TCD is the only non-invasive real-time neuroimaging study that can evaluate the characteristics of cerebral blood flow continuously. CT Angiography (CTA ) and Magnetic Resonance Angiography (MRA ) can do snapshots in time. Fluoroscopic angiography is the gold standard and can be used as a confirmatory tool.
Some of the applications of TCD are: stroke, vasomotor reactivity testing, emboli monitoring, shunt detection, increased intracranial pressure, circulatory arrest, monitoring vasospasm after spontaneous subarachnoid hemorrhage, and sickle cell anemia.
In my opinion TCD can be a good test in the assessment of brain death when the diagnosis is doubtful.
A good review paper in this topic is the following:
Tsivgoulis G, et al, Advances in Transcranial Doppler Ultrasonography. Current Neurology and Neuroscience Reports, 2009, 9:46-54
Adolfo Cotter, MD