What causes a Doppler shift?

• A. Change in amplitude of the wave
• B. Change in wavelength due to relative motion
• C. Change in tissue impedance
• D. Change in receiver gain

Answer: (B)

Doppler shift happens because of relative motion between the ultrasound wave and a moving scatterer, namely the red blood cells. When the transmitted wave hits moving blood, the echo that returns has a different frequency than what was sent. This frequency change, the Doppler shift, is driven by the velocity of the blood toward or away from the transducer, the angle between flow and the beam, and the transmitted frequency. A common way to think about it is that the shift in frequency (and thus the corresponding change in wavelength) is larger when blood is moving toward the transducer and smaller when it’s moving away. Amplitude, tissue impedance, or receiver gain don’t create this frequency shift; they affect signal strength, not the Doppler frequency.