View source for Hounsfield scale

{{Short description|Quantitative scale of radiodensity}}
The '''Hounsfield scale''' ({{IPAc-en|ˈ|h|aʊ|n|z|f|iː|l|d}} {{respell|HOWNZ|feeld}}), named after Sir [[Godfrey Hounsfield]], is a quantitative scale for describing [[radiodensity]]. It is frequently used in [[CT scan]]s, 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]] ([[Standard temperature and pressure|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 <math>\mu</math>, the corresponding HU value is therefore given by:<ref>{{Cite journal |last1=Levine |first1=Zachary H. |last2=Peskin |first2=Adele P. |last3=Holmgren |first3=Andrew D. |last4=Garboczi |first4=Edward J. |date=2018-12-20 |title=Preliminary X-ray CT investigation to link Hounsfield unit measurements with the International System of Units (SI) |journal=PLOS ONE |language=en |volume=13 |issue=12 |pages=e0208820 |doi=10.1371/journal.pone.0208820 |doi-access=free |issn=1932-6203 |pmc=6301669 |pmid=30571779}}</ref><ref>{{Cite journal |last1=Hurrell |first1=Michael Anthony |last2=Butler |first2=Anthony Philip Howard |last3=Cook |first3=Nicholas James |last4=Butler |first4=Philip Howard |last5=Ronaldson |first5=J. Paul |last6=Zainon |first6=Rafidah |date=2012-05-01 |title=Spectral Hounsfield units: a new radiological concept |url=https://link.springer.com/article/10.1007/s00330-011-2348-3 |journal=European Radiology |language=en |volume=22 |issue=5 |pages=1008–1013 |doi=10.1007/s00330-011-2348-3 |pmid=22134894 |issn=1432-1084}}</ref>

<math display=block>HU = 1000\times\frac{\mu - \mu_{\textrm{water}}}{\mu_{\textrm{water}} - \mu_{\textrm{air}}}</math>

where <math>\mu_{\textrm{water}}</math> and <math>\mu_{\textrm{air}}</math> 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.<ref name="IAEADiagRadPhys">{{cite book |title=Diagnostic radiology physics: a handbook for teachers and students |date=2014 |pages=1–682 |publisher=[[International Atomic Energy Agency]] |location=Vienna |isbn=978-92-0-131010-1 |url=https://www.iaea.org/publications/8841/diagnostic-radiology-physics}}</ref>{{rp|259}}

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]] [[Radiation treatment planning|treatment planning]], where HU is converted to [[electron density]].<ref>{{cite book |title=Radiation oncology physics: a handbook for teachers and students. |date=2005 |publisher=[[International Atomic Energy Agency]] |location=Vienna |isbn=92-0-107304-6 |page=230 |url=https://www.iaea.org/publications/7086/radiation-oncology-physics}}</ref> 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.<ref name="IAEADiagRadPhys" />{{rp|283}}<ref>{{cite book |title=AAPM Report No. 233: Performance Evaluation of Computed Tomography Systems - The Report of AAPM Task Group 233 |date=2019 |publisher=[[American Association of Physicists in Medicine]] |location=Alexandria, VA |isbn=978-1-936366-69-9 |url=https://www.aapm.org/pubs/reports/detail.asp?docid=186|doi=10.37206/186|last1=Samei |first1=Ehsan |last2=Bakalyar |first2=Donovan |last3=Boedeker |first3=Kirsten |last4=Brady |first4=Samuel |last5=Fan |first5=Jiahua |last6=Leng |first6=Shuai |last7=Myers |first7=Kyle |last8=Popescu |first8=Lucretiu |last9=Ramirez-Giraldo |first9=Juan Carlos |last10=Ranallo |first10=Frank |last11=Solomon |first11=Justin |last12=Vaishnav |first12=Jay |last13=Wang |first13=Jia |s2cid=214010953 }}</ref>

===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.'' [[human]]s.{{cn|date=July 2024}}

