The CT image is composed of a range of CT numbers.
The range of the numbers is referred to as the Window Width (WW).
The center of the range is the Window Level (WL).
The WW and WL can be adjusted and can alter the image contrast and brightness.
The process of changing the WW and WL is referred to as Windowing.
Windowing is the process in which the grayscale of a particular image can be adjusted. This is typically done by the technologist or radiologist to better demonstrate certain anatomy or pathology.
WW controls the contrast
WL controls the brightness
The window width is the range of the grayscale that can be displayed.
The center of grayscale range is referred to as the window level.
A large window width means there is a long grayscale and the transition from black to white will take longer.
A narrow window width means the transition from black to white will take place much faster. Some examples of WW and WL are listed below:
WW of 2000 HU could mean the grayscale ranges from -1000 HU to +1000 HU, with a WL of 0 HU.
WW of 100 HU could mean the grayscale only ranges from 0HU to +100 HU, with a WL of +50 HU.
WW of 600 HU could mean the grayscale ranges from -300 HU to +300 HU, with a WL of 0 HU.
It is important to adjust the WW settings so that the structures of interest are properly displayed. Including an unnecessarily wide WW range can reduce the ability to differentiate between differences in attenuation.
If the abdomen is the area that is being imaged, it is important to include a wide WW in order to properly display the various tissues and densities present in the anatomical area. It is common for abdomen imaging to contain fat, bone, air, and fluid.
If the area being imaged contains structures of similar densities, a narrow WW should be utilized to properly display the differences between the structures with similar attenuation.
The WL should be set so that it is near the CT number for the area of interest that is being imaged.
By adjusting the WW, the contrast of the image can be changed.
If the WW increases, the contrast decreases.
If the WW decreases, the contrast increases.
If the WW is decreased, the image goes from black to white quickly, with very few shades of gray in between.
In most instances, the contrast of an image is best with a medium WW setting. This will allow for most structures and gray shades to be displayed.