In Doppler ultrasound, the Doppler frequency shift is defined as the difference between which two frequencies?

• A. Received frequency and Transmitted frequency
• B. Transmitted frequency and pulse repetition frequency
• C. Pulse repetition frequency and pulse length
• D. Frequency bandwidth and pulse repetition frequency

Answer: (A)

Doppler frequency shift comes from motion of the reflecting structures causing the echo’s frequency to differ from the frequency that was transmitted. The amount of shift is the difference between the received echo frequency and the transmitted frequency. In practical terms, the emitted ultrasound has a fixed frequency, and motion of blood or tissue slightly increases or decreases the frequency of the returning signal, producing a Doppler shift that can be used to infer velocity and direction.

This shift is determined by factors like the transmitted frequency, the velocity of the moving target, and the angle between the flow and the beam, following the standard Doppler relation. The other items mentioned—how often pulses are sent (pulse repetition frequency), how long a pulse lasts (pulse length), and the overall bandwidth of the pulse—are related to imaging timing or resolution, not to the frequency difference that defines the Doppler shift.