CT scan: Difference between revisions

{{Aboutabout|X-ray computed tomography as used in medicine| cross-sectional images used in industry|Industrial computed tomography|means of tomography other than X-ray|Tomography}}

|Name = CT scan

|Image = Moderní výpočetní tomografie s přímo digitální detekcí rentgenového záření.jpg

|Caption = Modern CT scanner (2021), photon-counting CT (Siemens NAEOTOM Alpha)

| synonyms = X-ray computed tomography (X-ray CT), computerized axial tomography scan (CAT scan),<ref name="mayoclinic">{{Cite web |title=CT scan – Mayo Clinic |url=http://www.mayoclinic.org/tests-procedures/ct-scan/basics/definition/prc-20014610 |url-status=live |archive-url=https://web.archive.org/web/20161015182843/http://www.mayoclinic.org/tests-procedures/ct-scan/basics/definition/prc-20014610 |archive-date=15 October 2016 |access-date=20 October 2016 |publisher=mayoclinic.org}}</ref> computer aided tomography, computed tomography scan

|ICD10 = B?2

|ICD9 = {{ICD9proc|88.38}}

|MeshID = D014057

|MedlinePlus = 003330

|OPS301 = {{OPS301|3–20...3–26}}

|OtherCodes =

A '''computed tomography scan''' (usually abbreviated to '''CT scan'''; formerly called '''computed axial tomography scan''' or '''CAT scan''') is a [[medical imaging]] technique used to obtain detailed internal images of the body.<ref>{{Citation |lastlast1=Hermena |firstfirst1=Shady |title=CT-scan Image Production Procedures |date=2022 |work=StatPearls |url=https://www.ncbi.nlm.nih.gov/books/NBK574548/ |work=StatPearls |access-date=2023-11-24 |place=Treasure Island, (FL)Florida |publisher=StatPearls Publishing |pmid=34662062 |last2=Young |first2=Michael}}</ref> The personnel that perform CT scans are called [[radiographer]]s or radiology technologists.<ref>{{Cite web |title=Patient Page |url=https://www.arrt.org/Patient-Public/Patient-Page |archive-url=https://web.archive.org/web/20141109192141/https://www.arrt.org/Patient-Public/Patient-Page |archive-date=9 November 2014 |website=ARRT – The American Registry of Radiologic Technologists}}</ref><ref>{{Cite web |title=Individual State Licensure Information |url=http://www.asrt.org/main/standards-regulations/state-legislative-affairs/individual-state-licensure-info |url-status=live |archive-url=https://web.archive.org/web/20130718215951/http://www.asrt.org/main/standards-regulations/state-legislative-affairs/individual-state-licensure-info |archive-date=18 July 2013 |access-date=19 July 2013 |publisher=American Society of Radiologic Technologists}}</ref>

CT scanners use a rotating [[X-ray tube]] and a row of detectors placed in a [[gantry (medical)|gantry]] to measure X-ray [[Attenuation#Radiography|attenuations]] by different tissues inside the body. The multiple [[X-ray]] measurements taken from different angles are then processed on a computer using [[tomographic reconstruction]] algorithms to produce [[Tomography|tomographic]] (cross-sectional) images (virtual "slices") of a body. CT scanscans can be used in patients with metallic implants or pacemakers, for whom [[magnetic resonance imaging]] (MRI) is [[Contraindication|contraindicated]].

On the basis of image acquisition and procedures, various type of scanners are available in the market.

Sequential CT, also known as step-and-shoot CT, is a type of scanning method in which the CT table moves stepwise. The table increments to a particular location and then stops which is followed by the [[X-ray tube]] rotation and acquisition of a slice. Then theThe table then increments again, and another slice is taken. The table hasmovement tostops make a stop inwhile taking slices. This results in an increased time of scanning.<ref>{{Cite book |last1=Terrier |first1=F. |url=https://books.google.com/books?id=AV3wCAAAQBAJ&dq=Sequential+CT+scan&pg=PA4 |title=Spiral CT of the Abdomen |last2=Grossholz |first2=M. |last3=Becker |first3=C. D. |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-3-642-56976-0 |page=4 |language=en}}</ref>

[[File:Axial plane CT scan of the thorax illustrative image.jpg|thumb|CT scan of the thorax. The axial slice (right) is the image that corresponds to number 2/33 on the coronal slice (left).]]

Spinning tube, commonly called [[Spiral computed tomography|spiral CT]], or helical CT, is an imaging technique in which an entire [[X-ray tube]] is spun around the central axis of the area being scanned. These are the dominant type of scanners on the market because they have been manufactured longer and offer a lower cost of production and purchase. The main limitation of this type of CT is the bulk and inertia of the equipment (X-ray tube assembly and detector array on the opposite side of the circle) which limits the speed at which the equipment can spin. Some designs use two X-ray sources and detector arrays offset by an angle, as a technique to improve temporal resolution.<ref>{{Cite book |last1=Fishman |first1=Elliot K. |url=https://books.google.com/books?id=aWlrAAAAMAAJ&q=spiral+ct |title=Spiral CT: Principles, Techniques, and Clinical Applications |last2=Jeffrey |first2=R. Brooke |date=1995 |publisher=Raven Press |isbn=978-0-7817-0218-8 |language=en}}</ref><ref>{{Cite book |last=Hsieh |first=Jiang |url=https://books.google.com/books?id=JX__lLLXFHkC&q=spiral+ct&pg=PA265 |title=Computed Tomography: Principles, Design, Artifacts, and Recent Advances |date=2003 |publisher=SPIE Press |isbn=978-0-8194-4425-7 |page=265 |language=en}}</ref>

[[Electron beam tomography]] (EBT) is a specific form of CT in which a large enough X-ray tube is constructed so that only the path of the [[electron]]s, travelling between the [[cathode]] and [[anode]] of the X-ray tube, are spun using [[Deflection yoke|deflection coils]].<ref>{{Cite book |last=Stirrup |first=James |url=https://books.google.com/books?id=SarDDwAAQBAJ&q=ebct&pg=PA6 |title=Cardiovascular Computed Tomography |date=2020-01-02 |publisher=Oxford University Press |isbn=978-0-19-880927-2 |language=en}}</ref> This type had a major advantage since sweep speeds can be much faster, allowing for less blurry imaging of moving structures, such as the heart and arteries.<ref>{{Cite journal |last1=Talisetti |first1=Anita |last2=Jelnin |first2=Vladimir |last3=Ruiz |first3=Carlos |last4=John |first4=Eunice |last5=Benedetti |first5=Enrico |last6=Testa |first6=Giuliano |last7=Holterman |first7=Ai-Xuan L. |last8=Holterman |first8=Mark J. |date=December 2004 |title=Electron beam CT scan is a valuable and safe imaging tool for the pediatric surgical patient |journal=Journal of Pediatric Surgery |volume=39 |issue=12 |pages=1859–1862 |doi=10.1016/j.jpedsurg.2004.08.024 |issn=1531-5037 |pmid=15616951}}</ref> Fewer scanners of this design have been produced when compared with spinning tube types, mainly due to the higher cost associated with building a much larger X-ray tube and detector array and limited anatomical coverage.<ref>{{Cite journal |last=Retsky |first=Michael |date=31 July 2008 |title=Electron beam computed tomography: Challenges and opportunities |journal=Physics Procedia |volume=1 |issue=1 |pages=149–154 |bibcode=2008PhPro...1..149R |doi=10.1016/j.phpro.2008.07.090 |doi-access=free}}</ref>

