Compare pulsed-wave (PW) and continuous-wave (CW) Doppler in terms of depth specificity and velocity measurement capability.

• A. PW Doppler provides depth-specific velocity information at a defined range but is subject to aliasing at high velocities.
• B. PW Doppler provides depth-specific velocity but cannot measure velocity at any depth.
• C. CW Doppler provides depth-resolved velocity but cannot measure high velocities.
• D. CW Doppler and PW Doppler offer identical depth information.

Answer: (A)

The key idea is depth sampling versus continuous range measurement. Pulsed-wave Doppler sends short pulses and uses the time delay of echoes to sample a specific depth, giving velocity information at a defined location along the beam. This depth specificity is what allows you to measure flow at a particular phantom or tissue depth. However, because velocity measurements are limited by the pulse repetition frequency, there is a Nyquist constraint: velocities above half the PRF will wrap around, causing aliasing in the spectrum.

Continuous-wave Doppler transmits and receives continuously, so it has no range (depth) resolution and cannot tell you where along the beam the velocity is coming from. It can measure very high velocities without aliasing, but you lose depth information.

Therefore the statement that PW Doppler provides depth-specific velocity information at a defined range but is subject to aliasing at high velocities is the best answer.