The Doppler shift is a measure of which of the following?

• A. Ratio of incident frequency to reflector frequency
• B. Sum of incident frequency & reflected frequency
• C. Difference between incident frequency & reflected frequency
• D. Product of incident and reflected frequencies

Answer: (C)

Doppler shift is the change in frequency caused by motion of the scatterer, and what we measure is the difference between the frequency of the emitted ultrasound and the frequency of the echo that returns. In other words, the shift equals fR minus f0 (the incident frequency). If the reflector (like blood cells) moves toward the transducer, the returned frequency is higher; if it moves away, it’s lower. The larger the motion along the beam, the larger the difference. For a stationary target there is no shift. This two-way Doppler effect is what links the frequency difference to the velocity of flow through the tissue.