Color baseline. The position of the baseline on the color bar is indicated by a horizontal black line (yellow circles). When the baseline is adjusted, the relative position of this horizontal black line changes. Note that when the position of the baseline is changed, the color velocity range that is displayed on the color bar also changes (in this example, from 15.3 to 46.1 cm/sec above or below the baseline). The range of depicted velocities remains constant, but different flow velocities will be emphasized depending on their relative position on the color bar.


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