Abstract: In a coordinate system defined by a coordinate space digitizer, the location of an implanted symmetric object may automatically be determined based on differences between the characteristic acoustic impedances of at least a portion of an implanted object and one or more materials surrounding that object. An A-mode ultrasound transducer may be attached to a coordinate space digitizer or pointing device such that an attached computer tracks the position and orientation of the ultrasound transducer. The reflected radio-frequency (rf) ultrasound signals may automatically be analyzed along with the transducer position and orientation corresponding to each received rf signal to detect the position of the implanted object. The time delay between received ultrasound echoes is used to compute the depth of the object from the transducer. This allows for an accurate determination of the three-dimensional coordinates of the implanted object.