Dual Energy CT, also known as Spectral CT, is an advancement of Computed Tomography in which two energies are used to create two sets of data.<ref>{{Cite book |last1=Johnson |first1=Thorsten |url=https://books.google.com/books?id=Etvcnz0mjF4C&q=dual+energy+ct |title=Dual Energy CT in Clinical Practice |last2=Fink |first2=Christian |last3=Schönberg |first3=Stefan O. |last4=Reiser |first4=Maximilian F. |date=2011-01-18 |publisher=Springer Science & Business Media |isbn=978-3-642-01740-7 |language=en}}</ref> A Dual Energy CT may employ Dual source, Single source with dual detector layer, Single source with energy switching methods to get two different sets of data.<ref>{{Cite book |last1=Johnson |first1=Thorsten |url=https://books.google.com/books?id=Etvcnz0mjF4C&q=dual+energy+ct |title=Dual Energy CT in Clinical Practice |last2=Fink |first2=Christian |last3=Schönberg |first3=Stefan O. |last4=Reiser |first4=Maximilian F. |date=2011-01-18 |publisher=Springer Science & Business Media |isbn=978-3-642-01740-7 |page=8 |language=en}}</ref>

#'''Dual source CT''' is an advanced scanner with a two X-ray tube detector system, unlike conventional single tube systems.<ref>{{Cite book |last1=Carrascosa |first1=Patricia M. |url=https://books.google.com/books?id=wJ2oCgAAQBAJ&q=dual+source+ct |title=Dual-Energy CT in Cardiovascular Imaging |last2=Cury |first2=Ricardo C. |last3=García |first3=Mario J. |last4=Leipsic |first4=Jonathon A. |date=2015-10-03 |publisher=Springer |isbn=978-3-319-21227-2 |language=en}}</ref><ref>{{Cite journal |last1=Schmidt |first1=Bernhard |last2=Flohr |first2=Thomas |date=2020-11-01 |title=Principles and applications of dual source CT |url=https://www.sciencedirect.com/science/article/pii/S112017972030257X |journal=Physica Medica |series=125 Years of X-Rays |language=en |volume=79 |pages=36–46 |doi=10.1016/j.ejmp.2020.10.014 |issn=1120-1797 |pmid=33115699 |s2cid=226056088|doi-access=free}}</ref> These two detector systems are mounted on a single gantry at 90° in the same plane.<ref name="auto1">{{Cite book |last1=Seidensticker |first1=Peter R. |url=https://books.google.com/books?id=oUtHea3ZnJ0C&q=dual+source+ct |title=Dual Source CT Imaging |last2=Hofmann |first2=Lars K. |date=2008-05-24 |publisher=Springer Science & Business Media |isbn=978-3-540-77602-4 |language=en}}</ref> Dual Source CT scanners allow fast scanning with higher temporal resolution by acquiring a full CT slice in only half a rotation. Fast imaging reduces motion blurring at high heart rates and potentially allowing for shorter breath-hold time. This is particularly useful for ill patients having difficulty holding their breath or unable to take heart-rate lowering medication.<ref name="auto1"/><ref>{{Cite journal |last1=Schmidt |first1=Bernhard |last2=Flohr |first2=Thomas |date=2020-11-01 |title=Principles and applications of dual source CT |url=https://www.physicamedica.com/article/S1120-1797(20)30257-X/abstract |journal=Physica Medica: European Journal of Medical Physics |language=English |volume=79 |pages=36–46 |doi=10.1016/j.ejmp.2020.10.014 |issn=1120-1797 |pmid=33115699 |s2cid=226056088|doi-access=free}}</ref>

#'''Single Source with Energy switching''' is another mode of Dual energy CT in which a single tube is operated at two different energies by switching the energies frequently.<ref>{{Cite journal |last1=Mahmood |first1=Usman |last2=Horvat |first2=Natally |last3=Horvat |first3=Joao Vicente |last4=Ryan |first4=Davinia |last5=Gao |first5=Yiming |last6=Carollo |first6=Gabriella |last7=DeOcampo |first7=Rommel |last8=Do |first8=Richard K. |last9=Katz |first9=Seth |last10=Gerst |first10=Scott |last11=Schmidtlein |first11=C. Ross |last12=Dauer |first12=Lawrence |last13=Erdi |first13=Yusuf |last14=Mannelli |first14=Lorenzo |date=May 2018 |title=Rapid Switching kVp Dual Energy CT: Value of Reconstructed Dual Energy CT Images and Organ Dose Assessment in Multiphasic Liver CT Exams |journal=European Journal of Radiology |volume=102 |pages=102–108 |doi=10.1016/j.ejrad.2018.02.022 |issn=0720-048X |pmc=5918634 |pmid=29685522}}</ref><ref>{{Cite journal |last=Johnson |first=Thorsten R. C. |date=November 2012 |title=Dual-Energy CT: General Principles |url=https://www.ajronline.org/doi/10.2214/AJR.12.9116 |journal=American Journal of Roentgenology |language=en |volume=199 |issue=5_supplement |pages=S3–S8 |doi=10.2214/AJR.12.9116 |pmid=23097165 |issn=0361-803X}}</ref>

{{Mainmain|CT Perfusion}}

[[File:CT perfusion in M1 artery occlusion.png|thumb|CT Perfusion scan of the brain]]

CT perfusion imaging is a specific form of CT to assess flow through [[blood vessel]]s whilst injecting a [[contrast agent]].<ref name=":0">{{Cite journal |last1=Wittsack |first1=H.-J. |last2=Wohlschläger |first2=A.M. |last3=Ritzl |first3=E.K. |last4=Kleiser |first4=R. |last5=Cohnen |first5=M. |last6=Seitz |first6=R.J. |last7=Mödder |first7=U. |date=2008-01-01 |title=CT-perfusion imaging of the human brain: Advanced deconvolution analysis using circulant singular value decomposition |journal=Computerized Medical Imaging and Graphics |language=en |volume=32 |issue=1 |pages=67–77 |doi=10.1016/j.compmedimag.2007.09.004 |issn=0895-6111 |pmid=18029143}}</ref> Blood flow, blood transit time, and organ blood volume, can all be calculated with reasonable [[sensitivity and specificity]].<ref name=":0" /> This type of CT may be used on the [[heart]], although sensitivity and specificity for detecting abnormalities are still lower than for other forms of CT.<ref>{{Cite journal |last1=Williams |first1=M.C. |last2=Newby |first2=D.E. |date=2016-08-01 |title=CT myocardial perfusion imaging: current status and future directions |journal=Clinical Radiology |language=en |volume=71 |issue=8 |pages=739–749 |doi=10.1016/j.crad.2016.03.006 |issn=0009-9260 |pmid=27091433}}</ref> This may also be used on the [[brain]], where CT perfusion imaging can often detect poor brain perfusion well before it is detected using a conventional spiral CT scan.<ref name=":0" /><ref name=":1">{{Cite journal |last1=Donahue |first1=Joseph |last2=Wintermark |first2=Max |date=2015-02-01 |title=Perfusion CT and acute stroke imaging: Foundations, applications, and literature review |journal=Journal of Neuroradiology |language=en |volume=42 |issue=1 |pages=21–29 |doi=10.1016/j.neurad.2014.11.003 |issn=0150-9861 |pmid=25636991}}</ref> This is better for [[stroke]] diagnosis than other CT types.<ref name=":1" />

