The NYQUIST LIIMIT-It is a sampling phenomenon, which limits the maximum fiequemcy shift measurement to one half of the sampling frequency (PRF).

By its nature, pulse wave Doppler is a sampling system while continuous wave Doppler is not. The nyquist limit is a theoretical maximum frequency that a

If the frequency shift is higher than the Nyquist frequency, aliasing occurs; i.e., the Doppler spectrum is cut off the Nyquist frequency and the remaining frequency shift is recorded on the opposite side of baseline.

The PRF varies inversely with the depth of the sample volume the shallower the location of the sample volume, the higher the PRF and Nyquist frequency. In other words, higher velocities can be recorded without aliasing by pulse wave Doppler if the sample volume is closer to transducer.