In Doppler ultrasound, which incident angle results in no frequency shift?

• A. 0 degrees
• B. 45 degrees
• C. 90 degrees
• D. 180 degrees

Answer: (C)

Doppler frequency shift depends on the component of blood velocity that lies along the ultrasound beam. This is the velocity projected onto the beam, which scales with cos of the incident angle. When the beam is perpendicular to flow, the angle is 90 degrees and cos 90 degrees equals zero, so there is no velocity component along the beam and no Doppler frequency shift is produced. That’s why you don’t detect a shift in this geometry. If the beam is aligned with flow, you get the maximum shift in the flow direction; if it’s opposite to flow, you get a maximum shift in the opposite direction; at intermediate angles like 45 degrees you get a smaller shift proportional to cos 45. In practice, you aim for smaller angles to maximize sensitivity while accurately measuring the angle.