Hounsfield unit
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Number used to represent the mean x-ray attenuation associated with each elemental area of the CT image. Measured values of attenuation are transformed into HU (also known as CT numbers) using the Hounsfield scale:
[math]HU = \frac{(μ_{{material}} - μ_{{water}})} {μ_{{water}}} \cdot 1000[/math]
where μ is the effective linear attenuation coefficient of the measured material relative to water for the x-ray beam used. The scale is defined so that under standard conditions water has a value of 0 HU and air has a value of -1000 HU.
(adapted from ICRP Publication 129, 2015)
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