Hounsfield scale

 ( Radiology ) 

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

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Definition 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 0 Hounsfield units (HU), while the radiodensity of air at STP is defined as −1000 HU. In a voxel with average linear attenuation coefficient $\backslash mu$, the corresponding HU value is therefore given by:

$$HU\; =\; 1000\backslash times\backslash frac\{\backslash mu\; -\; \backslash mu\_\{\backslash textrm\{water\}\}\}\{\backslash mu\_\{\backslash textrm\{water\}\}\; -\; \backslash mu\_\{\backslash textrm\{air\}\}\}$$

where $\backslash mu\_\{\backslash textrm\{water\}\}$ and $\backslash mu\_\{\backslash textrm\{air\}\}$ 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.

Calibration tests of HU with reference to water and other materials may be done to ensure standardised response. This is particularly important for CT scans used in radiotherapy treatment planning, where HU is converted to electron density.

Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures.

(2025). 9781936366699, American Association of Physicists in Medicine. . ISBN 9781936366699

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Rationale

The above standards were chosen originally to encode the radiodensity of organic tissues relative to water for 12-bit processing on clinical CT scanners. A 12-bit encoding corresponds to 4096 ($2^\{12\}$) values, where the range (–1024 to 3071) encompasses HU values for air, soft tissue and bone.

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Values for different body tissues and material

HU-based differentiation of material applies to medical-grade dual-energy CT scans but not to cone beam computed tomography (CBCT) scans, as CBCT scans provide unreliable HU readings.

Values reported here are approximations. Different dynamics are reported from one study to another.

Exact HU dynamics can vary from one CT acquisition to another due to CT acquisition and reconstruction parameters (kV, filters, reconstruction algorithms, etc.). The use of modifies HU as well in some body parts (mainly blood).

Air		−1000
Fat		−120 to −90
Soft tissue on contrast CT		+100 to +300
+300 to +400
+500 to +1900
+75 to +100Fig 3 in:
+65 to +85
+35 to +40
+13 (2025). 9780781775540, Lippincott Williams & Wilkins. ISBN 9780781775540 to +50 F. W. Wright (2025). 9780415281416, CRC Press. ISBN 9780415281416
+50 (2025). 9781934559741, Demos Medical Publishing. ISBN 9781934559741 to +75
+2 to +15
+4 to +33
Chyle	−30 Page 342 in: Luca Saba (2025). 9781439893845, CRC Press. ISBN 9781439893845
Water	0
Urine	−5 to +15Page 83 in: Herbert Lepor (2025). 9780721674162, W.B. Saunders Company. ISBN 9780721674162
Bile	−5 to +15
CSF	+15
Abscess / Pus	0 or +20, to +40 or +45
Mucus	0 - 130 ("high attenuating" at over 70 HU)
Lung	−700 to −600 (2025). 9780781736558, Lippincott Williams & Wilkins. ISBN 9780781736558
Kidney	+20 to +45
Liver	60 ± 6 (2025). 9783540289777, Springer Science & Business Media. ISBN 9783540289777
	+10 to +20 G. Maatman (2025). 9789400942776, Springer Science & Business Media. ISBN 9789400942776
Muscle	+35 to +55
Thymus	+20 to +40 in children (2025). 9783642710766, Springer Science & Business Media. ISBN 9783642710766 +20 to +120 in adolescents
White matter	+20 to +30
Grey matter	+37 to +45
+30 to +100
+90 to +120
Foreign body	Windowpane glass| +500
	+2,100 to +2,300
	+2,800
	+14,000
	+17,000
	+20,000
	+30,000 (upper measurable limit)
<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.

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See also

• .

• Timothy G. Feeman (2025). 9780387927114, Springer. ISBN 9780387927114

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External links

• Hounsfield Unit - fpnotebook.com
• Imaging of deep brain stimulation leads using extended Hounsfield unit CT. Stereotact Funct Neurosurg. 2009;87(3):155-60. doi: 10.1159/000209296

Categories: Radiology

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