CT windows: one scan, many pictures
Why the same CT slice can look completely different: Hounsfield units, the limited greys of a screen, and why scans are read in more than one window.
Key points
- CT expresses each voxel in Hounsfield units, a scale defined so that water is 0 HU and air is -1000 HU.
- A CT voxel can take one of about 4096 values, but a standard screen shows far fewer grey shades, typically 256.
- So a window of HU is chosen and mapped to grey: values below it are black, values above it are white.
- Some structures and abnormalities can only be seen in a particular window, so the same scan is read in several.
- In a study of acute middle cerebral artery stroke, a narrow, adjusted brain window raised sensitivity for ischaemic brain from 57% to 71% without loss of specificity.
Same scan, two pictures
Take one chest CT slice and show it twice. In the first picture the heart, vessels and chest wall are in shades of grey and the lungs are black. In the second, the lungs are full of fine detail and the soft tissues are almost white. Nothing about the scan changed. Only the window did.
Hounsfield units
CT gives every voxel a number on the Hounsfield scale, defined so that water is 0 HU and air is -1000 HU.
Too many numbers for one screen
A CT voxel can take one of about 4096 values. A standard screen shows far fewer grey shades, typically 256. If all 4096 values were squeezed into 256 greys, small differences between tissues would disappear.
So the reader chooses a window: a range of Hounsfield values that is spread across the full grey scale. Everything below the window is shown black, everything above it white, and only the values inside it become shades of grey. A wide window shows a large range with little contrast; a narrow window shows a small range with strong contrast.
Why we read in several windows
Some structures and abnormalities can only be seen in a particular window. That is why the same scan is read in several: a soft-tissue or mediastinal window for the organs and vessels, a lung window for the lung, and others where needed. On a workstation the reader can change the window width and centre to bring out the contrast between tissues.
The difference is not cosmetic. In a study of acute middle cerebral artery stroke, reading the same unenhanced CT with a narrow, adjusted brain window raised sensitivity for ischaemic brain from 57% to 71%, without loss of specificity.
How to practise
Open any CT and change only the window. Watch which structures appear and which disappear. Then decide, for each question you are asking of the scan, which window answers it.
Check yourself
On the Hounsfield scale, what value is water?
Sources
- Chauvie S, Mazzoni LN, O'Doherty J. A review on the use of imaging biomarkers in oncology clinical trials: quality assurance strategies for technical validation. Tomography 2023;9(5):1876-1902. doi:10.3390/tomography9050149 · PMID 37888741
- Ebert LC, Franckenberg S, Sieberth T, et al. A review of visualization techniques of post-mortem computed tomography data for forensic death investigations. Int J Legal Med 2021;135(5):1855-1867. doi:10.1007/s00414-021-02581-4 · PMID 33931808
- Lev MH, Farkas J, Gemmete JJ, et al. Acute stroke: improved nonenhanced CT detection, benefits of soft-copy interpretation by using variable window width and center level settings. Radiology 1999;213(1):150-155. doi:10.1148/radiology.213.1.r99oc10150 · PMID 10540655
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For educational purposes only. Not medical advice.