Velocities recorded in a sample volume of the pulsed wave Doppler can be presented with a color. A color scale from blue to red is conventionally used. Blue color implies velocities (movement) away from the transducer and red color implies velocities (movement) towards the transducer. If many sample volumes are placed along several Doppler lines, then all velocities in the area can be presented with colors. The brighter the color, the higher the velocity. As shown in Figure 1, the Doppler sector is superimposed on the 2D image to facilitate the interpretation of the Doppler signals.
The main advantage of color Doppler is that it allows for rapid visualization of flows, velocitites and volumes. This is useful for detecting valvular regurgitation and defects in the atria or ventricles (Figure 2). In addition, color Doppler can be used to align the continuous Doppler.
Because color Doppler is a type of pulsed wave Doppler, it is limited by the Nyquist limit. In fact, color Doppler is limited more by the Nyquist limit (as compared with standard pulsed wave Doppler), which is explained by the fact that the pulse repetition frequency (PRF) is reduced when obtaining both a 2D image and Doppler signals simultaneously. If the blood flow velocity exceeds the Nyquist limit, aliasing occurs and the signal changes color (blue turns red, and red turns blue). Aliasing usually occurs at speeds above 0.5 m/s. Aliasing can be reduced by minimizing the distance between the transducer and the color sector (where sample volumes are recorded) and by using the smallest possible sector.
Note that color Doppler presents the average velocity in each sample volume (i.e not the maximum velocity). Large variations in velocities recorded within a single sample volume indicate turbulent flow; the ultrasound machine is programmed to depict such flows with green color to indicate that the flow is turbulent. Figure 1 displays green areas within the Doppler sector.