In color Doppler imaging, retrograde color flow seen below the baseline in a vessel with a stenosis is most likely caused by what?

• A. Retrograde flow due to post stenotic turbulence
• B. Aliasing
• C. Mirror imaging
• D. Incorrect setting of Doppler gain

Answer: (A)

The key idea is that turbulence after a narrowing can create local flow reversal. When a vessel has a stenosis, the flow accelerates through the narrow segment and then becomes disturbed as it moves downstream. This post-stenotic region often develops vortices and recirculation, so components of the flow move away from the transducer. On color Doppler, that shows up as retrograde color flow below the baseline, indicating flow in the opposite direction in parts of the vessel caused by turbulent, bidirectional flow downstream of the stenosis.

Aliasing is an imaging artifact from the velocity scale exceeding the Nyquist limit, which can make colors wrap around the baseline, but the retrograde signal described here reflects real turbulent flow after the stenosis. Mirror imaging is a ghost artifact, and incorrect Doppler gain changes brightness without altering the direction of flow.