The Hounsfield unit (HU) scale is a linear transformation of the original
linear attenuation coefficient measurement into one in which the
radiodensity of
distilled water at standard
pressure and
temperature (
STP) is defined as 0 Hounsfield units (HU), while the radiodensity of
air at STP is defined as −1000 HU. In a
voxel with average linear attenuation coefficient \mu, the corresponding HU value is therefore given by: HU = 1000\times\frac{\mu - \mu_{\textrm{water}}}{\mu_{\textrm{water}} - \mu_{\textrm{air}}} where \mu_{\textrm{water}} and \mu_{\textrm{air}} are respectively the linear attenuation coefficients of water and air. Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero. Calibration tests of HU with reference to water and other materials may be done to ensure standardised response. This is particularly important for CT scans used in
radiotherapy treatment planning, where HU is converted to
electron density. Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures.
Rationale The above standards were chosen originally to encode the radiodensity of organic tissues relative to water for
12-bit processing on clinical CT scanners. A 12-bit encoding corresponds to 4096 (2^{12}) values, where the range (–1024 to 3071) encompasses HU values for air, soft tissue and bone. ==Values for different body tissues and material ==