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.

(CWD) Continuous wave (CW) Doppler is an ultrasound imaging mode, which records blood flow velocities along the length of the beam. Continuous wave Doppler uses different crystals to send and receive the signal. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound waves of a single frequency while the other half is continuously receiving the reflected signals.
The advantages of a continuous wave transducer are a high sensitivity and no Nyquist limit. CW Doppler does not alias but has no depth precision and large gate. The beat frequency is the Doppler shift. CW Doppler echocardiography employs this technique to record the flow of blood through the cardiovascular system.

See also Cross Talk, Periorbital Doppler, and Mirror Artifact.

Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

The Nyquist frequency describes the highest frequency that can be sampled in a pulsed ultrasound system. To record higher frequencies the zero baseline can be shifted in the desired direction.

See also Nyquist Limit, and Sampling Rate.