==Values for different body tissues and material ==
[[File:CT Scan Thorax Lung -700 HU Window Level.jpg|thumb|CT scan of the thorax with window level set to -700 HU (lung)]]
[[File:CT Scan Thorax Air -1000 HU Window Level.jpg|thumb|CT scan of the thorax with window level set to -1,000 HU (air)]]
[[File:CT Scan Thorax Water 0 HU Window Level.jpg|thumb|CT scan of the thorax with window level set to 0 HU (water)]]
[[File:CT Scan Thorax Liver 60 HU Window Level.jpg|thumb|CT scan of the thorax with window level set to 60 HU (liver)]]
HU-based differentiation of material applies to [[medical-grade]] [[X-ray computed tomography|dual-energy CT scan]]s but not to [[cone beam computed tomography]] (CBCT) scans, as CBCT scans provide [[Cone_beam_computed_tomography#Bone_density_and_the_Hounsfield_scale|unreliable HU readings]].<ref name="DEVOS">{{cite journal |last1=De Vos| first1= W. |last2=Casselman|first2=J.|last3=Swennen|first3=G.R.J.|date=June 2009|title=Cone-beam computerized tomography (CBCT) imaging of the oral and maxillofacial region: A systematic review of the literature|journal=International Journal of Oral and Maxillofacial Surgery|volume=38|issue=6|pages=609–625| doi= 10.1016/j.ijom.2009.02.028|pmid=19464146}}</ref>

Values reported here are '''approximations'''. Different dynamics are reported from one study to another.{{cn|date=July 2024}}

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 agent]]s modifies HU as well in some body parts (mainly blood).