{{Mainmain|PET-CT}}

Positron emission tomography–computed tomography is a hybrid CT modality which combines, in a single gantry, a [[positron emission tomography]] (PET) scanner and an xX-ray computed tomography (CT) scanner, to acquire sequential images from both devices in the same session, which are combined into a single superposed ([[Image registration|co-registered]]) image. Thus, [[functional imaging]] obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning.<ref>{{Cite journal |last1=Blodgett |first1=Todd M. |last2=Meltzer |first2=Carolyn C. |last3=Townsend |first3=David W. |date=February 2007 |title=PET/CT: form and function |url=https://pubmed.ncbi.nlm.nih.gov/17255408/#:~:text=CT%20is%20complementary%20in%20the,identify%20and%20localize%20functional%20abnormalities. |journal=Radiology |volume=242 |issue=2 |pages=360–385 |doi=10.1148/radiol.2422051113 |issn=0033-8419 |pmid=17255408}}</ref>

Since its introduction in the 1970s,<ref>{{Cite book |last1=Curry |first1=Thomas S. |url=https://books.google.com/books?id=W2PrMwHqXl0C |title=Christensen's Physics of Diagnostic Radiology |last2=Dowdey |first2=James E. |last3=Murry |first3=Robert C. |date=1990 |publisher=Lippincott Williams & Wilkins |isbn=978-0-8121-1310-5 |pages=289 |language=en}}</ref> CT has become an important tool in [[medical imaging]] to supplement conventional [[X-ray]] imaging and [[medical ultrasonography]]. It has more recently been used for [[preventive medicine]] or [[screening (medicine)|screening]] for disease, for example, [[Virtual colonoscopy|CT colonography]] for people with a high risk of [[colon cancer]], or full-motion heart scans for people with a high risk of heart disease. Several institutions offer [[full-body scan]]s for the general population although this practice goes against the advice and official position of many professional organizations in the field primarily due to the [[radiation dose]] applied.<ref>{{Cite web |title=CT Screening |url=http://hps.org/documents/ctscreening_ps018-0.pdf |url-status=dead |archive-url=https://web.archive.org/web/20161013203907/http://hps.org/documents/ctscreening_ps018-0.pdf |archive-date=13 October 2016 |access-date=1 May 2018 |website=hps.org}}</ref>

The use of CT scans has increased dramatically over the last two decades in many countries.<ref name="Smith2009">{{Cite journal |vauthors=Smith-Bindman R, Lipson J, Marcus R, Kim KP, Mahesh M, Gould R, Berrington de González A, [[Diana Miglioretti|Miglioretti DL]] |date=December 2009 |title=Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer |journal=Arch.Archives Intern.of Med.Internal Medicine |volume=169 |issue=22 |pages=2078–862078–2086 |doi=10.1001/archinternmed.2009.427 |pmc=4635397 |pmid=20008690}}</ref> An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015.<ref name="Berrington2009">{{Cite journal |vauthors=Berrington de González A, Mahesh M, Kim KP, Bhargavan M, Lewis R, Mettler F, Land C |date=December 2009 |title=Projected cancer risks from computed tomographic scans performed in the United States in 2007 |journal=Arch. Intern. Med. |volume=169 |issue=22 |pages=2071–7 |doi=10.1001/archinternmed.2009.440 |pmc=6276814 |pmid=20008689}}</ref><ref>{{Cite web |title=Dangers of CT Scans and X-Rays – Consumer Reports |url=https://www.consumerreports.org/cro/magazine/2015/01/the-surprising-dangers-of-ct-sans-and-x-rays/index.htm |access-date=16 May 2018}}</ref>

{{Mainmain|Computed tomography of the head}}

CT scanning of the head is typically used to detect [[infarction]] ([[stroke]]), [[Neoplasm|tumors]], [[calcification]]s, [[haemorrhage]], and bone [[Major trauma|trauma]].<ref>{{Cite book |last1=Surgeons (AAOS) |first1=American Academy of Orthopaedic Surgeons |url=https://books.google.com/books?id=pUVwDwAAQBAJ&q=CT+scanning+of+the+head+is+typically+used+to+detect&pg=PA389 |title=Critical Care Transport |last2=Physicians (ACEP) |first2=American College of Emergency Physicians |last3=UMBC |date=2017-03-20 |publisher=Jones & Bartlett Learning |isbn=978-1-284-04099-9 |page=389 |language=en}}</ref> Of the above, [[hypodense]] (dark) structures can indicate [[edema]] and infarction, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. CT scanning of the head is also used in CT-[[image guided surgery|guided]] [[stereotactic surgery]] and [[radiosurgery]] for treatment of intracranial tumors, [[arteriovenous malformation]]s, and other surgically treatable conditions using a device known as the [[N-localizer]].<ref>{{Cite book |last=Galloway |first=RL Jr. |url=https://books.google.com/books?id=ioxongEACAAJ |title=Image-Guided Neurosurgery |publisher=Elsevier |year=2015 |isbn=978-0-12-800870-6 |editor-last=Golby |editor-first=AJ |location=Amsterdam |pages=3–4 |chapter=Introduction and Historical Perspectives on Image-Guided Surgery}}</ref><ref>{{Cite book |last1=Tse |first1=VCK |url=https://books.google.com/books?id=uEghr21XY6wC |title=Principles and Practice of Stereotactic Radiosurgery |last2=Kalani |first2=MYS |last3=Adler |first3=JR |publisher=Springer |year=2015 |isbn=978-0-387-71070-9 |editor-last=Chin |editor-first=LS |location=New York |page=28 |chapter=Techniques of Stereotactic Localization |editor-last2=Regine |editor-first2=WF}}</ref><ref>{{Cite book |last1=Saleh |first1=H |title=Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy |last2=Kassas |first2=B |publisher=CRC Press |year=2015 |isbn=978-1-4398-4198-3 |editor-last=Benedict |editor-first=SH |location=Boca Raton |pages=156–159 |chapter=Developing Stereotactic Frames for Cranial Treatment |editor-last2=Schlesinger |editor-first2=DJ |editor-last3=Goetsch |editor-first3=SJ |editor-last4=Kavanagh |editor-first4=BD |chapter-url=https://books.google.com/books?id=Pm3RBQAAQBAJ&q=Developing+Stereotactic+Frames+for+Cranial+Treatment&pg=PA153}}</ref><ref>{{Cite book |last1=Khan |first1=FR |url=https://books.google.com/books?id=mAN3MAEACAAJ&q=0444534970 |title=Brain Stimulation |last2=Henderson |first2=JM |series=Handbook of Clinical Neurology |publisher=Elsevier |year=2013 |isbn=978-0-444-53497-2 |editor-last=Lozano |editor-first=AM |volume=116 |location=Amsterdam |pages=28–30 |chapter=Deep Brain Stimulation Surgical Techniques |doi=10.1016/B978-0-444-53497-2.00003-6 |pmid=24112882 |editor-last2=Hallet |editor-first2=M}}</ref><ref>{{Cite book |last=Arle |first=J |url=https://books.google.com/books?id=cnF-2KCeR1sC&q=Textbook+of+Stereotactic+and+Functional+Neurosurgery |title=Textbook of Stereotactic and Functional Neurosurgery |publisher=Springer-Verlag |year=2009 |isbn=978-3-540-69959-0 |editor-last=Lozano |editor-first=AM |location=Berlin |pages=456–461 |chapter=Development of a Classic: the Todd-Wells Apparatus, the BRW, and the CRW Stereotactic Frames |editor-last2=Gildenberg |editor-first2=PL |editor-last3=Tasker |editor-first3=RR}}</ref><ref>{{Cite journal |vauthors=Brown RA, Nelson JA |date=June 2012 |title=Invention of the N-localizer for stereotactic neurosurgery and its use in the Brown-Roberts-Wells stereotactic frame |journal=Neurosurgery |volume=70 |issue=2 Supplement Operative |pages=173–176 |doi=10.1227/NEU.0b013e318246a4f7 |pmid=22186842 |s2cid=36350612}}</ref>

