Abstract: A system for externally locating a sensor in tissue, comprising an external probe including at least first and second electromagnetic output coils with non-parallel longitudinal axes; and output coil driver for alternately energizing the first and second output coils, for generating a time-varying magnetic field which penetrates the patient's skin; a sensor coil, having a longitudinal axis, for developing an induced electrical voltage in response to the time-varying magnetic field; a distance determinator, responsive to the induced voltage from the sensor coil, for determining from the induced voltage, the distance between the output coils and the sensor coil, independently of the relative angle, in a horizontal plane, between the sensor coil longitudinal axis, and the longitudinal axes of the output coils; and a direction determinator for determining and displaying the direction, in the horizontal plane, in which the sensor coil longitudinal axis is pointing.

Abstract: A system for externally locating the depth and orientation of a catheter in tissue with an external probe which generates a virtual rotating magnetic field. The catheter includes an inductive coil for developing an induced signal in response to the virtual rotating magnetic field. An indicating device such as a light bar display or digital readout indicates the strength of the induced signal for locating, independently of the relative angular orientation of the probe and the catheter, the depth in the tissue of the catheter.