What exactly happens in the body during a Doppler ultrasound examination?

Answer

The Doppler effect is a shift in frequency as the source moves toward or away from you. The same thing happens with a wave that reflects off a moving object. So you can use this shift in frequency to determine the speed of an object.

In medicine, the Doppler effect is used to determine the speed of the blood: ultrasound waves are emitted almost parallel to the bloodstream. Some of it will reflect and this reflected wave has shifted in frequency due to the speed of the blood. Often a contrast liquid (gas bubbles) is used that increases the reflection. The Doppler effect is also used to measure the speed of heart valves.

You have to be careful with the term “Doppler” in medicine. There are other techniques for measuring speeds (e.g. difference in arrival time). In medicine, the term has become so established that any device that measures speed is called “Doppler”, even if it does not use the Doppler effect.