Figure 12.

Aliasing. Aliasing is the major Doppler artifact. It occurs when transducer PRF is less than twice the frequency of Doppler shift. In other words, the PRF setting must be great enough to sample at least two times for each cycle the Doppler signal. If the PRF is less than twice the maximum Doppler shift, the aliasing will occur. PRF value equal to 2fD is known as the Nyquist limit on the basis of the Shannon theorem. In the upper part of figure, a sampling of two sine waves is represented. The black curve is sampled at discrete times indicated by arrows. The dotted red lines on the bottom are the resultant sample signal. The sampling is adequate on the right side because the sampling rate is over the Nyquist limit. On the left side, the sampling rate is below this limit and the frequency of the resultant is an alias of the real signal. CD image represents critical stenosis of the cephalic arch at the level of infra-clavicular Mohrenheim’s fossa. Aliasing of spectral analysis appears as an abrupt cut and inversion of waveform peak. The table lists the useful procedures to correct the aliasing.