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# Hounsfield scale

The Hounsfield scale is a quantitative scale for describing radiodensity.

## Definition

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

$\frac{\mu_X-\mu_{H_2O}}{\mu_{H_2O}-\mu_{air}}\times 1000$

where $\mu_{H_2O}$ and μair are the linear attenuation coefficients of water and air, respectively, at STP. Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient difference between water and air, or approximately 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.

### Rationale

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

Substance HU
Air -1000
Fat -120
Water 0
Muscle +40
Bone +400 or more

## History

It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineers and developers of computed axial tomography (CAT, or CT scans).

CT machines were the first imaging devices for detailed visualization of the internal three-dimensional anatomy of living creatures, initially only as tomographic reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-Ray images show only compressed two-dimensional images of complex anatomy, i.e. radiodensity shadows.