According to Shannon's sampling theorem, the sampling frequency should be twice the frequency being sampled. The nyquist frequency is the maximum frequency that can be sampled without aliasing. In ultrasound imaging, it is defined as half of the pulse repetition frequency.
NF = PRF/2 (nyquist frequency = pulse repetition frequency/2)
This is the so-called Nyquist limit. If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and appear to change direction. Color flow Doppler capitalizes on this effect. This allows detecting flow disturbances from laminar to turbulent flow.

See also Aliasing Artifact, Repetition Rate, and Sampling Rate.

Echoes of deep lying structures within the body do not always come from the latest emitted sound pulse and can produce an aliasing artifact. Aliasing lowers the frequency components when the pulse repetition frequency is less than 2 times the highest frequency of a Doppler signal. This artifact can be problematical at Spectral or Color Doppler examinations.
Aliasing of the data displayed in pulsed wave technology is utilized as a benefit in determining transitions from laminar to turbulent flow.

See also Ultrasound Imaging Modes.