©Walter Rasmussen, R.D.C.S.
Color Doppler ultrasound is the process of spatially displaying a range of blood flow velocities within an adjustable, movable trapezoidal area of the two dimensional image.
Color Doppler is based upon rapid automatic analysis of pulsed Doppler phase shift caused by movement of blood within the ultrasound beam. Since pulses take time to reach their target and return, the ultrasound machine has severe limitations in the maximum velocity it can assign an accurate directional color to and how quickly images are updated (temporal resolution). The goal of color Doppler is to observe the blood flow in motion moving through valves, chambers, vessels and restrictions.
All ultrasound machines conform to the established standard using a range of progressively brighter red colors to indicate blood moving toward the transducer and progressively brighter blue colors for blood flow moving away from the transducer. The brightness of the colors displayed indicates the strength of the signal, as well as velocity. Where there is turbulence or velocities that are too high for the machine to accurately gauge, a bright pixel-mosaic pattern of yellow and red colors are displayed.