Radiology for Residents

The 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. In a voxel with average linear attenuation coefficient μₓ , the corresponding HU value is therefore given by:

Stacks Image 68
μ water is the linear attenuation coefficients 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.


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.

[ Substance - HU]
  • Air −1000
  • Lung −500
  • Fat −100 to −50
  • Water 0
  • CSF -15
  • Kidney -30
  • Blood -+30 to +40
  • Muscle -+10 to +40
  • Grey matter -+37 to +45
  • White matter -+20 to +30
  • Liver -+40 to +60
  • J-O Blast +20 to +30
  • Soft Tissue, Contrast-+100 to +300
  • Bone -+700 (cancellous bone) to +3000 (dense bone)

A practical application of this is in evaluation of tumors, where, for example, an adrenal tumor with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign adrenal adenoma.


Feeman, Timothy G. (2010). The Mathematics of Medical Imaging: A Beginner's Guide. Springer Undergraduate Texts in Mathematics and Technology. Springer. ISBN 978-0387927114.

Who was Hounsfield

Sir Godfrey Newbold Hounsfield CBE, FRS, (28 August 1919 – 12 August 2004) was an English electrical engineer who shared the 1979 Nobel Prize for Physiology or Medicine with Allan McLeod Cormack for his part in developing the diagnostic technique of X-ray computed tomography (CT).

His name is immortalised in the Hounsfield scale, a quantitative measure of radiodensity used in evaluating CT scans. The scale is defined in Hounsfield units (symbol HU), running from air at −1000 HU, through water at 0 HU, and up to bone at +400 HU and more

Childhood and education

Hounsfield was born in Sutton-On-Trent, Nottinghamshire, England on 28 August 1919. He was the youngest of five children. (Two brothers, Two sisters) As a child he was fascinated by the electrical gadgets and machinery found all over his parents' farm. Between the ages of eleven and eighteen, he tinkered with his own electrical recording machines, launched himself off haystacks with his own home-made glider, and almost killed himself by using water filled tar barrels and acetylene to see how high they could be waterjet propelled. He attended the Magnus Grammar School (now Magnus Church of England School) in Newark-on-Trent and excelled in physics and arithmetic.


Shortly before World War II, he joined the Royal Air Force as a volunteer reservist where he learned the basics of electronics and radar. After the war, he attended Faraday House Electrical Engineering College in London, graduating with the DFH (Diploma of Faraday House). Faraday House was a specialist Electrical Engineering college that provided university level education and was established in 1890, before the advent of most university engineering departments. Faraday House pioneered the use of sandwich courses, combining practical experience with theoretical study.

The suggestion that Hounsfield lacked formal engineering education to the level of a Chartered Engineer does not reflect the nature of engineering education at the time when Hounsfield was a student, or the esteem in which Faraday House was held within the profession.

EMI and later years

In 1951, Hounsfield began work at EMI Ltd in their Central Research Laboratories in Hayes, Middlesex where he researched guided weapon systems and radar. There, he became interested in computers and in 1958, he helped design the first commercially available all-transistor computer made in Great Britain: the EMIDEC 1100. Shortly afterwards, he began work on the CT scanner at EMI. He continued to improve CT scanning, introducing a whole-body scanner in 1975, and was senior researcher (and after his retirement in 1984, consultant) to the laboratories.

Hounsfield received numerous awards in addition to the Nobel Prize. He was appointed Commander of the British Empire in 1976 and knighted in 1981. In 1975, he was elected to the Royal Society.
He never married and died in 2004.