The Nyquist limit in Doppler ultrasound is defined as which of the following?

• A. The speed of sound divided by the transducer diameter
• B. Half of the pulse repetition frequency
• C. The product of depth and velocity
• D. The square root of the PRF

Answer: (B)

Nyquist limit is the highest Doppler frequency shift you can measure without aliasing, and it equals half the pulse repetition frequency. The PRF is set by how fast you listen for echoes, which depends on depth: shallower sampling gives a higher PRF and thus a higher Nyquist limit, while deeper imaging lowers PRF and lowers the Nyquist limit. If the Doppler shift from blood flow exceeds this limit, the velocity reading wraps around and becomes misleading on the display. This is why half of the PRF best defines the Nyquist limit. The other options describe quantities that aren’t about sampling rate or aliasing, so they don’t define this limit.