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Transcranial Doppler ultrasonography is a noninvasive technology that uses the Doppler Effect to measure the velocity and direction of blood flow in the vessels. Just as a siren's pitch sounds higher when its source is moving toward you and lower as it moves away, so too will ultrasound waves change pitch, or frequency, as they bounce off the red blood cells moving in the blood. It is these pitch changes that produce the audible sounds during the exam.
The Transcranial Doppler (TCD) uses a handheld-pulsed low frequency Doppler transducer that enables recording of blood flow velocities from intracranial arteries through selected cranial foramina and thin regions of the skull. Analysis of the Doppler spectra allows display and calculation of peak systolic, peak diastolic, and mean velocities and pulsatility indices. Mapping of the sampled velocities as a color display of spectra in lateral, coronal and horizontal views locates the major brain arteries in three dimensions.