ABSTRACT
Ultrasound systems may be used to detect the motion of blood in arteries, veins and in the heart. For several decades clinical practice has been concerned with two display modes; spectral Doppler for the display of velocity-time waveforms and colour flow as used for the two-dimensional display of flow. The Doppler effect is observed regularly in daily lives. For example, it can be heard as the changing pitch of an ambulance siren as it passes by. The Doppler effect is the change in the observed frequency of the sound wave compared to the emitted frequency which occurs due to the relative motion between the observer and the source. Ultrasound can be used to assess blood flow by measuring the change in frequency of the ultrasound scattered from the moving blood. The detected Doppler shift also depends on the cosine of the angle between the path of the ultrasound beam and the direction of the blood flow.
