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Color Flow Imaging Basics: Frequency Aliasing

Color Flow Imaging is a form of pulsed wave (PW) Doppler and follows the same principles. In PW Doppler, when the frequency shift of the blood flow exceeds the Nyquist limit, the display shows an aliased signal. The same thing happens in color flow imaging, where aliasing appears as color reversal. As illustrated on the color bar below, when the velocity of blood flow moving toward the transducer causes aliasing the color changes from yellow to blue. When the velocity of blood flow moving away from the transducer causes aliasing, the color changes from blue to yellow.

In the example below, because the blood flow in the center of the jet is moving faster than light blue indicates, you see color reversal, from blue, through yellow, to orange. Notice the lightest blue and the lightest yellow are adjacent at the aliasing point. You can sometimes use this smooth progression of color around the color bar to easily identify a high velocity, laminar flow area.