Windowing (CT)

Windowing, also known as gray-level mapping, contrast stretching, histogram modification or contrast enhancement is the process in which the CT image grayscale component of an image is manipulated via the CT numbers; doing this will change the appearance of the picture to highlight particular structures.The brightness of the image is, adjusted via the window width. The contrast is adjusted via the window level.

Window width

The window width as the name suggest is the measure of the range of CT numbers that an image contains.

A wider window width (2000), therefore, will display a wider range of CT numbers. Consequently, the transition of dark to light structures will occur over a larger transition area to that of a narrow window width (<1000).

Accordingly, it is important to note, that a significantly wide window displaying all the CT numbers will result in different attenuations between soft tissues to become obscured ¹.

Wide window

Defined as 400-2000 HU best used in areas of acute differing attenuation values, a good example is lungs or cortical tissue, where air and vessels will sit side by side. 

Narrow window

Defined as 50-350 HU are excellent when examining areas of similar attenuation, for example, soft tissue.

Window level/centre

The window level, often also referred to as window centre, is the midpoint of the range of the CT numbers displayed.

When the window level is decreased the CT image will be brighter and vice versa.

Typical window width and level values

Although this varies somewhat from institution to institution and vendor to vendor, window width and centres are generally fairly similar. The values below are written as width and level (W:x L:y) in Hounsfield units (HU). 

• head and neck
  • brain W:80 L:40
  • subdural W:130-300 L:50-100
  • stroke W:8 L:32 or W:40 L:40 ³
  • temporal bones W:2800 L:600
  • soft tissues: W:350–400 L:20–60 ⁴
• chest
  • lungs W:1500 L:-600
  • mediastinum W:350 L:50
• abdomen
  • soft tissues W:400 L:50
  • liver W:150 L:30
• spine
  • soft tissues W:250 L:50
  • bone W:1800 L:400