{| class="wikitable"
|-
! colspan="2" | Substance !! HU
|-
| colspan="2" | [[Air]]
| −1000
|-
| colspan="2" | [[Fat]]
| −120 to −90<ref name="Lepor2000" />
|-
| colspan="2" | Soft tissue on [[contrast CT]]
| +100 to +300
|-
| rowspan="2" | [[Bone]] || Cancellous || +300 to +400<ref name="BirurPatrick2017">{{cite journal|last1=Birur|first1=N. Praveen |last2= Patrick |first2= Sanjana |last3= Gurushanth |first3= Keerthi|last4=Raghavan|first4=AShubhasini|last5=Gurudath|first5=Shubha|display-authors= 3 |title=Comparison of gray values of cone-beam computed tomography with hounsfield units of multislice computed tomography: An in vitro study|journal=Indian Journal of Dental Research |volume= 28 |issue=1|year=2017|pages=66–70|issn=0970-9290|doi=10.4103/ijdr.IJDR_415_16|pmid=28393820|doi-access=free}}</ref>
|-
| Cortical || +500 to +1900<ref>{{Cite journal|last1=Lim Fat|first1=Daren|last2=Kennedy|first2=Jim|last3=Galvin|first3=Rose|last4=O’Brien|first4=Fergal|last5=Mc Grath |first5= Frank|last6=Mullett|first6=Hannan|display-authors= 3 |date=2012-05-01|title=The Hounsfield value for cortical bone geometry in the proximal humerus—an in vitro study| url= https://doi.org/10.1007/s00256-011-1255-7|journal=Skeletal Radiology|language=en|volume=41|issue=5|pages=557–568|doi=10.1007/s00256-011-1255-7| pmid= 21932054 |s2cid= 23463451|issn=1432-2161}}</ref><ref name="BirurPatrick2017" /><ref>{{Cite journal|last1=Aamodt|first1=A.|last2=Kvistad|first2=K. A.| last3= Andersen |first3=E.|last4=Lund-Larsen |first4=J.|last5=Eine|first5=J.|last6=Benum|first6=P.|last7=Husby|first7=O. S.|display-authors= 3 |date=January 1999| title=Determination of Hounsfield value for CT-based design of custom femoral stems|journal=The Journal of Bone and Joint Surgery. British Volume |volume=81|issue=1|pages=143–147|doi=10.1302/0301-620X.81B1.0810143|issn=0301-620X|pmid=10068022|doi-access=free}}</ref>
|-
| rowspan="3" | [[Subdural hematoma]] || First hours || +75 to +100<ref name="Rao2016">Fig 3 in: {{cite journal|last1=Rao|first1=Murali Gundu|title=Dating of Early Subdural Haematoma: A Correlative Clinico-Radiological Study|journal=Journal of Clinical and Diagnostic Research|volume=10|issue=4|pages=HC01–5|year=2016|issn=2249-782X |doi= 10.7860/JCDR/2016/17207.7644| pmid=27190831|pmc=4866129}}</ref>
|-
| After 3 days || +65 to +85<ref name="Rao2016" />
|-
| After 10–14 days || +35 to +40<ref>{{cite web|url=https://radiopaedia.org/articles/subdural-haemorrhage|first1= Rohit |last1= Sharma| first2= Frank |last2= Gaillard |access-date=2018-08-14|website=[[Radiopaedia]]|title=Subdural haemorrhage}}</ref>
|-
| rowspan="2" | Other [[blood]] || Unclotted || +13<ref name="fosbinder2011">{{cite book |title=Essentials of Radiologic Science| first1= Robert |last1= Fosbinder| first2= Denise |last2= Orth| page= [https://books.google.com/books?id=jRVg5ilM1UEC&pg=PA263 263]|publisher=Lippincott Williams & Wilkins|year=2011|isbn=9780781775540}}</ref> to +50<ref>{{cite book|title=Radiology of the Chest and Related Conditions| first= F. W. |last= Wright| page= [https://books.google.com/books?id=Bif0zpmEWtAC&pg=SA20-PA17 20.17] |publisher=CRC Press|year=2001|isbn=9780415281416}}</ref>
|-
| [[Coagulation|Clotted]] || +50<ref name="fast2006">{{cite book|title=Navigating the Adult Spine: Bridging Clinical Practice and Neuroradiology| first1= Avital |last1= Fast| first2= Dorith |last2= Goldsher| page= [https://books.google.com/books?id=4HaPJYaXXSAC&pg=PA17 17] |publisher=Demos Medical Publishing|year=2006|isbn=9781934559741}}</ref> to +75<ref name="fosbinder2011" /><ref name="fast2006" />
|-
| rowspan="2" | [[Pleural effusion]] || [[Transudate]] || +2 to +15 <ref name="CulluKalemci2013">{{cite journal |last1= Cullu |first1= Nesat |last2= Kalemci |first2= Serdar |last3= Karakas |first3=Omer|last4=Eser|first4=Irfan|last5=Yalcin|first5=Funda|last6=Boyaci|first6=Fatma Nurefsan|last7=Karakas|first7=Ekrem | display-authors= 3 |title= Efficacy of CT in diagnosis of transudates and exudates in patients with pleural effusion|journal=Diagnostic and Interventional Radiology|volume=20|issue=2|pages=116–20|year=2013|issn=1305-3825|doi=10.5152/dir.2013.13066|pmid=24100060|pmc=4463296}}</ref> 
|-
| [[Exudate]] || +4 to +33<ref name="CulluKalemci2013" />
|-
| rowspan="7" | Other fluids || [[Chyle]]
| −30<ref>[https://books.google.com/books?id=QmLNBQAAQBAJ&pg=PA342 Page 342] in: {{cite book|title=Multi-Detector CT Imaging: Principles, Head, Neck, and Vascular Systems, Volume 1|author=Luca Saba|author2=Jasjit S. Suri|publisher=CRC Press|year=2013|isbn=9781439893845}}</ref>
|-
|[[Water]]
| 0
|-
| [[Urine]]
| −5 to +15<ref name="Lepor2000">Page 83 in: {{cite book|title=Prostatic Diseases|author=Herbert Lepor|publisher=W.B. Saunders Company|year=2000|isbn=9780721674162}}</ref>
|-
| [[Bile]]
| −5 to +15<ref name="Lepor2000" />
|-
|[[Cerebrospinal fluid|CSF]]
| +15
|-
| [[Abscess]] / [[Pus]]
| 0<ref name="ecr">{{cite journal| url= http://pdf.posterng.netkey.at/download/index.php?module=get_pdf_by_id&poster_id=119826| title= 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| first1= P.| last1= Sanchez de Medina Alba| first2= C.| last2= Santos Montón| first3= N.| last3= Calvo| first4= K.| last4= El Karzazi| first5= T.| last5= González de la Huebra Labrador| first6= R.| last6= Corrales| first7= N.| last7= Alegre Borge| place= Salamanca| display-authors= 3| doi= 10.1594/ecr2014/C-1927| website= ECR 2014| year= 2014| access-date= 2019-01-12| archive-date= 2019-01-12| archive-url= https://web.archive.org/web/20190112195058/http://pdf.posterng.netkey.at/download/index.php?module=get_pdf_by_id&poster_id=119826| url-status= dead}}</ref> or +20,<ref name="SasakiMiyata2014">{{cite journal| last1= Sasaki| first1= Toru| last2= Miyata |first2=Rie |last3=Hatai| first3=Yoshiho |last4=Makita|first4=Kohzoh|last5=Tsunoda|first5=Koichi |display-authors= 3| title= Hounsfield unit values of retropharyngeal abscess-like lesions seen in Kawasaki disease|journal=Acta Oto-Laryngologica|volume=134|issue=4|year=2014|pages=437–440|issn=0001-6489| doi= 10.3109/00016489.2013.878475| pmid= 24512428| s2cid=6248808}}</ref> to +40<ref name="SasakiMiyata2014" /> or +45<ref name="ecr" />