A CT scan can be used for detecting both acute and chronic changes in the [[Parenchyma#Lung parenchyma|lung parenchyma]], the tissue of the [[lung]]s.<ref>{{Cite book |url=https://books.google.com/books?id=rQlDDwAAQBAJ |title=Computed Tomography of the Lung |publisher=Springer Berlin Heidelberg |year=2007 |isbn=978-3-642-39518-5 |pages=40, 47}}</ref> It is particularly relevant here because normal two-dimensional X-rays do not show such defects. A variety of techniques are used, depending on the suspected abnormality. For evaluation of chronic interstitial processes such as [[Pneumatosis#Lungs|emphysema]], and [[Pulmonary fibrosis#|fibrosis]],<ref>{{Cite book |url=https://books.google.com/books?id=VKATAQAAMAAJ&q=ct+of+lungs |title=High-resolution CT of the Lung |publisher=Lippincott Williams & Wilkins |year=2009 |isbn=978-0-7817-6909-9 |pages=81,568}}</ref> thin sections with high spatial frequency reconstructions are used; often scans are performed both on inspiration and expiration. This special technique is called [[high resolution CT]] that produces a sampling of the lung, and not continuous images.<ref>{{Cite book |last1=Martínez-Jiménez |first1=Santiago |url=https://books.google.com/books?id=QjouDwAAQBAJ&q=HRCT |title=Specialty Imaging: HRCT of the Lung E-Book |last2=Rosado-de-Christenson |first2=Melissa L. |last3=Carter |first3=Brett W. |date=2017-07-22 |publisher=Elsevier Health Sciences |isbn=978-0-323-52495-7 |language=en}}</ref>

[[File:Bronchial wall thickness (T) and diameter (D).svg|thumb|110pxupright|Bronchial wall thickness (T) and diameter of the bronchus (D)]]

[[Computed tomography angiography]] (CTA) is a type of [[contrast CT]] to visualize the [[arteries]] and [[vein]]s throughout the body.<ref>{{Citation |last1=McDermott |first1=M. |title=Chapter 10 – Critical care in acute ischemic stroke |date=2017-01-01 |journal=Handbook of Clinical Neurology |volume=140 |pages=153–176 |editor-last=Wijdicks |editor-first=Eelco F. M. |series=Critical Care Neurology Part I |publisher=Elsevier |language=en |doi=10.1016/b978-0-444-63600-3.00010-6 |pmid=28187798 |last2=Jacobs |first2=T. |last3=Morgenstern |first3=L. |editor2-last=Kramer |editor2-first=Andreas H.}}</ref> This ranges from arteries serving the [[brain]] to those bringing blood to the [[lung]]s, [[kidney]]s, [[arm]]s and [[leg]]s. An example of this type of exam is [[CT pulmonary angiogram]] (CTPA) used to diagnose [[pulmonary embolism]] (PE). It employs computed tomography and an [[iodinated contrast|iodine-based contrast agent]] to obtain an image of the [[pulmonary artery|pulmonary arteries]].<ref>{{Cite web |date=19 November 2019 |title=Computed Tomography Angiography (CTA) |url=https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/computed-tomography-angiography-cta |access-date=2021-03-21 |website=www.hopkinsmedicine.org |language=en}}</ref><ref>{{Cite journal |last1=Zeman |first1=R K |last2=Silverman |first2=P M |last3=Vieco |first3=P T |last4=Costello |first4=P |date=1995-11-01 |title=CT angiography. |journal=American Journal of Roentgenology |volume=165 |issue=5 |pages=1079–1088 |doi=10.2214/ajr.165.5.7572481 |issn=0361-803X |pmid=7572481 |doi-access=free}}</ref><ref>{{Cite book |last1=Ramalho |first1=Joana |url=https://books.google.com/books?id=FKdMAgAAQBAJ&q=cta+is+an+imaging |title=Vascular Imaging of the Central Nervous System: Physical Principles, Clinical Applications, and Emerging Techniques |last2=Castillo |first2=Mauricio |date=2014-03-31 |publisher=John Wiley & Sons |isbn=978-1-118-18875-0 |page=69}}</ref> CT scans can reduce the risk of angiography by providing clinicians with more information about the positioning and number of clots prior to the procedure.<ref>{{Cite journal |last=Jones |first=Daniel A. |last2=Beirne |first2=Anne-Marie |last3=Kelham |first3=Matthew |last4=Rathod |first4=Krishnaraj S. |last5=Andiapen |first5=Mervyn |last6=Wynne |first6=Lucinda |last7=Godec |first7=Thomas |last8=Forooghi |first8=Nasim |last9=Ramaseshan |first9=Rohini |last10=Moon |first10=James C. |last11=Davies |first11=Ceri |last12=Bourantas |first12=Christos V. |last13=Baumbach |first13=Andreas |last14=Manisty |first14=Charlotte |last15=Wragg |first15=Andrew |date=2023-10-31 |title=Computed Tomography Cardiac Angiography Before Invasive Coronary Angiography in Patients With Previous Bypass Surgery: The BYPASS-CTCA Trial |url=https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.123.064465 |journal=Circulation |language=en |volume=148 |issue=18 |pages=1371–1380 |doi=10.1161/CIRCULATIONAHA.123.064465 |issn=0009-7322 |pmc=11139242 |pmid=37772419}}</ref><ref>{{Cite journal |date=6 August 2024 |title=CT scan reduces the complications of angiography after bypass surgery |url=https://evidence.nihr.ac.uk/alert/ct-scan-reduces-the-complications-of-angiography-after-bypass-surgery/ |journal=NIHR Evidence}}</ref>

*[[Coronary CT angiography]] (CCTA): the use of CT to assess the [[coronary artery|coronary arteries]] of the [[heart]]. The subject receives an [[intravenous injection]] of [[radiocontrast]], and then the heart is scanned using a high-speed CT scanner, allowing radiologists to assess the extent of occlusion in the coronary arteries, usually to diagnose coronary artery disease.<ref>{{Cite book |last=Passariello |first=Roberto |url=https://books.google.com/books?id=eR5USB6sRU4C&q=ct+angiography |title=Multidetector-Row CT Angiography |date=2006-03-30 |publisher=Springer Science & Business Media |isbn=978-3-540-26984-7 |language=en}}</ref><ref>{{Cite web |last=Radiology (ACR) |firstlast1=Radiological Society of North America (RSNA) and |last2=American College of Radiology |title=Coronary Computed Tomography Angiography (CCTA) |url=https://www.radiologyinfo.org/en/info.cfm?pg=angiocoroct |access-date=2021-03-19 |website=www.radiologyinfo.org |language=en}}</ref>

*[[Coronary CT calcium scan]]: also used for the assessment of severity of coronary artery disease. Specifically, it looks for calcium deposits in the coronary arteries that can narrow arteries and increase the risk of a heart attack.<ref name="mayo">{{Cite web |title=Heart scan (coronary calcium scan) |url=http://www.mayoclinic.org/tests-procedures/heart-scan/basics/definition/prc-20015000 |url-status=live |archive-url=https://web.archive.org/web/20150905084216/http://www.mayoclinic.org/tests-procedures/heart-scan/basics/definition/prc-20015000 |archive-date=5 September 2015 |access-date=9 August 2015 |publisher=Mayo Clinic}}</ref> A typical coronary CT calcium scan is done without the use of radiocontrast, but it can possibly be done from contrast-enhanced images as well.<ref name="van der BijlJoemai2010">{{Cite journal |last1=van der Bijl |first1=Noortje |last2=Joemai |first2=Raoul M. S. |last3=Geleijns |first3=Jacob |last4=Bax |first4=Jeroen J. |last5=Schuijf |first5=Joanne D. |last6=de Roos |first6=Albert |last7=Kroft |first7=Lucia J. M. |year=2010 |title=Assessment of Agatston Coronary Artery Calcium Score Using Contrast-Enhanced CT Coronary Angiography |journal=American Journal of Roentgenology |volume=195 |issue=6 |pages=1299–1305 |doi=10.2214/AJR.09.3734 |issn=0361-803X |pmid=21098187}}</ref>

