Transcranial Doppler (TCD) uses low-frequency pulsed wave Doppler (typically 2 MHz) to insonate intracranial arteries through natural bony windows, primarily the temporal window. It is used to evaluate the middle cerebral artery (MCA), anterior cerebral artery (ACA), posterior cerebral artery (PCA), and vertebrobasilar system. Clinical applications include detection of intracranial stenosis, monitoring for cerebral vasospasm after subarachnoid hemorrhage, embolus detection during carotid endarterectomy, and evaluation of cerebral autoregulation. TCD requires specialized training and is a component of advanced vascular technology practice.