|-
| [[Mucus]]
| 0<ref>{{Cite journal|url=http://medind.nic.in/ibn/t06/i2/ibnt06i2p191.pdf|title=Mucocoele of the Appendix|first1=K|last1=Saggar|first2=A|last2=Ahluwalia|first3=P|last3=Sandhu|first4=V|last4=Kalia|journal=Ind J Radiol Imag|year=2006|volume=16|issue=2}}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> - 130<ref name="GaetaVinci2001">{{cite journal |last1= Gaeta |first1= Michele |last2= Vinci |first2= Sergio |last3= Minutoli |first3= Fabio |last4= Mazziotti |first4= Silvio |last5= Ascenti| first5= Giorgio |last6= Salamone |first6= Ignazio |last7= Lamberto |first7=Salvatore|last8=Blandino|first8=Alfredo|title=CT and MRI findings of mucin-containing tumors and pseudotumors of the thorax: pictorial review| display-authors= 3| journal=European Radiology|volume=12|issue=1|year=2001|pages=181–189|issn=0938-7994|doi=10.1007/s003300100934|pmid=11868096|s2cid=28755973}}</ref> ("high attenuating" at over 70 HU)<ref>{{cite journal|title=High-Attenuation Mucus Impaction in Patients With Allergic Bronchopulmonary Aspergillosis: Objective Criteria on High-Resolution Computed Tomography and Correlation With Serologic Parameters| first1=Subash S. |last1= Phuyal| first2= Mandeep Kumar MK |last2= Garg| first3=Ritesh R |last3= Agarwal| first4=Pankaj P |last4= Gupta| first5= Arunaloke A |last5= Chakrabarti| first6= Manavjit Singh MS |last6=Sandhu| first7= Niranjan N |last7= Khandelwal| display-authors= 3| date=2015-09-02| journal= Current Problems in Diagnostic Radiology}}</ref><ref name="AgarwalSehgal2016">{{cite journal |last1=Agarwal |first1=Ritesh| last2=Sehgal| first2=Inderpaul Singh| last3= Dhooria| first3=Sahajal|last4=Aggarwal|first4=Ashutosh|title=Radiologic Criteria for the Diagnosis of High-Attenuation Mucus in Allergic Bronchopulmonary Aspergillosis |journal=Chest|volume=149|issue=4|year=2016|pages=1109–1110|issn=0012-3692|doi=10.1016/j.chest.2015.12.043|pmid=27055707|doi-access=free}}</ref>
|-
| rowspan="8" | [[Parenchyma]] || [[Lung]]
| −700 to −600<ref>{{cite book|title=Cardiopulmonary Imaging|volume=4|first1= Ella A. |last1= Kazerooni |first2= Barry H.|last2= Gross|publisher=Lippincott Williams & Wilkins | page= [https://books.google.com/books?id=IJwZHPrDQYUC&pg=PA379 379] |year=2004|isbn=9780781736558}}</ref>
|-
| [[Kidney]]
| +20 to +45<ref name="Lepor2000" />
|-
| [[Liver]]
| 60 ± 6<ref>{{cite book|title=Hepatology, Principles and Practice: History, Morphology, Biochemistry, Diagnostics, Clinic, Therapy|first1= Erwin |last1= Kuntz| first2= Hans-Dieter |last2= Kuntz|page= [https://books.google.com/books?id=62jMOh67D8UC&pg=PA210 210] |publisher=Springer Science & Business Media|year=2006|isbn=9783540289777}}</ref>
|-
| [[Lymph node]]s
| +10 to +20<ref>{{cite book|title=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| first= G.| last= Maatman| page= [https://books.google.com/books?id=mfYkBAAAQBAJ&pg=PA58 58] |publisher=Springer Science & Business Media|year=2012|isbn=9789400942776}}</ref>
|-
|[[Muscle]]
| +35 to +55<ref name="Lepor2000" />
|-
| [[Thymus]]
|
*+20 to +40 in children<ref name="givel2012">{{cite book|title=Surgery of the Thymus: Pathology, Associated Disorders and Surgical Technique| first1= Jean-Claude |last1= Givel| first2=Marco |last2= Merlini| first3=David B. |last3= Clarke| first4= Michael |last4= Dusmet |page= [https://books.google.com/books?id=AsTcBQAAQBAJ&pg=PT488 488] |publisher= Springer Science & Business Media|year=2012|isbn=9783642710766}}</ref>
*+20 to +120 in adolescents<ref name="givel2012" />
|-
|[[White matter]]
| +20 to +30
|-
|[[Grey matter]]
| +37 to +45
|-
| rowspan="2" | [[Gallstone]] || Cholesterol stone || +30 to +100<ref name="pmid2121509">{{cite journal|vauthors=Rambow A, Staritz M, Wosiewitz U, Mildenburger P, Thelen M, Meyer zum Büschenfelde KH| title=Analysis of radiolucent gallstones by computed tomography for in vivo estimation of stone components | journal=Eur J Clin Invest | year= 1990 | volume= 20 | issue= 4 | pages= 475–8 | pmid=2121509 | doi= 10.1111/j.1365-2362.1990.tb01887.x| s2cid=39138974 }}</ref>
|-
| Bilirubin stone || +90 to +120<ref name="pmid2121509" />
|-
| rowspan="7" | [[Foreign body]]<ref name="BolligerOesterhelweg2009">{{cite journal|last1=Bolliger|first1=Stephan A. |last2= Oesterhelweg| first2= Lars |last3= Spendlove |first3= Danny |last4=Ross|first4=Steffen|last5=Thali|first5=Michael J.|display-authors= 3 |title=Is Differentiation of Frequently Encountered Foreign Bodies in Corpses Possible by Hounsfield Density Measurement?|journal=Journal of Forensic Sciences|volume=54|issue=5|year=2009|pages=1119–1122|issn=0022-1198|doi=10.1111/j.1556-4029.2009.01100.x |pmid= 19627414| s2cid=40300162}}</ref> 
|| 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)
|-
| colspan="2" | [[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]].<ref>{{cite web| url= http://emedicine.medscape.com/article/376240 |title= Adrenal Adenoma Imaging| first= Perry J.| last= Horwich| editor= Eugene C Lin| date= December 20, 2018| website= Medscape | access-date= July 28, 2023}}</ref>

