Hounsfield unit

Syllable breakdown

Dictionary meaning

the quantitative unit of X-ray attenuation used in computed tomography, named after Sir Godfrey Hounsfield (1919–2004), the British electrical engineer and inventor of CT scanning who shared the 1979 Nobel Prize in Physiology or Medicine; the Hounsfield scale is a linear scale normalised to the attenuation of water (defined as 0 HU) and air (defined as −1000 HU), with values ranging from approximately −1000 to +3000 in clinical practice; different tissues have characteristic Hounsfield unit ranges: air (−1000 HU), fat (−100 to −50 HU), water (0 HU), soft tissue (20–80 HU), blood (50–80 HU), bone (400–1000 HU), and calcified structures or metal implants at even higher values; these characteristic ranges allow radiologists to characterise tissue types on CT — determining whether a lesion is a simple cyst (water density, ~0 HU), a lipoma (fat density, negative HU values), or a hyperdense haematoma (elevated HU); window and level settings on CT viewing systems are set in Hounsfield units

Examples

• The CT report described a lesion with a Hounsfield unit value of −80, consistent with a fat-containing adrenal myelolipoma.
• The right basal ganglia haemorrhage appeared hyperdense on CT with Hounsfield unit values of approximately 70 HU, typical of acute blood.

Synonyms: HU, CT number, attenuation unit