Patent ID: 8500651

Claim:
A device for measuring the movement of a patient's body part comprising: (a) first and second matched pairs of fixed coils; (b) a patient coil configured to wrap elastically around the patient's body part; (c) a current source coupled to, and adapted to energize, either the matched pairs of fixed coils or the patient coil; and (d) a volume sensing element comprising whichever of the matched pairs of fixed coils or the patient coil that is not coupled to the current source; wherein the first and second matched pairs of fixed coils are arranged (i) to reduce a sum of magnetic noise signals from remote sources to zero when the current source is coupled to, and adapted to energize, the patient coil and the volume sensing element comprises the matched pairs of fixed coils and (ii) to generate a relatively homogeneous magnetic field in a spatial volume occupied by the patient's body part and to reduce potential interference with other electronic instruments when the current source is coupled to, and adapted to energize, the matched pairs of fixed coils and the volume sensing element comprises the patient coil.