing through the second electrodes;

electrically bypassing the patch measurement circuits using a switch; and

thereafter determining respective crosstalk signals experienced by the second electrodes using the patch measurement circuits.

4. The method according to claim 3 , wherein determining respective crosstalk signals comprises determining for each of the second electrodes phases between currents and voltages experienced therein, the currents and voltages being coupled from transmitters connected respectively to the at least one first electrode.

5. The method according to claim 2 , wherein the one or more medical devices comprise an ablator linked to the probe, and a third electrode on the surface of the body, wherein calibrating the measurements comprises calculating respective leakage currents flowing in a path extending from the at least one first electrode through the ablator and the third electrode to the second electrodes, and wherein computing the position is performed while the ablator is linked to the body and to a generator of one of the mapping electrical currents.

6. The method according to claim 5 , wherein calculating respective leakage currents comprises:

disconnecting the ablator from the probe;

determining an ablator leakage current passing from the generator of one of the mapping electrical currents through the ablator and the third electrode; and

calculating a relation between the ablator leakage current and the mapping electrical currents at respective working frequencies of the mapping electrical currents.

7. The method according to claim 5 , wherein calculating respective leakage currents comprises:

disconnecting the ablator from the probe;

determining an ablator leakage current passing from the generator of one of the mapping electrical currents through the ablator and the third electrode; and

determining respective components of the ablator leakage current at the second electrodes; and

calculating ratios between the components and the ablator leakage current, respectively.

8. The method according to claim 5 , wherein calculating respective leakage currents is performed after positioning the probe in the body, and comprises determining a ratio between each of the mapping electrical currents and a summation of the mapping electrical currents passing through the second electrodes.

9. The method according to claim 8 , wherein determining the ratio is performed at respective working frequencies of the mapping electrical currents in the second electrodes, and further comprises extrapolating from a portion of the working frequencies to others of the working frequencies.

10. The method according to claim 1 wherein calibrating the measurements further comprises linking the second electrodes to respective body surface receivers and body surface generators, and using the body surface receivers and the body surface generators to determine a patch-to-patch conductance matrix among the second electrodes.

11. The method according to claim 10 , further comprising applying the patch-to-patch conductance matrix to perform frequency compensation of currents measured by the body surface receivers.

12. An apparatus for sensing a position of an object, comprising:

a probe adapted to be inserted into a body of a subject;

at least one first electrode disposed near a distal end of the probe;

a plurality of second electrodes adapted to be coupled at respective locations to a surface of the body;

at least one electrode transmitter connected to the first electrode for passing mapping electrical currents between the first electrode on the probe and the second electrodes on the surface of the body; and

a control unit, adapted for making measurements of the mapping electrical currents, for calibrating the measurements so as to compensate for one or more non-ideal features of the measurements including effects of system-dependent electrical coupling to one or more medical devices other than the first electrode and the second electrodes, and for computing the position of the probe in the body using the calibrated measurements.

13. The apparatus according to claim 12 , wherein calibrating the measurements comprises calculating the effects of system-dependent electrical coupling, and calculating mapping generator-induced crosstalk.

14. The apparatus according to claim 13 , wherein calculating the effects of system-dependent electrical coupling is performed prior to positioning the probe in the body, and comprises the steps of:

providing respective patch measurement circuits to determine respective portions of the mapping electrical currents passing through the second electrodes;

electrically bypassing the patch measurement circuits using a switch; and

thereafter determining respective crosstalk signals experienced by the second electrodes using the patch measurement circuits.

15. The apparatus according to claim 14 , wherein determining respective crosstalk signals comprises determining for each of the second electrodes phases between currents and voltages experienced therein, the currents and voltages being coupled from transmitters connected respectively to the at least one first electrode.

16. The apparatus according to claim 13 , further comprising an ablator, and a third electrode on the surface of the body, wherein calibrating the measurements comprises calculating respective leakage currents flowing in a path extending from the at least one first electrode through the ablator and the third electrode to the second electrodes, and wherein computing the position is performed while the ablator is linked to the body and to a generator of one of the mapping electrical currents.

17. The apparatus according to claim 16 , wherein calculating respective leakage currents comprises:

disconnecting the ablator from the probe;

determining an ablator leakage current passing from the generator of one of the mapping electrical currents through the ablator and the third electrode; and

calculating a relation between the ablator leakage current and the mapping electrical currents at respective working frequencies of the mapping electrical currents.

18. The apparatus according to claim 16 , wherein calculating respective leakage currents comprises:

disconnecting the ablator from the probe;

determining an ablator leakage current passing from the generator of one of the mapping electrical currents through the ablator and the third electrode; and

determining respective components of the ablator leakage current at the second electrodes; and

calculating ratios between the components and the ablator leakage current, respectively.

19. The apparatus according to claim 16 , wherein calculating respective leakage currents is performed after positioning the probe in the body, and comprises determining a ratio between each of the mapping electrical currents and a summation of the mapping electrical currents passing through the second electrodes.

20. The apparatus according to claim 19 , wherein determining the ratio is performed at respective working frequencies of the mapping electrical currents in the second electrodes, and further comprises extrapolating from a portion of the working frequencies to others of the working frequencies.

21. The apparatus according to claim 12 wherein calibrating the measurements further comprises linking the second electrodes to respective body surface receivers and body surface generators, and using the body surface receivers and the body surface generators to determine a patch-to-patch conductance matrix among the second electrodes.

22. The apparatus according to claim 21 , further wherein the control unit is operative for applying the patch-to-patch conductance matrix to perform frequency compensation of currents measured by the body surface receivers.