RESUMO
The circle of Willis may be examined using low frequency pulsed Doppler ultrasound transmitted through the thin squamous temporal bone or 'acoustic windows' of the skull. The thickness and shape of these windows affect the characteristics of the ultrasound beam and partly determine the spatial resolution of transcranial Doppler scanning. We have studied the effects of temporal bone on the ultrasound beam and sample volume characteristics of the EME Transcan 2 MHz transducer using a stepper motor, computer-controlled ultrasound plotting system. The lateral and axial spatial dimensions of the sample volume, and a direct estimate of the lateral resolution of the system, were determined at various depths of interest using Doppler targets. The results in this initial study of a small number of specimens indicates a decrease in sample width, and hence an increase in lateral resolution, presumably due to the lensing effect of the skull cusping. The estimated lateral resolution of the system is of the order of 4 mm.
