Changes

<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.