In pulsed wave Doppler, same piezoelectric crystal is used to transmit and receive the echo from the sample volume. Hence the signals are sent out in pulses and the intervals between the pulses are used to receive the echoes. In continuous wave Doppler, one piezoelectric crystal transmits continuously and another one receives continuously. As the transmission and reception are continuous, it is not possible to find out the depth from which the return signals are received. At the same time continuous wave Doppler can analyze higher velocities while pulsed wave Doppler can analyze only lower velocities.
In case of pulsed Doppler, the maximum velocity which can be analyzed is limited by the Nyquist limit. Nyquist limit is half the pulse repetition frequency. When the velocity of the signal being analyzed, is beyond the Nyquist limit, aliasing occurs so that the exact direction of the signal will be masked.
In this image, aortic regurgitation jet is seen as aliased
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