Hounsfield scale

The Hounsfield scale /ˈhaʊnzˌfiːld/, 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.

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

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

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

It is the definition for CT scanners that are calibrated with reference to water.

Rationale

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.

Values for different body tissues and material

HU applies to medical-grade dual-energy CT scans but not to cone beam computed tomography (CBCT) scans.

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 contrast agents modifies HU as well in some body parts (mainly blood).

Substance		HU
Air		−1000
Fat		−120 to −90
Soft tissue on contrast CT		+100 to +300
Bone	Cancellous	+300 to +400
	Cortical	+500 to +1900
Subdural hematoma	First hours	+75 to +100
	After 3 days	+65 to +85
	After 10–14 days	+35 to +40
Other blood	Unclotted	+13 to +50
	Clotted	+50 to +75
Pleural effusion	Transudate	+2 to +15
	Exudate	+4 to +33
Other fluids	Chyle	−30
	Water	0
	Urine	−5 to +15
	Bile	−5 to +15
	CSF	+15
	Abscess / Pus	0 or +20, to +40 or +45
	Mucus	0 - 130 ("high attenuating" at over 70 HU)
Parenchyma	Lung	−700 to −600
	Kidney	+20 to +45
	Liver	60 ± 6
	Lymph nodes	+10 to +20
	Muscle	+35 to +55
	Thymus	+20 to +40 in children +20 to +120 in adolescents
	White matter	+20 to +30
	Grey matter	+37 to +45
Gallstone	Cholesterol stone	+30 to +100
	Bilirubin stone	+90 to +120
Foreign body	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.

See also

• Cone beam computed tomography: Bone density and the Hounsfield scale.

References

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

External links

• "Hounsfield unit". Medcyclopaedia. GE. Archived from the original on 2012-04-04.
• Hounsfield Unit - fpnotebook.com
• "Introduction to CT physics" (PDF). elsevierhealth.com. Archived from the original (PDF) on 2007-09-26.
• Imaging of deep brain stimulation leads using extended Hounsfield unit CT. Stereotact Funct Neurosurg. 2009;87(3):155-60. doi: 10.1159/000209296