18.104.22.168 Principles of Color Doppler
The advent of color Doppler was a breakthrough in medical ultrasound. With this technique it became possible to directly "observe" blood flow within the heart. Such visualization is achieved by color-encoding Doppler information and displaying the colors as an overlay on the 2D image of the heart. The colors represent the speed and direction of blood flow within a certain area of the image (color box). The color box is divided into small sample regions (one color pixel). Each of these represent the mean velocity within the region as measured by multiple PW sample volumes. The mean velocity is then converted into a specific color. By definition, flow towards the transducer is depicted in red while flow away from the transducer is shown in blue. Different shades of red and blue are used to display velocity. Lighter shades of color are assigned to higher velocities. The exact manner in which velocities are displayed is defined by the so-called color map. Ultrasound scanners are usually provided with several maps from which the investigator may choose those he/she prefers. Some maps encode turbulent flow in yellow or green. Turbulent flow is present when there are large variations in flow velocity within the sample region. The information is consistently updated from frame to frame, thus providing a clear view of how blood "flows". The main advantage of color Doppler is the fact that one can view blood flow simultaneously in many regions of the heart. However, in contrast to PW and CW Doppler, color Doppler only permits semiquantitative assessment of blood flow velocities.
Color Doppler showing a mitral regurgitation (MR) and an aortic regurgitation (AR) jet. Note that the AR jet is red (flow towards the transducer) while the MR jet is blue (the jet is directed away from the transducer):
Since color Doppler is based on the same principles as PW Doppler it is also subject to aliasing. When the velocity of blood flow exceeds the Nyquist limit (0.4-0.7 m/s) it becomes impossible to measure the direction of blood flow. This condition results in an abrupt shift of color from red to blue or vice versa. Note that the brightest shades of red and blue lie adjacent to each other at the aliasing point. This accentuates regions of aliased flow for the eye and helps the investigator to detect high-velocity jets.
Color Doppler and Aliasing
When the Nyquist limit is reached the color changes abruptly (red to blue, or blue to red). As variations in velocity are quite marked in regions of turbulent flow, you will find a "mosaic" pattern of colors.
The phenomenon of aliasing and "mosaic colors" provides a good delineation of jets (and measurement of PISA)
As with PW Doppler, the Nyquist limit (the velocity at which aliasing occurs) also depends on the imaging depth and the ultrasound frequency one is using. The Nyquist limit can be adjusted (within a certain range) by changing the pulse repetition frequency (PRF).GET FREE ECHO LECTURES