Color Doppler Scale Key
©Walter Rasmussen, R.D.C.S.
A color Doppler scale key, (pictured), is placed in the data area of the display for the purpose of showing which colors are being used to represent the range of velocities from low to high, as well as the maximum velocity that can accurately be recorded, in centimeters per second.
The established convention for color Doppler is a red spectrum, indicating blood moving toward the transducer and a blue spectrum indicating blood moving away from the transducer. The top of the rule is bright red with yellow mixed in and the bottom of the rule is bright blue with yellow mixed in, producing a green. Slower velocities are marked with a darker and more pure red or blue.
In the middle of the color key bar is a black area called the baseline and indicates zero Doppler shift. The baseline on color Doppler does not indicate a lack of blood flow but simply that if no movement is detected, no colors are displayed. The baseline can be shifted up or down so that higher velocities can be displayed accurately in primarily one direction or the other, however, the baseline is not generally moved when performing an Echocardiogram and the colors are allowed to alias (become ambiguous) when they exceed what is termed the, “Nyquist limit”. Unintentional shifts of the baseline can happen, and the sonographer should always make certain that the machine is set with the baseline in the center and the maximum velocity numbers equal, in order to avoid confusion.
As with all Doppler systems, for the greatest sensitivity and accuracy, the transducer axis should be aligned as parallel as possible to the blood flow. Beam penetration should be maximized by using a transducer position that produces an image that is as clear as possible in the area of interest.
Multiple viewpoints are necessary for complete assessment of blood flow with color Doppler during an exam. For example, an atrial-septal defect is often best-seen from the subcostal view because the septum is perpendicular to the transducer position and the shunt direction will likely be parallel to the ultrasound beam. In contrast, the ASD is more likely to be invisible from the apical window, where the septum is parallel to the ultrasound beam and the shunt is typically perpendicular to it.