Doppler
Named after the Austrian physicist Doppler. The Doppler effect makes use of the fact that any wave-form energy (sound, light) reflected from a moving object, will change in frequency causing higher frequency (higher tones, colour toward violet) if the object moves toward the observer and lower frequency (deeper tones, colour toward red) if the object moves away from the observer. This property can be used to calculate the speed of the moving object. In ultrasound, sound waves are reflected from thousands of blood cells moving through the blood vessels, enabling us to calculate the speed and direction of the blood in that particular vessel.
GE Healthcare Glossary