Infographics / CT Physics

Hounsfield unit scale & calibration

A labelled vertical Hounsfield unit scale defined by HU = 1000 times (mu minus mu-water) divided by mu-water. Water is 0 HU (reference) and air is minus 1000 HU; these two are the only calibration points. Fat is about minus 100 HU, aerated lung minus 500 to minus 900 HU, dense cortical bone about plus 1000 HU, and metal or concentrated iodinated contrast above plus 1000 HU. An expanded soft-tissue panel shows water 0, white matter 20 to 30, grey matter 35 to 45, blood 30 to 45 and organ soft tissue 20 to 70 HU clustered just above water, with overlapping ranges. A windowing schematic shows that window width and level select the band of HU that is spread across the display grey scale.

CT number (Hounsfield unit) scale A labelled vertical Hounsfield unit scale defined by HU = 1000 times (mu minus mu-water) divided by mu-water. Water is 0 HU (reference) and air is minus 1000 HU; these two are the only calibration points. Fat is about minus 100 HU, aerated lung minus 500 to minus 900 HU, dense cortical bone about plus 1000 HU, and metal or concentrated iodinated contrast above plus 1000 HU. An expanded soft-tissue panel shows water 0, white matter 20 to 30, grey matter 35 to 45, blood 30 to 45 and organ soft tissue 20 to 70 HU clustered just above water, with overlapping ranges. A windowing schematic shows that window width and level select the band of HU that is spread across the display grey scale. CT number (Hounsfield unit) scale HU = 1000 × (μ − μwater) / μwater · CT number ≡ Hounsfield unit water = 0 HU and air = −1000 HU by definition; values shift slightly with kV / filtration +1000 +500 0 −500 −1000 HU metal / iodinated contrast > +1000 HU dense cortical bone ≈ +1000 HU soft tissues 0–70 HU — see detail water = 0 HU (reference) fat ≈ −100 HU aerated lung −500 to −900 HU air = −1000 HU (reference) soft-tissue detail (0–70 HU expanded) +50 0 −50 −100 soft tissue / organs 20–70 grey matter 35–45 blood 30–45 white matter 20–30 water 0 HU fat ≈ −100 HU ranges overlap; values vary with kV / filtration windowing (display mapping) W = HU range mapped to full grey scale L = window centre above W → white below W → black Calibration references: water (0 HU) and air (−1000 HU) only — bone, fat and iodine are not fixed reference points.
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