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1. The CT number scale typically covers a 12 bit range: 12 bits => 4096 shades of grey (–1024 to 3071).
2. The human eye cannot distinguish this many shades of grey (its range is about 50).
3. If we displayed –1000 as black and +3000 as white then very little differentiation would be seen between soft tissues (whose CT numbers are within a limited range).
4. We therefore use windowing.
5. Many different presets depending on the region of interest. We set a CT number level that we’re interested in and a range either side. Anything below this window is black and above is white.
6. Window level = CT no. of mid-grey; window width = contrast.


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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