Which of these Doppler techniques allows for continuous operation and avoids aliasing?

Continuous-wave Doppler is the technique that allows for continuous operation and effectively avoids aliasing. This method employs two crystals: one that continuously emits sound waves while the other continuously receives them. This setup enables it to measure high-velocity blood flow without the limitations imposed by pulsed-wave Doppler, which can only sample velocities up to a certain limit before aliasing begins to occur.

Aliasing arises in pulsed-wave Doppler when the frequency of the blood flow exceeds the Nyquist limit, which is determined by the pulse repetition frequency. Since continuous-wave Doppler does not have this limitation, it is particularly useful in situations involving high-velocity flows, such as in certain valvular heart diseases or vascular stenoses.

Other techniques like color Doppler and power Doppler are more suited for visualizing blood flow within a given area rather than measuring specific velocities. Although they can display flow directions and patterns, they will not provide the same continuous measurement capabilities or avoid aliasing challenges associated with high-velocity flows as effectively as continuous-wave Doppler.