Adjusting the color velocity scale. (a) Color Doppler flow US image obtained with the color velocity scale set too high (69.2 cm/sec) demonstrates apparent absence of flow in the portal vein. (b) Color Doppler flow US image obtained after the scale was reduced to 30.7 cm/sec demonstrates normal flow in a widely patent portal vein. (c) Color Doppler flow US image obtained after the scale was set even lower (2.3 cm/sec) shows aliasing of color flow in all branches of the portal vein, which results in meaningless data concerning flow direction. Thus, the color velocity scale should be increased to increase the sampling rate.

http://radiographics.rsna.org/content/24/3/657.figures-only