Hounsfield scale

The Hounsfield scale, named after Sir Godfrey Newbold Hounsfield, is a quantitative scale for describing radiodensity.
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 zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU.
It is the definition for CT scanners that are calibrated with reference to water.

The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. humans.

The HU of common substances

The Hounsfield scale applies to medical grade CT scans but not to cone beam computed tomography (CBCT) scans.

SubstanceHU
Air-1000
Fat-120
Water0
Blood+30 to +45
Muscle+40
Contrast+130
Bone+400 or more

HU
Lungs-950 to -550
Kidneys+20 to +40
Heart+40 to +100
Liver+50 to +70
Spongiose Bone+50 to +200
Compact bone+250 to +1000

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