The Nyquist limit represents the maximum Doppler shift frequency that can be correctly measured without resulting in aliasing in color or pulsed wave ultrasound.
Physics
The Nyquist limit always equals Pulse Repetition Frequency (PRF)/2 3. The US machine can display the Nyquist limit either as the maximum measurable blood flow velocity or in kHz at the top and bottom of a velocity range on a color scale. If the unit is in kHz, it represents the maximum measurable Doppler shift 4.
If the blood flow velocity exceeds this limit the device will incorrectly register the direction and velocity of the flow, resulting in color or spectral Doppler aliasing artifacts 1,2. The maximum measurable velocity of pulsed Doppler is 1 m/s at 6 cm depth 4.
In continuous wave Doppler, there is no pulse repetition frequency, thus no Nyquist limit to restrict the maximum measurable blood flow velocity 4.
Practical points
Choosing the appropriate PRF for the interrogated vessel is key to avoiding errors caused by exceeding the sampling limit. For high-velocity flow the PRF should be increased accordingly. As a rule of thumb the lowest PRF without aliasing should be chosen. In spectral Doppler this can be ensured not only by changing the PRF, but also by correctly setting the baseline of the Doppler curve 1.
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