Which of the following statements about Doppler frequency shift is true?

• A. It is the sum of the transmitted and reflected frequencies.
• B. It is the difference between transmitted and reflected frequencies.
• C. It is the ratio of transmitted to reflected frequencies.
• D. It is the product of transmitted and reflected frequencies.

Answer: (B)

Doppler frequency shift is the change in frequency caused by relative motion between the ultrasound source and the reflective tissue. When the transducer emits at a frequency f0 and the target is moving, the frequency of the wave that returns (the reflected frequency) differs from f0. The amount of this difference, Δf, is the Doppler frequency shift and is defined as the received (reflected) frequency minus the transmitted frequency: Δf = fR − f0. This is why the statement that the shift is the difference between transmitted and reflected frequencies is correct.

In practice, the shift grows with the target’s velocity and depends on the angle between motion and the beam: Δf ≈ 2 f0 v cos θ / c, where c is the speed of sound in tissue. The factor of two reflects the round-trip path (to the target and back). If the target moves toward the transducer, Δf is positive (upshift); if it moves away, Δf is negative (downshift).