In Doppler ultrasound, the frequency difference between the transmitted and received frequencies is called

• A. Beat frequency
• B. Doppler frequency shift
• C. Phase shift
• D. Signal delay

Answer: (B)

The key idea is the Doppler frequency shift. In Doppler ultrasound, moving blood cells cause the frequency of the echoed signal to be different from the transmitted frequency. That difference between the received and transmitted frequencies is the Doppler frequency shift, and it’s the quantity used to estimate blood velocity. The shift depends on the transmitted frequency, the speed of the target, and the angle between the flow and the ultrasound beam (and is inversely related to the speed of sound in tissue). This specific frequency difference is what distinguishes Doppler information from other concepts. While beat frequency is a general name for the difference between two frequencies, in Doppler ultrasound the standard term for this motion-induced difference is Doppler frequency shift. Phase shift refers to a change in the waveform’s phase rather than its frequency, and signal delay is a time lag, neither of which describe the Doppler-induced frequency change.