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 are obtained from a linear transformation of the measured attenuation coefficients 1. This transformation (figure 1) is based on the arbitrarily-assigned densities of air and pure water:

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

STP: standard temperature is 0 °C and pressure is 105 pascals (i.e. sea-level).

This results in a scale running from -1000 HU for air to +~2000 HU for very dense bone (cochlea). 

The software of all CT scanners and PACSs has the ability to measure the density of a region of interest (ROI) electronically overlaid the image.

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 <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.

History and etymology

Hounsfield units are named after Sir Godfrey Hounsfield, the Nobel prize winning, developer of CT.

Physics and imaging technology: CT
Share article

Article information

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

Support Radiopaedia and see fewer ads

Cases and figures

  • Figure 1: formula
    Drag here to reorder.
  • Updating… Please wait.

     Unable to process the form. Check for errors and try again.

     Thank you for updating your details.