Which statement correctly describes a condition that avoids aliasing in Doppler sampling?

• A. Sampling frequency is equal to highest Doppler frequency
• B. Sampling frequency is less than the Nyquist limit
• C. Sampling frequency is at least twice the highest Doppler frequency
• D. Aliasing cannot be avoided in Doppler measurements

Answer: (C)

Understanding aliasing in Doppler sampling comes from the Nyquist criterion: to capture all frequency content without folding, the sampling rate must be at least twice the highest frequency you’re trying to measure. In Doppler ultrasound, that highest frequency is the maximum Doppler shift you intend to detect. If you sample at a rate equal to or less than that maximum, frequency components above half the sampling rate will wrap around and appear at incorrect frequencies, distorting velocity and even direction. So the condition to avoid aliasing is that the sampling frequency is at least twice the highest Doppler frequency. For example, if the maximum Doppler frequency is 8 kHz, you’d want a sampling rate of 16 kHz or higher. The other statements either undercut the required rate or state something that’s not true about Doppler aliasing.