Position sensing and detection of skin impedance
Apparatus and methods are provided for determining in near realtime the position of a probe placed within a living body. Electric currents are driven between one or more electrodes on the probe and electrodes placed on the body surface. The impedance between the probe and each of the body surface electrodes is measured, and three-dimensional position coordinates of the probe are determined based on the impedance measurements. Dynamic compensation is provided for changing impedance of the body surface and its interface with the electrodes, resulting from such causes as electrode peel-off and changes in moisture and temperature. The compensation improves the accuracy of, inter alia, medical procedures, such as mapping the heart or performing ablation to treat cardiac arrhythmias.
1. An apparatus for position sensing, comprising:
a probe, comprising at least one probe electrode, which is adapted to be inserted into a body of a living subject, said body having a body surface;
a plurality of electroconductive body surface patches, which are adapted to be fixed to said body surface at respective surface locations, wherein said body surface patches each comprise a first electrode and a second electrode, said second electrode being electrically isolated from said first electrode;
electrical circuitry having at least one switch for arranging said first electrode in a series circuit with said second electrode in a calibration mode of operation and for arranging said first electrode in a parallel circuit with said second electrode in a sensing mode of operation; and
a controller, operative to control said electrical circuitry, wherein said controller is adapted to be coupled to said probe and to said body surface patches so as to pass first electrical signals through said first electrode and said second electrode thereof in said calibration mode of operation, and to pass respective second electrical signals through the body between said probe electrode and said body surface patches, wherein said controller is operative in said calibration mode of operation to determine a local electrical impedance of said body surface and of an interface thereof with a respective one of said body surface patches, and to determine position coordinates of said probe in said sensing mode of operation by measuring respective impedance characteristics of said second electrical signals, and wherein said impedance characteristics of said second electrical signals are adjusted by said controller according to respective instances of said local electrical impedance of said body surface.
2. The apparatus according to claim 1 , wherein said controller is adapted to maintain a constant voltage between each of the surface locations and said probe electrode, and to measure said respective impedance characteristics by measuring currents at said constant voltage.
3. The apparatus according to claim 1 , wherein said controller is adapted to maintain a constant current between each of the surface locations and said probe electrode, and to measure respective said impedance characteristics by measuring voltages at said constant current.
4. The apparatus according to claim 1 , wherein said probe is adapted to perform a medical treatment on said subject.
5. The apparatus according to claim 1 , wherein said body surface patches comprise an adhesive layer that contacts said body surface.
6. The apparatus according to claim 1 , further comprising a timer, wherein said controller is operative to alternate said calibration mode of operation and said sensing mode of operation responsively to time intervals measured by said timer.