The formula is:
hu = pixel_value * slope + intercept
Normally, these values are stored in the DICOM file itself. The tags are
generally called the Rescale Slope and Rescale Intercept, and typically have
values of 1 and -1024, respectively.
The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement in one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU. For a material X with linear attenuation coefficient μX, the corresponding HU value is therefore given by where is the linear attenuation coefficient of water. 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. It is the definition for CT scanners that are calibrated with reference to water.
| Tissue | CT number (HU) |
|---|---|
| Bone | 1000 |
| Liver | 40 - 60 |
| White matter | ~20-30 |
| Grey matter | ~37-45 |
| Blood | 40 |
| Muscle | 10 - 40 |
| Kidney | 30 |
| Cerebrospinal fluid | 15 |
| Water | 0 |
| Fat | -50 - 100 |
| Air | -1000 |
sources: