Hounsfield scale explained

The Hounsfield scale, 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 0 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:

$$HU\; =\; 1000\backslash times\backslash frac$$

where

\mu _{rm{water} } and \mu _{rm{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.^[1]

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.^[2] 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.^[3]

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-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.^[4]

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
	Cancellous	+300 to +400^[5]
	Cortical	+500 to +1900^[6] ^[7]
	First hours	+75 to +100^[8]
	After 3 days	+65 to +85
	After 10–14 days	+35 to +40^[9]
	Unclotted	+13^[10] to +50^[11]
		+50^[12] to +75
		+2 to +15 ^[13]
		+4 to +33
Other fluids	Chyle	−30^[14]
	Water	0
	Urine	−5 to +15^[15]
	Bile	−5 to +15
	CSF	+15
	Abscess / Pus	0^[16] or +20,^[17] to +40 or +45
	Mucus	0^[18] - 130^[19] ("high attenuating" at over 70 HU)^[20] ^[21]
	Lung	−700 to −600^[22]
	Kidney	+20 to +45
	Liver	60 ± 6^[23]
	Lymph nodes	+10 to +20^[24]
	Muscle	+35 to +55
	Thymus	+20 to +40 in children^[25] +20 to +120 in adolescents
	White matter	+20 to +30
	Grey matter	+37 to +45
	Cholesterol stone	+30 to +100^[26]
	Bilirubin stone	+90 to +120
Foreign body^[27]	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)
		<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.^[28]

See also

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

References

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

External links

• Web site: Hounsfield unit. https://archive.today/20120404030558/http://www.medcyclopaedia.com/library/topics/volume_i/h/hounsfield_unit.aspx. dead. 2012-04-04. GE. Medcyclopaedia.
• Hounsfield Unit - fpnotebook.com
• Web site: Introduction to CT physics. elsevierhealth.com. https://web.archive.org/web/20070926231241/http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf. 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

