APPLIED PHYSICS CORNER Here in  practical application of a  physics principle will be discussed. ALIASING It is an artefact. When the sampling rate is too low, the direction of flow is presented wrongly, this is called as Aliasing It is caused when a signal is sampled at less than double its frequency (less than twice the maximum shift produced by the movement of the target (the Nyquist limit)> gives rise to > ambiguous signal of incorrect(aliased) frquency. HOW DOES IT LOOK On colour doppler : in addition to normal red and blue colours,intervening colour shades are seen-mosaic pattern (a transition of unsaturated colours). On spectrum : aliasing appears as a "wrap-around" effect, the higher frequencies are displayed below the baseline. PRACTICAL APPLICATION -Encountered in high velocity flow: allows identification of high velocity jets  Eg. ductus venosus, stenosis   site. -To reach deep vessels, low PRF is needed > more prone to aliasing if high velocities are present TO REDUCE ALIASING: -Increase PRF                                                      Note : on increasing the pulse repetition frequency (PRF), aliasing is alleviated, however, sensitivity to low velocities is diminished. eg. carotid artery is filled in systole and not in diastole, jugular vein is not filled in. -Shift the baseline downwards (increases the PRF)        -Increase doppler angle within limits (not beyond 60 degree) -Use low frequency transducer  -Scan at shallower depth -Power doppler does not suffer from aliasing