Usual colour Doppler echocardiogram is superimposition of colour Doppler images on a two dimensional echocardiogram. Colour M-Mode is superimposition of colour Doppler images on an M-Mode echocardiogram. The principle of Doppler is that the frequency of sound wave coming from an object which is moving towards the ultrasound probe increases while that from an object moving away from the probe decreases. It is similar to the difference in the sound of a train whistle, which is different when the train is moving towards you than when it is moving away from you. In Doppler echocardiography, the target is moving red blood cells.
