The colors displayed on the flow map image contain useful information.
By convention, Doppler color flow systems assign a given color
to the direction of flow; red is flow toward, and blue is flow
away from the transducer. Three typical color bars from a color
flow imaging device are shown in Figure
4.2 and give an initial frame of reference to the meaning
of colors. Such color reference bars always appear on the screen
of Doppler flow imaging devices. The center of the standard color
bar on the left is black (white center reference mark) and represents
In addition to simple direction, velocity information is also displayed. Progressively increasing velocities are encoded in varying hues of either red or blue. The more dull the hue, the slower the velocity. The brighter the hue, the faster the relative velocity.
In the color bar shown in the center of Figure 4.2, the colors have been "enhanced" so that the hues of red are increased from very dull red to bright yellow and the hues of blue are increased from very dull blue to a bright pale blue. The enhanced map helps a beginner to understand the relationship between velocity and color. The bar at the right demonstrates variance.
As will be seen later, color is also used to display turbulent flow and allows an operator to discriminate between normal and abnormal flow states.