Cardiology MCQ: Nyquist limit

Relation of Nyquist limit to Pulse Repetition Frequency (PRF):

A. Nyquist limit is half the pulse repetition frequency

B. Nyquist limit is twice the pulse repetition frequency

C. Nyquist limit is three times the pulse repetition frequency

D. Nyquist limit is one third of the pulse repetition frequency

Correct Answer: A. Nyquist limit is half the pulse repetition frequency

Nyquist limit is the maximum velocity of blood flow that can be estimated by pulsed Doppler. It can be increased by increasing the pulse repetition frequency of the machine until the physical limit is reached. If the velocity is higher, the spectrum display will appear to wrap around or ‘alias’. That is a signal which has to be shown above the baseline will be shown partly above the baseline and remaining part below the baseline.