Which color would represent an aliased Doppler signal from flow moving away from the transducer?

• A. Dark red
• B. Dark blue
• C. Black
• D. Yellow/orange

Answer: (D)

Color Doppler encodes velocity on a limited color scale, with flow toward the transducer usually shown in red tones and flow away shown in blue tones. Aliasing happens when the actual velocity exceeds the Nyquist limit set by the color Doppler’s pulse repetition frequency. When that happens, the measured velocity wraps around to the opposite end of the color map. For flow moving away, this wrap often appears as colors at the high-velocity end of the spectrum, which on many systems show up as yellow or orange instead of blue. So an aliased away-flow signal is typically seen as yellow/orange. To reduce aliasing, you can raise the color scale (PRF) or use spectral Doppler to measure high velocities directly.