Why should the Doppler angle be kept near 0 or 180 degrees for velocity measurements?

• A. To maximize cos θ and minimize error
• B. To maximize sin θ
• C. To minimize Doppler shift
• D. To increase PRF

Answer: (A)

The Doppler shift that we measure is tied to the velocity component along the ultrasound beam, which is proportional to cos θ. Keeping the Doppler angle near 0 degrees (or 180 degrees, where the velocity component along the beam is still large in magnitude) makes cos θ close to ±1. That gives the strongest Doppler signal for a given true velocity and keeps the velocity estimate from being overly sensitive to small errors in angle. Mathematically, fD ∝ v cos θ, and the velocity you calculate often involves dividing by cos θ; when θ is near 0 or 180, cos θ is large in magnitude, so the calculation is more stable and less prone to error. If θ drifts toward 90 degrees, cos θ shrinks toward zero, the Doppler shift becomes small, and even tiny angle errors cause large errors in the velocity estimate.

The other ideas don’t fit because Doppler shift depends on cos θ, not sin θ, so maximizing sin θ would weaken the signal. Minimizing the Doppler shift would make the measurement less reliable. Increasing PRF affects sampling and aliasing, not the angular sensitivity of velocity measurements.