A Doppler ultrasound (or duplex scanning) is a medical test that uses reflected sound waves to estimate the speed and direction of blood as it flows through a blood vessel. It helps doctors evaluate blood flow through arteries and veins.

Doppler ultrasound can be used to evaluate the vessels in several areas of the body. For example, a Doppler ultrasound test of the blood vessels in the neck (carotid ultrasound) can be done to estimate a person's risk of a stroke from blockage in those arteries. Renal ultrasound can help detect kidney problems. Lower extremity ultrasound can be used to detect a blood clot in the deep veins of the legs. A Doppler ultrasound also may be used to evaluate problems with the blood flow to the placenta and umbilical cord in a pregnant woman; problems with this blood flow may indicate fetal distress.