The significance of sonography can be increased considerably by the use of the Doppler effect. There is a distinction between the one-dimensional method (pulsed-wave Doppler, continuous-wave Doppler; also known as D-mode) and two-dimensional, colour-coded applications (colour Doppler - F-mode). The combination of B-picture with PW Doppler (Pulsed Wave Doppler) is also called duplex.
Doppler methods are used to determine blood flow velocities, for the detection and analysis of cardiac or heart-valve defects, strictures (stenosis), closures or fistulae (shunts) in which colour-coded Doppler sonography for a large area of a conventional ultrasound image (colour window), the localised Doppler frequency (= mean flow velocity) and the range of their deviations are determined. Thus we can hopefully estimate the turbulence of the flow. Due to the statistical motion of the scattered particles, however, the variation in the flow rate is always greater than the turbulence. The result is superposed in false-colour on the B-picture, that is, in shades of red and blue for different blood velocity and green for turbulence. The colour red is usually used to represent movement towards the transducer, while flows away from the probe are coded in blue. Areas of zero speed are suppressed electronically.