A diagnostic technique that utilizes the fact that the frequency of sound or light waves changes when they are reflected from a moving surface (the Doppler effect). It is used to study the flow in blood vessels and the movement of blood in the heart. The frequency detector may be part of an ultrasound imaging probe, which displays an image of the anatomy on a monitor. Simultaneously the Doppler signal from a particular point on the ultrasound image can be displayed next to the anatomical position using a split screen (duplex imaging). Using electronic techniques, direction and velocity of blood flow can each be allocated different colours and displayed on a colour monitor over the anatomical image (colour flow ultrasound imaging). Power Doppler, a modification of this technique, is more sensitive at detecting flow but does not give information on direction of flow. Doppler ultrasound is a valuable technique to detect vessel thrombosis, such as deep vein thrombosis, and can be safely used in pregnancy without the risk of ionizing radiation. Doppler measurement of the fetal middle cerebral vessels (MCA Doppler) can predict fetal anaemia as the resistance in these vessels is increased. This noninvasive technique has supplanted serial amniocentesis/fetal cord blood sampling in cases of haemolytic disease of the newborn. Doppler of other fetal vessels (e.g. the ductus venosus) is used as a screening method for fetal chromosomal and cardiac defects and, later, fetal growth restriction. C. J. Doppler (1803–53), Austrian physicist
Subjects: Medicine and Health.