Window level direction: it depends on the viewing software, so check your own system
Key takeaways
- It depends on the viewer. Bontrager says some PACS go darker as level rises and others go brighter.
- Bushong, Merrill's and ARRT's outlines give no fixed direction.
- Window level sets brightness. Window width sets contrast.
- Narrow width: short scale, high contrast. Wide width: long scale, low contrast.
- Windowing doesn't change dose, or an exposure indicator or deviation index worked out before display processing.
It depends on the viewing software. Bontrager says that in some PACS a higher window level makes the image darker, and in others brighter. No textbook we opened sets one direction, so check which way your own system works.
The part you can rely on everywhere is the pairing: level sets brightness and width sets contrast.
Why would a higher level look darker?
Level is the center of the slice of pixel values being displayed. Raise it and the slice moves up the scale. A pixel that sat in the middle now sits low in the window, so it displays darker. That is our reasoning, not a line from a textbook.
It assumes higher values display brighter, as in Bontrager’s CT table, where cortical bone at +1000 is white and air at -1000 is black. Change that display mapping and the direction flips.
Why isn’t it a flat rule?
Sources don’t settle it for every radiography viewer. Bontrager says the effect depends on the viewing software: in some PACS a higher level darkens the image, and in others it brightens it. Bushong explains what window and level do but gives no direction. Merrill’s CT chapter says level sets the midpoint and controls brightness, and gives no direction either. ARRT® lists “brightness (e.g., window level)” in its 2027 outline and states no direction.
We found no source that makes this direction a fixed exam rule, and ARRT doesn’t publish its questions. Try it on your department’s viewer, and use the convention your program teaches.
What does window width do?
- Narrow: few values spread across the grays. Short scale, high contrast.
- Wide: many values across the grays. Long scale, low contrast.
Bontrager gives CT examples: wide width and low contrast in chest imaging, narrow width and high contrast in cranial imaging.
What doesn’t windowing change?
It doesn’t change the patient’s dose. It also doesn’t change an exposure indicator or deviation index worked out before display processing, as AAPM Report 116 proposes. Some older vendor scales differ: the report says Canon’s REX depends on the brightness and contrast settings, and Konica’s S-value follows its gradation processing, which the operator can change. Windowing also can’t add detail the exposure never recorded. To change noise or subject contrast, you change the technique on the next exposure.
More in the Image Production guide, the Digital Image Processing chapter and the Equipment QA set.
Frequently asked questions
Does a higher window level make the image brighter or darker?
It depends on the viewing software. Bontrager notes that some PACS make the image darker and others brighter when the level goes up, and no textbook we opened sets one direction. Check your own system and use the convention your program teaches.
What is the difference between window width and window level?
Width is how wide a slice of pixel values is displayed, and it sets contrast. Level is where that slice sits, and it sets brightness.
Which window width gives high contrast?
A narrow width gives short scale and high contrast. A wide width gives long scale and low contrast.
Does windowing change patient dose?
No. Windowing changes the display only. The exposure was already made.
Sources
Found an error? Email [email protected] with the page and the source.
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