# 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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