Doppler shifts depend on which of the following?

• A. Only frequency affects Doppler shift
• B. Doppler shifts depend on relative motion and angle to the beam
• C. Doppler shifts depend only on tissue depth
• D. Doppler shifts depend on distance to the reflector

Answer: (B)

Doppler shifts arise because moving scatterers reflect the ultrasound with a frequency that changes based on how fast they are moving toward or away from the transducer. The amount of shift depends on the component of that motion along the ultrasound beam, so the angle between the flow direction and the beam is crucial. When the motion is aligned with the beam, the shift is greatest; when the motion is perpendicular to the beam, the shift is zero because there is no velocity component along the beam. The transmitted frequency also influences the magnitude of the shift, but the defining factors here are the relative motion and the beam-angle projection. Depth or distance to the reflector doesn’t determine the Doppler shift; those factors affect timing and signal strength, not the frequency change itself.