Infographics / Ultrasound Physics

Pulsed-wave vs continuous-wave Doppler

A two-column comparison. Left: pulsed-wave Doppler uses one crystal that alternates transmit and receive, with a range gate selecting a single depth (depth-specific), but a Nyquist limit of PRF over 2 so high velocities alias. Right: continuous-wave Doppler uses two angled crystals that transmit and receive continuously with overlapping beams; it has no Nyquist limit and no aliasing so it measures very high velocities, but the whole overlap is sampled so there is no depth information (range ambiguity).

Pulsed-wave versus continuous-wave Doppler A two-column comparison. Left: pulsed-wave Doppler uses one crystal that alternates transmit and receive, with a range gate selecting a single depth (depth-specific), but a Nyquist limit of PRF over 2 so high velocities alias. Right: continuous-wave Doppler uses two angled crystals that transmit and receive continuously with overlapping beams; it has no Nyquist limit and no aliasing so it measures very high velocities, but the whole overlap is sampled so there is no depth information (range ambiguity). Pulsed-wave Doppler depth-specific, but aliasing-limited Continuous-wave Doppler no aliasing, but no depth depth 1 crystal range gate (sample volume) only this depth is sampled transmit receive beams overlap whole beam summed, no depth – one crystal, alternates transmit / receive – range gated → depth-specific – Nyquist limit = PRF / 2 – aliases at high velocity – max sampling depth ∝ 1 / PRF – two angled crystals, separate send / receive – transmits and receives continuously – no Nyquist limit → no aliasing – measures very high velocities – no depth info → range ambiguity
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