What is aliasing?

Aliasing is the result of flow velocity exceeding the measuring ability of pulsed-wave Doppler. To measure a velocity, pulsed Doppler must sample the moving blood rapidly. In stenosed areas, blood velocity may exceed sampling ability. The high-velocity blood is assigned an incorrect velocity by the system. On spectral Doppler, this is seen as wraparound (arrows), with high velocity on the bottom of the scale rather than at the top.
On color Doppler, it is seen as an inversion of color (blue within an area of red or vice versa).