Doppler ultrasonography may be used to assess the patency of both intracranial and extracranial vessels.
Either a continuous wave or a pulsed system is used. The latter may be interfaced with B mode scanning to provide a duplex scan. This provides additional information to the continuous wave regarding plaque morphology. Colour coded doppler is also available; flow away from the transducer is blue, towards it it red, and flow velocity is proportional to colour saturation. It is especially useful in pseudo-occlusion for example, high grade carotid stenosis may be misinterpreted as total occlusion with standard methods.
Frequencies of 5-10 MHz are typical in extracranial ultrasonography. A lower frequency, 2 MHz, is more usual in intracranial ultrasound.
One major disadvantage of doppler ultrasonography as a whole is operator dependence. At least 6 months must be spent training before reliable data can be obtained.
Last reviewed 01/2018