ABSTRACT
Since its introduction by Aaslid and colleagues in 1982, transcranial Doppler (TCD)
ultrasonography has rapidly evolved as a noninvasive technique for measuring blood flow
velocity (FV) in the major intracranial arteries. TCD has been described as a stethoscope
of the brain, allowing the clinician to “listen” for the changes in cerebral circulation. The
main advantages are that it is safe, noninvasive, easy to use, portable, reproducible, and
versatile; provides continuous online beat-to-beat information; and does not involve the
use of radioactive substances. It is particularly useful in the investigation of vascular
reactivity because it can detect rapid changes in FV instantaneously (1).