Answer #54

The correct answer is D: Shift the baseline. Aliasing occurs when the Pulsed Wave Doppler cannot resolve the Doppler frequencies it receives, because they are above the Nyquist limit. This is equal to the pulse repetition frequency (PRF), which is how often the transducer sends out a pulse, divided by two. Events occurs much faster in time that this are not resolvable and cause aliasing, as shown in the figure below [figure.PW.aliasing]. Note that signal is both above and below the baseline in a haphazard fashion.

All of the techniques listed above can minimize aliasing except for shifting the baseline which will not remove true aliasing. CW Doppler (Choice A) is not really subject to aliasing and can resolving high velocity shifts. A lower frequency transducer (Choice B) will shrink the available spectrum and cause less aliasing as higher frequency transducers will cause more Doppler shift (recall that doppler shift is proportional to transducer frequency) and more aliasing. Chaning the avaliable scale on the machine (Choice C) to increase the Nyquist limit should decrease aliasing as will changing the angle of insonification. If you angle away from a higher velocity jet (Choice E) and are off axis, there will naturally be less aliasing.