Color Doppler aliasing occurs when the pulse repetition frequency is too low to sample high-velocity flow.

• A. High PRF
• B. Low PRF
• C. High gain
• D. Narrow color box

Answer: (B)

Color Doppler aliasing happens because the system only samples flow at discrete time points. The pulse repetition frequency sets the maximum unambiguous velocity that can be measured—the Nyquist limit (PRF/2). When the actual flow velocity creates a Doppler shift larger than that limit, the measured velocity wraps around to the opposite direction, causing aliasing. A low PRF lowers this Nyquist limit, so aliasing occurs more readily for faster flow. The way to prevent or reduce aliasing is to increase PRF (use a higher color scale), which raises the Nyquist limit. The other options don’t address the sampling rate: changing gain affects brightness, and narrowing the color box confines where color is shown but doesn’t fix aliasing.