Design considerations for a transcranial Doppler coupled to a stereotactic position arm.

We present the localization error analysis for a stereotactic positioning arm, transcranial Doppler (TCD) system. Localization displacement errors were determined with a custom phantom head. A bubble apparatus was built to measure sample volume (SV). From interface plane relationships, biological material properties, and human computerized tomography (CT) scan data, an algorithm for computing the actual TCD signal path was developed. Localization resolution was diminished by constraints intrinsic to the TCD instrument (+/-1 mm) and the positioning arm (1.9+/-0.9 mm). The overall SV localization error ranged from 3.3 mm from the target, to complete signal loss at a critical cumulative angle of 31, one third of which may be reached at the TCD probe/skin interface. Any stereotactic ultrasound system must include correction for the beam path. The "failed ultrasound window," may be predicted by signal loss based on refraction alone. The use of stereotactic TCD may enhance the clinical applications of TCD.