CT is an accurate technique for diagnosis of [[Human abdomen|abdominal]] diseases like [[Crohn's disease]],<ref>{{Cite journal |last1=Furukawa |first1=Akira |last2=Saotome |first2=Takao |last3=Yamasaki |first3=Michio |last4=Maeda |first4=Kiyosumi |last5=Nitta |first5=Norihisa |last6=Takahashi |first6=Masashi |last7=Tsujikawa |first7=Tomoyuki |last8=Fujiyama |first8=Yoshihide |last9=Murata |first9=Kiyoshi |last10=Sakamoto |first10=Tsutomu |date=2004-05-01 |title=Cross-sectional Imaging in Crohn Disease |journal=RadioGraphics |volume=24 |issue=3 |pages=689–702 |doi=10.1148/rg.243035120 |issn=0271-5333 |pmid=15143222 |doi-access=free}}</ref> GIT bleeding, and diagnosis and staging of cancer, as well as follow-up after cancer treatment to assess response.<ref>{{Cite book |url=https://books.google.com/books?id=r3uK7sSZUmcC |title=CT of the Acute Abdomen |publisher=Springer Berlin Heidelberg |year=2011 |isbn=978-3-540-89232-8 |pages=37}}</ref> It is commonly used to investigate [[acute abdominal pain]].<ref>{{Cite book |last1=Jay P Heiken |chapter-url=https://books.google.com/books?id=CSy5BQAAQBAJ&pg=PA3 |title=Diseases of the Abdomen and Pelvis |last2=Douglas S Katz |publisher=Springer Milan |year=2014 |isbn=9788847056596978-88-470-5659-6 |editor-last=J. Hodler |page=3 |chapter=Emergency Radiology of the Abdomen and Pelvis: Imaging of the Nontraumatic and Traumatic Acute Abdomen |editor-last2=R. A. Kubik-Huch |editor-last3=G. K. von Schulthess |editor-last4=Ch. L. Zollikofer}}</ref>

CT is used in [[biomechanics]] to quickly reveal the geometry, anatomy, [[density]] and [[Modulus of elasticity|elastic moduli]] of biological tissues.<ref>{{Cite journal |last=Keaveny |first=Tony M. |date=March 2010 |title=Biomechanical computed tomography-noninvasive bone strength analysis using clinical computed tomography scans |journal=Annals of the New York Academy of Sciences |volume=1192 |issue=1 |pages=57–65 |bibcode=2010NYASA1192...57K |doi=10.1111/j.1749-6632.2009.05348.x |issn=1749-6632 |pmid=20392218 |s2cid=24132358}}</ref><ref>{{Cite book |last1=Barber |first1=Asa |url=https://books.google.com/books?id=shSMDwAAQBAJ&q=CT+is+used+in+biomechanics+to |title=Computed Tomography Based Biomechanics |last2=Tozzi |first2=Gianluca |last3=Pani |first3=Martino |date=2019-03-07 |publisher=Frontiers Media SA |isbn=978-2-88945-780-9 |page=20 |language=en}}</ref>

[[Industrial CT scanning]] (industrial computed tomography) is a process which utilizesuses X-ray equipment to produce 3D representations of components both externally and internally. Industrial CT scanning has been utilizedused in many areas of industry for internal inspection of components. Some of the key uses for CT scanning have been flaw detection, failure analysis, metrology, assembly analysis, image-based finite element methods<ref>{{Cite journal |last1=Evans |first1=Ll. M. |last2=Margetts |first2=L. |last3=Casalegno |first3=V. |last4=Lever |first4=L. M. |last5=Bushell |first5=J. |last6=Lowe |first6=T. |last7=Wallwork |first7=A. |last8=Young |first8=P. |last9=Lindemann |first9=A. |date=2015-05-28 |title=Transient thermal finite element analysis of CFC–Cu ITER monoblock using X-ray tomography data |url=https://www.researchgate.net/publication/277338941 |url-status=live |journal=[[Fusion Engineering and Design]] |volume=100 |pages=100–111 |doi=10.1016/j.fusengdes.2015.04.048 |archive-url=https://web.archive.org/web/20151016091649/http://www.researchgate.net/publication/277338941_Transient_thermal_finite_element_analysis_of_CFCCu_ITER_monoblock_using_X-ray_tomography_data |archive-date=2015-10-16 |doi-access=free|bibcode=2015FusED.100..100E |hdl=10871/17772 |hdl-access=free }}</ref> and reverse engineering applications. CT scanning is also employed in the imaging and conservation of museum artifacts.<ref>{{Cite journal |last=Payne, Emma Marie |year=2012 |title=Imaging Techniques in Conservation |url=http://discovery.ucl.ac.uk/1443164/1/56-566-2-PB.pdf |journal=Journal of Conservation and Museum Studies |volume=10 |issue=2 |pages=17–29 |doi=10.5334/jcms.1021201 |doi-access=free}}</ref>

