What is the effect of increasing PRF (color scale) on color Doppler?

• A. Increases aliasing
• B. Reduces aliasing but may miss slow flow
• C. Eliminates color
• D. Has no effect

Answer: (B)

Raising the color Doppler PRF expands the velocity range that can be displayed without wraparound. In practical terms, the Nyquist limit goes up, so high-velocity signals are less likely to alias. But this comes with a trade-off: the higher the PRF, the coarser the velocity increments and the less sensitive the system becomes to very slow flow. Small Doppler shifts may fall below the display threshold and be effectively missed, so slow or microvascular flow can disappear from the color map. That combination—less aliasing on fast flow but potential loss of slow flow—explains why increasing PRF reduces aliasing but may miss slow flow.