==See also==
* [[Cone beam computed tomography#Bone density and the Hounsfield scale|Cone beam computed tomography: Bone density and the Hounsfield scale]].

==References==
{{reflist}} 
*{{cite book
|last=Feeman
|first=Timothy G.
|title=The Mathematics of Medical Imaging: A Beginner's Guide
|series=Springer [[Undergraduate Texts in Mathematics]] and Technology
|publisher=Springer
|year=2010
|isbn=978-0387927114
}}

==External links==
*{{cite web|url=http://www.medcyclopaedia.com/library/topics/volume_i/h/hounsfield_unit.aspx|archive-url=https://archive.today/20120404030558/http://www.medcyclopaedia.com/library/topics/volume_i/h/hounsfield_unit.aspx|url-status=dead|archive-date=2012-04-04|title=Hounsfield unit|publisher=[[General Electric|GE]]|website=Medcyclopaedia}}
*[http://www.fpnotebook.com/Rad/CT/HnsfldUnt.htm Hounsfield Unit] - fpnotebook.com
*{{cite web|url=http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf|title=Introduction to CT physics|publisher=elsevierhealth.com|archive-url=https://web.archive.org/web/20070926231241/http://www.intl.elsevierhealth.com/e-books/pdf/940.pdf|archive-date=2007-09-26}}
* [https://www.researchgate.net/publication/24234798_Imaging_of_deep_brain_stimulation_leads_using_extended_Hounsfield_unit_CT Imaging of deep brain stimulation leads using extended Hounsfield unit CT. Stereotact Funct Neurosurg. 2009;87(3):155-60. doi: 10.1159/000209296]

{{Medical imaging}}
[[Category:Radiology]]