CT scanning has also found an application in transport security (predominantly [[airport security]]) where it is currently used in a materials analysis context for explosives detection [[CTX (explosive-detection device)]]<ref>{{Cite book |last1=P. Babaheidarian |title=Anomaly Detection and Imaging with X-Rays (ADIX) III |last2=D. Castanon |editor-first1=Joel A. |editor-first2=Michael E. |editor-first3=Mark A. |editor-first4=Amit |editor-last1=Greenberg |editor-last2=Gehm |editor-last3=Neifeld |editor-last4=Ashok |date=2018 |isbn=978-1-5106-1775-9 |pages=12 |chapter=Joint reconstruction and material classification in spectral CT |doi=10.1117/12.2309663 |s2cid=65469251}}</ref><ref name="jin12securityct">{{Cite book |last1=P. Jin |title=Second International Conference on Image Formation in X-Ray Computed Tomography |last2=E. Haneda |last3=K. D. Sauer |last4=C. A. Bouman |date=June 2012 |chapter=A model-based 3D multi-slice helical CT reconstruction algorithm for transportation security application |access-date=2015-04-05 |chapter-url=https://engineering.purdue.edu/~bouman/publications/orig-pdf/CT-2012a.pdf |archive-url=https://web.archive.org/web/20150411000659/https://engineering.purdue.edu/~bouman/publications/orig-pdf/CT-2012a.pdf |archive-date=2015-04-11 |url-status=dead}}</ref><ref name="jin12securityctprior">{{Cite book |last1=P. Jin |title=Signals, Systems and Computers (ASILOMAR), 2012 Conference Record of the Forty Sixth Asilomar Conference on |last2=E. Haneda |last3=C. A. Bouman |date=November 2012 |publisher=IEEE |pages=613–636 |chapter=Implicit Gibbs prior models for tomographic reconstruction |access-date=2015-04-05 |chapter-url=https://engineering.purdue.edu/~bouman/publications/pdf/Asilomar-2012-Pengchong.pdf |archive-url=https://web.archive.org/web/20150411025559/https://engineering.purdue.edu/~bouman/publications/pdf/Asilomar-2012-Pengchong.pdf |archive-date=2015-04-11 |url-status=dead}}</ref><ref name="kisner13securityct">{{Cite book |last1=S. J. Kisner |title=Security Technology (ICCST), 2013 47th International Carnahan Conference on |last2=P. Jin |last3=C. A. Bouman |last4=K. D. Sauer |last5=W. Garms |last6=T. Gable |last7=S. Oh |last8=M. Merzbacher |last9=S. Skatter |date=October 2013 |publisher=IEEE |chapter=Innovative data weighting for iterative reconstruction in a helical CT security baggage scanner |access-date=2015-04-05 |chapter-url=https://engineering.purdue.edu/~bouman/publications/pdf/iccst2013.pdf |archive-url=https://web.archive.org/web/20150410234541/https://engineering.purdue.edu/~bouman/publications/pdf/iccst2013.pdf |archive-date=2015-04-10 |url-status=dead}}</ref> and is also under consideration for automated baggage/parcel security scanning using [[computer vision]] based object recognition algorithms that target the detection of specific threat items based on 3D appearance (e.g. guns, knives, liquid containers).<ref name="megherbi10baggage">{{Cite book |last1=Megherbi, N. |title=Proc. International Conference on Image Processing |last2=Flitton, G.T. |last3=Breckon, T.P. |date=September 2010 |publisher=IEEE |isbn=978-1-4244-7992-4 |pages=1833–1836 |chapter=A Classifier based Approach for the Detection of Potential Threats in CT based Baggage Screening |citeseerx=10.1.1.188.5206 |doi=10.1109/ICIP.2010.5653676 |access-date=5 November 2013 |chapter-url=http://www.durham.ac.uk/toby.breckon/publications/papers/megherbi10baggage.pdf |s2cid=3679917 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="megherbi12baggage">{{Cite book |last1=Megherbi, N. |title=Proc. International Conference on Image Processing |last2=Han, J. |last3=Flitton, G.T. |last4=Breckon, T.P. |date=September 2012 |publisher=IEEE |isbn=978-1-4673-2533-2 |pages=3109–3112 |chapter=A Comparison of Classification Approaches for Threat Detection in CT based Baggage Screening |citeseerx=10.1.1.391.2695 |doi=10.1109/ICIP.2012.6467558 |access-date=5 November 2013 |chapter-url=http://www.durham.ac.uk/toby.breckon/publications/papers/megherbi12baggage.pdf |s2cid=6924816 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="flitton13interestpoint">{{Cite journal |last1=Flitton, G.T. |last2=Breckon, T.P. |last3=Megherbi, N. |date=September 2013 |title=A Comparison of 3D Interest Point Descriptors with Application to Airport Baggage Object Detection in Complex CT Imagery |url=http://www.durham.ac.uk/toby.breckon/publications/papers/flitton13interestpoint.pdf |journal=Pattern Recognition |volume=46 |pages=2420–2436 |bibcode=2013PatRe..46.2420F |doi=10.1016/j.patcog.2013.02.008 |access-date=5 November 2013 |number=9 |hdl=1826/15213 |s2cid=3687379 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Its usage in airport security pioneered at [[Shannon Airport]] in March 2022 has ended the ban on liquids over 100&nbsp;ml there, a move that [[Heathrow Airport]] plans for a full roll-out on 1 December 2022 and the TSA spent $781.2 million on an order for over 1,000 scanners, ready to go live in the summer.<ref>{{Cite web |last=Russell |first=Molly |date=30 March 2022 |title=How Next Generation Airport Scanners Are Ending The 100ml Liquid Rule |url=https://simpleflying.com/airport-scanners-ending-100ml-liquid-rule/ |access-date=11 April 2022 |website=Simple Flying }}</ref>

X-ray CT is used in geological studies to quickly reveal materials inside a drill core.<ref>{{Cite web |title=Laboratory {{!}} About Chikyu {{!}} The Deep-sea Scientific Drilling Vessel CHIKYU |url=http://www.jamstec.go.jp/chikyu/e/about/laboratory.html |access-date=2019-10-24 |website=www.jamstec.go.jp}}</ref> Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images.<ref>{{Cite journal |last1=Tonai |first1=Satoshi |last2=Kubo |first2=Yusuke |last3=Tsang |first3=Man-Yin |last4=Bowden |first4=Stephen |last5=Ide |first5=Kotaro |last6=Hirose |first6=Takehiro |last7=Kamiya |first7=Nana |last8=Yamamoto |first8=Yuzuru |last9=Yang |first9=Kiho |last10=Yamada |first10=Yasuhiro |last11=Morono |first11=Yuki |date=2019 |title=A New Method for Quality Control of Geological Cores by X-Ray Computed Tomography: Application in IODP Expedition 370 |journal=Frontiers in Earth Science |language=English |volume=7 |doi=10.3389/feart.2019.00117 |issn=2296-6463 |doi-access=free |s2cid=171394807|hdl=2164/12811 |hdl-access=free}}</ref>

Varied types of fungus can degrade wood to different degrees, one Belgium research group has been used X-ray CT 3 dimension with sub-micron resolution unveiled fungi can penetrate micropores of 0.6 μm<ref>{{Cite journal |last1=Bulcke |first1=Jan Van den |last2=Boone |first2=Matthieu |last3=Acker |first3=Joris Van |last4=Hoorebeke |first4=Luc Van |date=October 2009 |title=Three-Dimensional X-Ray Imaging and Analysis of Fungi on and in Wood |url=https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/abs/threedimensional-xray-imaging-and-analysis-of-fungi-on-and-in-wood/1461E6965BD5061B6549308E5EEE1ADE |journal=Microscopy and Microanalysis |language=en |volume=15 |issue=5 |pages=395–402 |doi=10.1017/S1431927609990419 |pmid=19709462 |bibcode=2009MiMic..15..395V |hdl=1854/LU-675607 |s2cid=15637414 |issn=1435-8115}}</ref> under certain conditions.

Sawmills use industrial CT scanners to detect round defects, for instance knots, to improve total value of timber productions. Most sawmills are planning to incorporate this robust detection tool to improve productivity in the long run, however initial investment cost is considerably high.

[[File:CT presentation as thin slice, projection and volume rendering.jpg|thumb|300pxupright=1.4|Types of presentations of CT scans: <br />-− Average intensity projection<br />-− [[Maximum intensity projection]]<br />-− Thin slice ([[median plane]])<br />-− [[Volume rendering]] by high and low threshold for [[radiodensity]]]]

*Slices (of varying thickness). Thin slice is generally regarded as planes representing a thickness of less than 3 [[Millimetre|mm]].<ref name="Goldman2008">{{Cite journal |last=Goldman |first=L. W. |year=2008 |title=Principles of CT: Multislice CT |journal=Journal of Nuclear Medicine Technology |volume=36 |issue=2 |pages=57–68 |doi=10.2967/jnmt.107.044826 |issn=0091-4916 |pmid=18483143 |doi-access=free}}</ref><ref name=":2">{{Cite journal |last1=Reis |first1=Eduardo Pontes |last2=Nascimento |first2=Felipe |last3=Aranha |first3=Mateus |last4=Mainetti Secol |first4=Fernando |last5=Machado |first5=Birajara |last6=Felix |first6=Marcelo |last7=Stein |first7=Anouk |last8=Amaro |first8=Edson |date=29 July 2020 |title=Brain Hemorrhage Extended (BHX): Bounding box extrapolation from thick to thin slice CT images v1.1 |journal=PhysioNet |language=en |volume=101 |issue=23 |pages=215–220 |doi=10.13026/9cft-hg92}}</ref> Thick slice is generally regarded as planes representing a thickness between 3&nbsp;mm and 5&nbsp;mm.<ref name=":2" /><ref>{{Cite journal |last1=Park |first1=S. |last2=Chu |first2=L.C. |last3=Hruban |first3=R.H. |last4=Vogelstein |first4=B. |last5=Kinzler |first5=K.W. |last6=Yuille |first6=A.L. |last7=Fouladi |first7=D.F. |last8=Shayesteh |first8=S. |last9=Ghandili |first9=S. |last10=Wolfgang |first10=C.L. |last11=Burkhart |first11=R. |last12=He |first12=J. |last13=Fishman |first13=E.K. |last14=Kawamoto |first14=S. |date=2020-09-01 |title=Differentiating autoimmune pancreatitis from pancreatic ductal adenocarcinoma with CT radiomics features |journal=Diagnostic and Interventional Imaging |language=en |volume=101 |issue=9 |pages=555–564 |doi=10.1016/j.diii.2020.03.002 |issn=2211-5684 |pmid=32278586 |s2cid=215751181|doi-access=free }}</ref>