Notes and References

1. Book: Diagnostic radiology physics: a handbook for teachers and students . 2014 . . Vienna . 978-92-0-131010-1 .
2. Book: Radiation oncology physics: a handbook for teachers and students. . 2005 . . Vienna . 92-0-107304-6 . 230 .
3. Book: AAPM Report No. 233: Performance Evaluation of Computed Tomography Systems - The Report of AAPM Task Group 233 . 2019 . . Alexandria, VA . 978-1-936366-69-9 . 10.37206/186. Samei . Ehsan . Bakalyar . Donovan . Boedeker . Kirsten . Brady . Samuel . Fan . Jiahua . Leng . Shuai . Myers . Kyle . Popescu . Lucretiu . Ramirez-Giraldo . Juan Carlos . Ranallo . Frank . Solomon . Justin . Vaishnav . Jay . Wang . Jia . 214010953 .
4. De Vos. W. . Casselman. J.. Swennen. G.R.J.. June 2009. Cone-beam computerized tomography (CBCT) imaging of the oral and maxillofacial region: A systematic review of the literature. International Journal of Oral and Maxillofacial Surgery. 38. 6. 609–625. 10.1016/j.ijom.2009.02.028. 19464146.
5. Birur. N. Praveen . Patrick . Sanjana . Gurushanth . Keerthi. Raghavan. AShubhasini. Gurudath. Shubha. 3 . Comparison of gray values of cone-beam computed tomography with hounsfield units of multislice computed tomography: An in vitro study. Indian Journal of Dental Research . 28 . 1. 2017. 66–70. 0970-9290. 10.4103/ijdr.IJDR_415_16. 28393820. free.
6. Lim Fat. Daren. Kennedy. Jim. Galvin. Rose. O’Brien. Fergal. Mc Grath . Frank. Mullett. Hannan. 3 . 2012-05-01. The Hounsfield value for cortical bone geometry in the proximal humerus—an in vitro study. Skeletal Radiology. en. 41. 5. 557–568. 10.1007/s00256-011-1255-7. 21932054 . 23463451. 1432-2161.
7. Aamodt. A.. Kvistad. K. A.. Andersen . E.. Lund-Larsen . J.. Eine. J.. Benum. P.. Husby. O. S.. 3 . January 1999. Determination of Hounsfield value for CT-based design of custom femoral stems. The Journal of Bone and Joint Surgery. British Volume . 81. 1. 143–147. 10.1302/0301-620X.81B1.0810143. 0301-620X. 10068022. free.
8. Fig 3 in: Rao. Murali Gundu. Dating of Early Subdural Haematoma: A Correlative Clinico-Radiological Study. Journal of Clinical and Diagnostic Research. 10. 4. HC01–5. 2016. 2249-782X . 10.7860/JCDR/2016/17207.7644. 27190831. 4866129.
9. Web site: Subdural haemorrhage. Rohit . Sharma. Frank . Gaillard . 2018-08-14. Radiopaedia.
10. Book: Essentials of Radiologic Science. Robert . Fosbinder. Denise . Orth. 263. Lippincott Williams & Wilkins. 2011. 9780781775540.
11. Book: Wright, F. W. . Radiology of the Chest and Related Conditions. 20.17 . CRC Press. 2001. 9780415281416.
12. Book: Navigating the Adult Spine: Bridging Clinical Practice and Neuroradiology. Avital . Fast. Dorith . Goldsher. 17 . Demos Medical Publishing. 2006. 9781934559741.
13. Cullu . Nesat . Kalemci . Serdar . Karakas . Omer. Eser. Irfan. Yalcin. Funda. Boyaci. Fatma Nurefsan. Karakas. Ekrem . 3 . Efficacy of CT in diagnosis of transudates and exudates in patients with pleural effusion. Diagnostic and Interventional Radiology. 20. 2. 116–20. 2013. 1305-3825. 10.5152/dir.2013.13066. 24100060. 4463296.
14. https://books.google.com/books?id=QmLNBQAAQBAJ&pg=PA342 Page 342
15. Page 83 in: Book: Prostatic Diseases. Herbert Lepor. W.B. Saunders Company. 2000. 9780721674162.
16. Liver abscesses: where do they come from? A review of the main types of liver abscesses and the correlation between their causes and the radiologic findings. P.. Sanchez de Medina Alba. C.. Santos Montón. N.. Calvo. K.. El Karzazi. T.. González de la Huebra Labrador. R.. Corrales. N.. Alegre Borge. Salamanca. 3. 10.1594/ecr2014/C-1927. ECR 2014. 2014. 2019-01-12. 2019-01-12. https://web.archive.org/web/20190112195058/http://pdf.posterng.netkey.at/download/index.php?module=get_pdf_by_id&poster_id=119826. dead.
17. Sasaki. Toru. Miyata . Rie . Hatai. Yoshiho . Makita. Kohzoh. Tsunoda. Koichi . 3. Hounsfield unit values of retropharyngeal abscess-like lesions seen in Kawasaki disease. Acta Oto-Laryngologica. 134. 4. 2014. 437–440. 0001-6489. 10.3109/00016489.2013.878475. 24512428. 6248808.
18. Mucocoele of the Appendix. K. Saggar. A. Ahluwalia. P. Sandhu. V. Kalia. Ind J Radiol Imag. 2006. 16. 2.
19. Gaeta . Michele . Vinci . Sergio . Minutoli . Fabio . Mazziotti . Silvio . Ascenti. Giorgio . Salamone . Ignazio . Lamberto . Salvatore. Blandino. Alfredo. CT and MRI findings of mucin-containing tumors and pseudotumors of the thorax: pictorial review. 3. European Radiology. 12. 1. 2001. 181–189. 0938-7994. 10.1007/s003300100934. 11868096. 28755973.
20. High-Attenuation Mucus Impaction in Patients With Allergic Bronchopulmonary Aspergillosis: Objective Criteria on High-Resolution Computed Tomography and Correlation With Serologic Parameters. Subash S. . Phuyal. Mandeep Kumar MK . Garg. Ritesh R . Agarwal. Pankaj P . Gupta. Arunaloke A . Chakrabarti. Manavjit Singh MS . Sandhu. Niranjan N . Khandelwal. 3. 2015-09-02. Current Problems in Diagnostic Radiology.
21. Agarwal . Ritesh. Sehgal. Inderpaul Singh. Dhooria. Sahajal. Aggarwal. Ashutosh. Radiologic Criteria for the Diagnosis of High-Attenuation Mucus in Allergic Bronchopulmonary Aspergillosis . Chest. 149. 4. 2016. 1109–1110. 0012-3692. 10.1016/j.chest.2015.12.043. 27055707. free.
22. Book: Cardiopulmonary Imaging. 4. Ella A. . Kazerooni . Barry H.. Gross. Lippincott Williams & Wilkins . 379 . 2004. 9780781736558.
23. Book: Hepatology, Principles and Practice: History, Morphology, Biochemistry, Diagnostics, Clinic, Therapy. Erwin . Kuntz. Hans-Dieter . Kuntz. 210 . Springer Science & Business Media. 2006. 9783540289777.
24. Book: Maatman, G.. High-Resolution Computed Tomography of the Paranasal Sinuses and Pharynx and Related Regions: Impact of CT identification on diagnosis and patient management. Volume 12 of Series in Radiology. 58 . Springer Science & Business Media. 2012. 9789400942776.
25. Book: Givel, Jean-Claude . Surgery of the Thymus: Pathology, Associated Disorders and Surgical Technique. Marco . Merlini. David B. . Clarke. Michael . Dusmet . 488 . Springer Science & Business Media. 2012. 9783642710766.
26. Rambow A, Staritz M, Wosiewitz U, Mildenburger P, Thelen M, Meyer zum Büschenfelde KH. Analysis of radiolucent gallstones by computed tomography for in vivo estimation of stone components . Eur J Clin Invest . 1990 . 20 . 4 . 475–8 . 2121509 . 10.1111/j.1365-2362.1990.tb01887.x. 39138974 .
27. Bolliger. Stephan A. . Oesterhelweg. Lars . Spendlove . Danny . Ross. Steffen. Thali. Michael J.. 3 . Is Differentiation of Frequently Encountered Foreign Bodies in Corpses Possible by Hounsfield Density Measurement?. Journal of Forensic Sciences. 54. 5. 2009. 1119–1122. 0022-1198. 10.1111/j.1556-4029.2009.01100.x . 19627414. 40300162.
28. Web site: Adrenal Adenoma Imaging. Perry J.. Horwich. Eugene C Lin. December 20, 2018. Medscape . July 28, 2023.