Velocity estimation in Doppler ultrasound is based on the measurement of which parameter?

• A. Pulsed wave frequency
• B. Flow velocity
• C. Doppler angle of incidence
• D. Pulse repetition frequency

Answer: (C)

Velocity estimation in Doppler ultrasound hinges on turning the measured Doppler frequency shift into a true flow speed. That Doppler shift is determined by the component of the velocity along the ultrasound beam, which is V cos θ, where θ is the angle between the flow direction and the beam. To recover the actual velocity, you must know this angle—the Doppler angle of incidence. The relationship can be written roughly as V ≈ (Δf c) / (2 f0 cos θ) for common Doppler formulas, so the cosine of that angle is the key factor in converting the observed shift into speed. If θ is near 90 degrees, cos θ is small and velocity estimates become unreliable; if θ is 0 degrees, you get the full velocity along the beam. The other aspects, like the transmitted or pulsed-wave frequency, aren’t what you use directly to compute velocity, and pulse repetition frequency affects sampling and aliasing rather than the velocity calculation itself.