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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.