What is Doppler ultrasound?

Abstract

The term Doppler should be capitalized because it refers to Christian Johann Doppler, an Austrian physicist (1803–1853). He described a phenomenon whereby the frequency of sound changes when it is reflected off a moving object. If the object is moving towards the observer, reflected sound frequency is increased (blue shift), while if the reflecting object is moving away from the observer, the sound frequency decreases (red shift). This is analogous to a moving ambulance with its siren on; the siren pitch increases as ambulance approaches and decreases as it receeds. The Doppler Effect may be used also in ultrasound. With normal grey scale ultrasound we rely on amplitude of reflected sound waves and the reflectivity of anatomic structures is proportional to the intensity (or amplitude) of the reflected sound and hence brightness on the scanner display. If frequency (rather than amplitude) is analysed, one can detect motion on the basis of Doppler shifts. These frequency shifts can be quantified reasonably accurately according to direction, velocity and also acceleration, all of which provide specific signatures that help identify both normal and diseased blood vessels.