Hounsfield unit

Hounsfield units (HU) are a dimensionless unit universally used in computed tomography (CT) scanning to express CT numbers in a standardised and convenient form. Hounsfield units, created by and named after Sir Godfrey Hounsfield, are obtained from a linear transformation of the measured attenuation coefficients 1. This transformation (figure 1) is based on the arbitrary definitions of air and water:

  • radiodensity of distilled water at standard temperature and pressure (STP) = 0 HU
  • radiodensity of air at STP = -1000 HU

NB: standard temperature is 0 °C and pressure is 105 pascals.

Hounsfield units are measured and reported in a variety of clinical applications. One well known use is the evaluation of the fat content of the liver, with fatty liver diagnosed by the presence of a liver-to-spleen ratio less than 1.0 or 0.8 2.  Other less common uses include assessing bone mineral density (BMD) 3, predicting the presence of anaemia 4, and guiding the management of kidney stones 5.


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Article information

rID: 38181
Section: Physics
Tag: ct
Synonyms or Alternate Spellings:
  • Hounsfield units (HUs)
  • Hounsfield unit (HU)

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    Figure 1: formula
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