Transcranial Doppler sonography is a noninvasive technique that uses a 2-MHz, pulsed Doppler transducer to measure the velocity of blood flow within the circle of Willis and vertebrobasilar system through regions of temporal calvarial thinning or through the orbits or foramen magnum. By using spectral analysis of the Doppler frequency shifts from insonated red blood cells moving through a preselected arterial sample volume, transcranial Doppler calculates and displays the peak systolic and diastolic velocity, the mean velocity, and the pulsatility index of blood flow within the interrogated vessel. Vessel identification is based on standard criteria, including the cranial window used, transducer position, depth of sample volume, direction of blood flow, relationship to the terminal internal carotid artery, and response to common carotid artery compression. Diagnoses made with transcranial Doppler sonography are based on the detection of increased or decreased blood flow velocity, absence of blood flow, or changes in pulsatility.