Hounsfield scale

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The Hounsfield scale /ˈhnzˌfld/, named after Sir Godfrey Hounsfield, is a quantitative scale for describing radiodensity. It is frequently used in CT scans, where its value is also termed CT number.


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 , the corresponding HU value is therefore given by:

where and are respectively the linear attenuation coefficients of water and air.

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.

Value in parts of the body[edit]

The Hounsfield scale applies to medical-grade CT scans but not to cone beam computed tomography (CBCT) scans.[1]

Substance HU
Air −1000
Fat −120 to −90[2]
Soft tissue on contrast CT +100 to +300
Bone Cancellous +300 to +400[3]
Cortical +1800 to +1900[3]
Subdural hematoma First hours +75 to +100[4]
After 3 days +65 to +85[4]
After 10-14 days +35 to +40[5]
Other blood Unclotted +13[6] to +50[7]
Clotted +50[8] to +75[6][8]
Pleural effusion Transudate +2 to +15 [9]
Exudate +4 to +33[9]
Other fluids Chyle −30[10]
Water 0
Urine -5 to +15[2]
Bile -5 to +15[2]
CSF +15
Abscess / Pus 0[11] or +20[12], to +40[12] or +45[11]
Mucus 0[13] - 130[14] ("high attenuating" at over 70 HU)[15][16]
Parenchyma Lung -700 to −600[17]
Kidney +20 to +45[2]
Liver 60 ± 6[18]
Lymph nodes +10 to +20[19]
Muscle +35 to +55[2]
  • +20 to +40 in children[20]
  • +20 to +120 in adolescents[20]
White matter +20 to +30
Grey matter +37 to +45
Gallstone Cholesterol stone +30 to +100[21]
Bilirubin stone +90 to +120[21]
Foreign body[22] Windowpane glass +500
Aluminum, tarmac, car window glass, bottle glass, and other rocks +2,100 to +2,300
Limestone +2,800
Copper +14,000
Silver +17,000
Steel +20,000
Gold, steel, and brass +30,000 (upper measurable limit)
Earwax <0

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.[23]

See also[edit]


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


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  2. ^ a b c d e Page 83 in: Herbert Lepor (2000). Prostatic Diseases. W.B. Saunders Company. ISBN 9780721674162.
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External links[edit]