What the Colors Mean on a Kidney Ultrasound
The invention of color Doppler was a breakthrough in medical ultrasounds. This technique made it possible to directly observe blood flow within certain organs, such as the kidney and heart and the cardiovascular system. Such visualization is made possible by color encoding Doppler information and displaying the colors as an overlay on the image of the organ. The colors represent the speed and direction of blood flow within a certain area of the image called the color box. What the colors mean on a kidney ultrasound are the same as an ultrasound of the heart, which we will discuss here.
The Color Box
The colors on a kidney ultrasound represent the speed and flow direction of blood within a certain area (known as velocity flow). The color box is divided into small sample regions. Each one represents the mean velocity with the region as measured by multiple sample volumes. The mean velocity is then changed into a specific color. Flow toward the transducer is depicted in red, while flow away is shown in blue. Different shades of the colors are used to display flow velocity. Lighter shades of the color are assigned to higher velocities. The exact manner that velocities are displayed is defined by the color map, but more importantly, by the actual velocity of the blood flow.
The Color Map
Ultrasound technicians are provided with several color maps so they can choose the colors that they prefer. Some maps show turbulent flows in yellow or green. Turbulent flow is present when there are big variations in flow velocity in the same sample region. The information is consistently updated from frame to frame, providing a clear view of how the blood flows. The main advantage of color Doppler is that the technician can view blood flow simultaneously in many regions.
Aliasing and Color
Color Doppler is based on the same principles as PW Doppler and is therefore subject to aliasing. When the velocity of blood flow exceeds the Nyquist limit, it becomes impossible to measure the direction of the flow. This results in a quick shift in color from blue to red and vice versa. When the Nyquist limit is reached and the colors change fast, variations in velocity are marked in regions of turbulent flow. There will then be a mosaic pattern of colors. This phenomenon of aliasing and mosaic colors provides a good delineation of jets and turbulent flow issues within the area being scanned.
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