Patent ID: 8843189

Claim:
A method of determining a position of an instrument in a patient, comprising: moving the instrument into a volume of the patient; determining a position of a position element only with a position sensing system, including: providing a set of axis generation electrodes on an exterior surface of the patient establishing an axis and a conductive path therebetween; selecting a first frequency for injection of a current into the patient between the axis generation electrodes; injecting an axis current into the patient at the selected first frequency between the set of axis generation electrodes and generating a voltage gradient between the set of axis generation electrodes; and evaluating an electrical property including sensing a voltage at the position element between the set of axis generation electrodes; determining a position of the position element based at least on the sensed voltage along the voltage gradient between the set of axis generation electrodes; measuring for an anomalous current frequency within the patient at a frequency other than the selected first frequency; determining if the measured anomalous current frequency is an interfering frequency; and changing the selected first frequency to a selected second frequency for injection of the axis current between the axis generation electrodes if it is determined that the measured anomalous current frequency is an interfering frequency.