The use of Doppler ultrasound to estimate blood flow velocity was described in 19601 but it was only in the 1980s that it was appreciated that sufficient ultrasound would pass through the skull to allow the detection of blood flow within the intracranial circulation.2 To achieve sufficient bone penetration low frequency ultrasound is used; 2 MHz is conventional. This increases tissue penetration at the cost of poor spatial resolution. Therefore, even with more recent transcranial duplex scanners two dimensional B mode images are of low spatial resolution, and in adults the technique primarily provides useful information about blood flow velocity. Using various bone windows, where the barrier to the ultrasound is thinnest, it is possible to insonate the proximal middle, anterior, and posterior cerebral arteries, the distal internal carotid artery, and the intracranial vertebrobasilar system.3 Localisation is achieved by the use of pulsed Doppler ultrasound allowing determination of the depth of the insonated vessel. Recent colour flow duplex systems, which can display the Doppler information from a number of vessels simultaneously, improve the ease with which the intracranial vessels can be identified. They also allow angle correction and therefore more accurate blood velocity measurements.

Transcranial Doppler has a number of advantages as a method of evaluating cerebral haemodynamics. It is relatively cheap and non-invasive, allowing repeated measurements and continuous monitoring. It has a high temporal resolution making it ideal to study rapid changes in cerebral haemodynamics. On the negative side it relies on a trained technician, and in about 5%-10% of cases sufficient ultrasound cannot be transmitted through the skull to obtain a signal. In such cases the use of intravenous contrast agents, which contain stabilised microbubbles which pass into the arterial circulation and result in increased backscattering of ultrasound, may enable vessel identification.4 It should …