Although considerable technological growth has occurred with regards to the
noninvasive assessment of the extracranial circulation, very little attention
had, until recently, been paid to the development of noninvasive techniques
for assessing the intracranial circulation. For many years it had been
assumed that the cranium was largely impenetrable to ultrasound, making
interrogation of the intracranial circulation seemingly impossible.
Aaslid et al., in 1982,however, demonstrated this assumption to be incorrect
by describing a noninvasive method of obtaining the blood flow velocities in
the major basal cerebral arteries using 2-MHz pulsed Doppler ultrasound
through natural cranial windows. The technique described subsequently became
known as transcranial Doppler sonography.
Transcranial Doppler sonography, relying on innovations in both technology
and technique, has proven to be a clinically useful tool in a variety of
established and developing applications.