Technically, all volume renderings become projections when viewed on a [[Display device#Full-area 2-dimensional displays|2-dimensional display]], making the distinction between projections and volume renderings a bit vague. The epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations.<ref name="SilversteinParsad2008">{{Cite journal |last1=Silverstein |first1=Jonathan C. |last2=Parsad |first2=Nigel M. |last3=Tsirline |first3=Victor |year=2008 |title=Automatic perceptual color map generation for realistic volume visualization |journal=Journal of Biomedical Informatics |volume=41 |issue=6 |pages=927–935 |doi=10.1016/j.jbi.2008.02.008 |issn=1532-0464 |pmc=2651027 |pmid=18430609}}</ref><ref>{{Cite book |last=Kobbelt |first=Leif |url=https://books.google.com/books?id=zndnSzkfkXwC |title=Vision, Modeling, and Visualization 2006: Proceedings, November 22-24, 2006, Aachen, Germany |date=2006 |publisher=IOS Press |isbn=978-3-89838-081-2 |pages=185 |language=en}}</ref>

Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet.<ref name="auto" /> Hence, the left side of the image is to the patient's right and vice versa, while anterior in the image also is the patient's anterior and vice versa. This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients.<ref>{{Cite journal |last1=Schmidt |first1=Derek |last2=Odland |first2=Rick |date=September 2004 |title=Mirror-Image Reversal of Coronal Computed Tomography Scans |journal=The Laryngoscope |language=en |volume=114 |issue=9 |pages=1562–1565 |doi=10.1097/00005537-200409000-00011 |issn=0023-852X |pmid=15475782 |s2cid=22320649}}</ref>

[[Pixel]]s in an image obtained by CT scanning are displayed in terms of relative [[radiodensity]]. The pixel itself is displayed according to the mean [[attenuation]] of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on the [[Hounsfield scale]]. A [[pixel]] is a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a [[voxel]], which is a three-dimensional unit.<ref>{{Cite book |url=https://books.google.com/books?id=63xxDwAAQBAJ |title=Brant and Helms' fundamentals of diagnostic radiology |date=2018-07-19 |publisher=Lippincott Williams & Wilkins |isbn=978-1-4963-6738-9 |edition=Fifth |pages=1600 |access-date=24 January 2019}}</ref> Water has an attenuation of 0 [[Hounsfield units]] (HU), while air is −1,000&nbsp;HU, cancellous bone is typically +400&nbsp;HU, and cranial bone can reach 2,000&nbsp;HU.<ref>{{Cite book |title=Brain mapping: the methods |date=2002 |publisher=Academic Press |isbn=0-12-693019-8 |editor-last=Arthur W. Toga |edition=2nd |location=Amsterdam |oclc=52594824 |editor-last2=John C. Mazziotta}}</ref> The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of +1000&nbsp;HU, iron steel can completely block the X-ray and is, therefore, responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics.<ref name="...">{{Cite book |last1=Jerrold T. Bushberg |title=The essential physics of medical imaging |last2=J. Anthony Seibert |last3=Edwin M. Leidholdt |last4=John M. Boone |date=2002 |publisher=Lippincott Williams & Wilkins |isbn=0-683-30118-7 |edition=2nd |location=Philadelphia, PA |page=358 |oclc=47177732}}</ref>

CT data sets have a very high [[dynamic range]] which must be reduced for display or printing. This is typically done via a process of "windowing", which maps a range (the "window") of pixel values to a grayscale ramp. For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU. Pixel values of 0 and lower, are displayed as black; values of 80 and higher are displayed as white; values within the window are displayed as a gray intensity proportional to position within the window.<ref>{{Cite book |last1=Kamalian |first1=Shervin |last2=Lev |first2=Michael H. |last3=Gupta |first3=Rajiv |date=2016-01-01 |titlechapter=Computed tomography imaging and angiography – principles |date=2016-01-01 |title=Neuroimaging Part I |series=Handbook of Clinical Neurology |language=en |volume=135 |pages=3–20 |doi=10.1016/B978-0-444-53485-9.00001-5 |isbn=978-0-444-53485-9 |issn=0072-9752 |pmid=27432657}}</ref> The window used for display must be matched to the X-ray density of the object of interest, in order to optimize the visible detail.<ref>{{Cite book |last=Stirrup |first=James |url=https://books.google.com/books?id=SarDDwAAQBAJ&q=windowing+in+ct&pg=PA136 |title=Cardiovascular Computed Tomography |date=2020-01-02 |publisher=Oxford University Press |isbn=978-0-19-880927-2 |page=136 |language=en}}</ref> Window width and window level parameters are used to control the windowing of a scan.<ref>{{Cite book |last=Carroll |first=Quinn B. |url=https://books.google.com/books?id=iTwYI5rzeRMC&dq=window+width+and+window+level&pg=PA512 |title=Practical Radiographic Imaging |date=2007 |publisher=Charles C Thomas Publisher |isbn=978-0-398-08511-7|page=512 |language=en}}</ref>

Multiplanar reconstruction (MPR) is the process of converting data from one [[anatomical plane]] (usually [[Transverse plane|transverse]]) to other planes. It can be used for thin slices as well as projections. Multiplanar reconstruction is possible as present CT scanners provide almost [[isotropy|isotropic]] resolution.<ref name="ref3">{{Cite book |last1=Udupa |first1=Jayaram K. |url=https://books.google.com/books?id=aR6PHYluq4oC&q=3D+Imaging+in+Medicine%2C+2nd+Edition |title=3D Imaging in Medicine, Second Edition |last2=Herman |first2=Gabor T. |date=1999-09-28 |publisher=CRC Press |isbn=978-0-8493-3179-4 |language=en}}</ref>

MPR is used almost in every scan. The spine is frequently examined with it.<ref>{{Cite journal |last1=Krupski |first1=Witold |last2=Kurys-Denis |first2=Ewa |last3=Matuszewski |first3=Łukasz |last4=Plezia |first4=Bogusław |date=2007-06-30 |title=Use of multi-planar reconstruction (MPR) and 3-dimentional &#91;sic&#93; (3D) CT to assess stability criteria in C2 vertebral fractures |url=http://www.jpccr.eu/Use-of-multi-planar-reconstruction-MPR-and-3-dimentional-3D-CT-to-assess-stability,71238,0,2.html |journal=Journal of Pre-Clinical and Clinical Research |language=english |volume=1 |issue=1 |pages=80–83 |issn=1898-2395}}</ref> An image of the spine in axial plane can only show one vertebral bone at a time and cannot show its relation with other vertebral bones. By reformatting the data in other planes, visualization of the relative position can be achieved in sagittal and coronal plane.<ref>{{Cite journal |last=Tins |first=Bernhard |date=2010-10-21 |title=Technical aspects of CT imaging of the spine |journal=Insights into Imaging |volume=1 |issue=5–6 |pages=349–359 |doi=10.1007/s13244-010-0047-2 |issn=1869-4101 |pmc=3259341 |pmid=22347928}}</ref>

