What medical technique uses the Doppler effect to image blood flow?

The medical technique that uses the Doppler effect to image blood flow is Doppler ultrasound, also called Doppler ultrasonography.

It sends high-frequency sound waves into the body and analyzes the frequency change in echoes returning from moving blood cells. The size and direction of that shift allow the system to estimate whether blood is moving toward or away from the probe and how fast it is traveling.

Doppler ultrasound is especially useful for examining arteries, veins, and the heart. It can help identify narrowed vessels, blocked circulation, reversed flow, and abnormal flow across heart valves. Many clinical scans combine ordinary B-mode ultrasound, which displays anatomy, with Doppler information layered over the image; this combined approach is often called duplex ultrasonography.

It is not the same as a CT scan, MRI, or X-ray. Those technologies use different forms of radiation or magnetic fields and are generally focused on anatomy rather than directly measuring moving blood. A practical limitation is that the ultrasound beam should be aligned fairly closely with the direction of blood flow for the velocity estimate to be reliable.