Which artifact occurs when the Doppler sampling rate is too low, causing high velocities to be displayed as lower velocities or reversed?

Which artifact occurs when the Doppler sampling rate is too low, causing high velocities to be displayed as lower velocities or reversed?

• A. Aliasing
• B. Spectral mirroring
• C. Reverberation
• D. Enhancement

Answer: (A)

When the Doppler sampling rate is too low, the system can’t unambiguously distinguish fast motion from slower motion. This limit is set by the Nyquist principle: you can only reliably measure velocities up to half the sampling rate (the Nyquist limit). If actual velocities exceed this limit, the measured values wrap around and appear as lower velocities or even flip to the opposite direction. That wrapping effect is known as aliasing, and it is exactly the artifact described. Other artifacts are related to different phenomena: reverberation comes from multiple echoes between interfaces, enhancement is due to strong backscatter in a region, and spectral mirroring is a separate display issue that isn’t caused by undersampling of the Doppler signal. The scenario of high velocities being displayed as lower or reversed directly points to aliasing.

When the Doppler sampling rate is too low, the system can’t unambiguously distinguish fast motion from slower motion. This limit is set by the Nyquist principle: you can only reliably measure velocities up to half the sampling rate (the Nyquist limit). If actual velocities exceed this limit, the measured values wrap around and appear as lower velocities or even flip to the opposite direction. That wrapping effect is known as aliasing, and it is exactly the artifact described.

Other artifacts are related to different phenomena: reverberation comes from multiple echoes between interfaces, enhancement is due to strong backscatter in a region, and spectral mirroring is a separate display issue that isn’t caused by undersampling of the Doppler signal. The scenario of high velocities being displayed as lower or reversed directly points to aliasing.