Which Doppler modality samples velocity along a line at a specific depth?

• A. Continuous wave Doppler
• B. Pulsed wave Doppler
• C. Color Doppler imaging
• D. Power Doppler

Answer: (B)

Pulsed Doppler uses short pulses and time-gated reception to sample Doppler shifts from echoes coming from a specific depth along the ultrasound beam. The depth is determined by the time between emission and reception, so the Doppler information comes from a defined “gate” or sample volume at one line and depth. Move the gate along the beam to sample velocity at different depths. This depth-specific sampling is what lets you measure flow at a precise location.

Continuous wave Doppler, by contrast, transmits and receives continuously and collects Doppler shifts along the entire beam path, so there’s no inherent depth discrimination. Color Doppler and power Doppler use similar principles but apply them across multiple lines or regions of the image rather than a single depth-focused gate, and power Doppler does not measure velocity at all.