New software allows the reconstruction of data in non-orthogonal (oblique) planes, which help in the visualization of organs which are not in orthogonal planes.<ref>{{Cite web |title=CT imaging: Where are we going? (Proceedings) |url=https://www.dvm360.com/view/ct-imaging-where-are-we-going-proceedings |access-date=2021-03-21 |website=DVM 360|date=April 2010 }}</ref><ref>{{Cite book |last1=Wolfson |first1=Nikolaj |url=https://books.google.com/books?id=8Y5FDAAAQBAJ&q=Modern+software+allows+reconstruction+in+non-orthogonal&pg=PA373 |title=Orthopedics in Disasters: Orthopedic Injuries in Natural Disasters and Mass Casualty Events |last2=Lerner |first2=Alexander |last3=Roshal |first3=Leonid |date=2016-05-30 |publisher=Springer |isbn=978-3-662-48950-5 |language=en}}</ref> It is better suited for visualization of the anatomical structure of the bronchi as they do not lie orthogonal to the direction of the scan.<ref>{{Cite journal |last1=Laroia |first1=Archana T |last2=Thompson |first2=Brad H |last3=Laroia |first3=Sandeep T |last4=van Beek |first4=Edwin JR |date=2010-07-28 |title=Modern imaging of the tracheo-bronchial tree |journal=World Journal of Radiology |volume=2 |issue=7 |pages=237–248 |doi=10.4329/wjr.v2.i7.237 |issn=1949-8470 |pmc=2998855 |pmid=21160663 |doi-access=free }}</ref>

Curved-plane reconstruction (or curved planar reformation = CPR) is performed mainly for the evaluation of vessels. This type of reconstruction helps to straighten the bends in a vessel, thereby helping to visualize a whole vessel in a single image or in multiple images. After a vessel has been "straightened", measurements such as cross-sectional area and length can be made. This is helpful in preoperative assessment of a surgical procedure.<ref>{{Cite journal |last1=Gong |first1=Jing-Shan |last2=Xu |first2=Jian-Min |date=2004-07-01 |title=Role of curved planar reformations using multidetector spiral CT in diagnosis of pancreatic and peripancreatic diseases |journal=World Journal of Gastroenterology |volume=10 |issue=13 |pages=1943–1947 |doi=10.3748/wjg.v10.i13.1943 |issn=1007-9327 |pmc=4572236 |pmid=15222042 |doi-access=free }}</ref>

For 2D projections used in [[radiation therapy]] for quality assurance and planning of [[external beam radiotherapy]], including digitally reconstructed radiographs, see [[Beam's eye view]].

|Useful for identifying the internal structures of a solid organ or the walls of hollow structures, such as intestines.

A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). With the help of [[edge detection]] image processing algorithms a 3D model can be constructed from the initial data and displayed on screen. Various thresholds can be used to get multiple models, each anatomical component such as muscle, bone and cartilage can be differentiated on the basis of different colours given to them. However, this mode of operation cannot show interior structures.<ref>{{Cite journal |last1=Calhoun |first1=Paul S. |last2=Kuszyk |first2=Brian S. |last3=Heath |first3=David G. |last4=Carley |first4=Jennifer C. |last5=Fishman |first5=Elliot K. |date=1999-05-01 |title=Three-dimensional Volume Rendering of Spiral CT Data: Theory and Method |url=https://pubs.rsna.org/doi/full/10.1148/radiographics.19.3.g99ma14745 |journal=RadioGraphics |volume=19 |issue=3 |pages=745–764 |doi=10.1148/radiographics.19.3.g99ma14745 |issn=0271-5333 |pmid=10336201}}</ref>

Surface rendering is limited technique as it displays only the surfaces that meet a particular threshold density, and which are towards the viewer. However, In volume rendering, transparency, colours and [[Phong shading|shading]] are used which makes it easy to present a volume in a single image. For example, Pelvic bones could be displayed as semi-transparent, so that, even viewing at an oblique angle one part of the image does not hide another.<ref>{{Cite journal |last1=van Ooijen |first1=P. M. A. |last2=van Geuns |first2=R. J. M. |last3=Rensing |first3=B. J. W. M. |last4=Bongaerts |first4=A. H. H. |last5=de Feyter |first5=P. J. |last6=Oudkerk |first6=M. |date=January 2003 |title=Noninvasive Coronary Imaging Using Electron Beam CT: Surface Rendering Versus Volume Rendering |url=http://www.ajronline.org/doi/10.2214/ajr.180.1.1800223 |journal=American Journal of Roentgenology |language=en |volume=180 |issue=1 |pages=223–226 |doi=10.2214/ajr.180.1.1800223 |issn=0361-803X |pmid=12490509}}</ref>

An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. In general, higher radiation doses result in higher-resolution images,<ref name="Crowther">{{Cite journal |last1=R. A. Crowther |last2=D. J. DeRosier |last3=A. Klug |year=1970 |title=The Reconstruction of a Three-Dimensional Structure from Projections and its Application to Electron Microscopy |journal=Proc. R. Soc. Lond. A |volume=317 |issue=1530 |pages=319–340 |bibcode=1970RSPSA.317..319C |doi=10.1098/rspa.1970.0119 |s2cid=122980366}}</ref> while lower doses lead to increased image noise and unsharp images. However, increased dosage raises the adverse side effects, including the risk of [[radiation-induced cancer]] – a four-phase abdominal CT gives the same radiation dose as 300 chest X-rays.<ref>{{Cite journal |last1=Nickoloff |first1=Edward L. |last2=Alderson |first2=Philip O. |date=August 2001 |title=Radiation Exposures to Patients from CT: Reality, Public Perception, and Policy |url=http://www.ajronline.org/doi/10.2214/ajr.177.2.1770285 |journal=American Journal of Roentgenology |language=en |volume=177 |issue=2 |pages=285–287 |doi=10.2214/ajr.177.2.1770285 |issn=0361-803X |pmid=11461846}}</ref> Several methods that can reduce the exposure to ionizing radiation during a CT scan exist.<ref name="ata">Barkan, O; Weill, J; Averbuch, A; Dekel, S. [http://www.cv-foundation.org/openaccess/content_cvpr_2013/papers/Barkan_Adaptive_Compressed_Tomography_2013_CVPR_paper.pdf "Adaptive Compressed Tomography Sensing"] {{webarchive |url=https://web.archive.org/web/20160313133222/http://www.cv-foundation.org/openaccess/content_cvpr_2013/papers/Barkan_Adaptive_Compressed_Tomography_2013_CVPR_paper.pdf |date=2016-03-13 }}. In Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition 2013 (pp. 2195–2202).</ref>

# New software technology can significantly reduce the required radiation dose. New [[Iterative reconstruction|iterative]] [[tomographic reconstruction]] algorithms (''e.g.'', [[SAMV (algorithm)|iterative Sparse Asymptotic Minimum Variance]]) could offer [[Super-resolution imaging|super-resolution]] without requiring higher radiation dose.<ref>{{Cite book |url=https://books.google.com/books?id=hclVAAAAMAAJ&q=iterative+construction+gives+super+resolution |title=Proceedings |date=1995 |publisher=IEEE |page=10 |isbn=9780780324985 |language=en978-0-7803-2498-5}}</ref>

# Individualize the examination and adjust the radiation dose to the body type and body organ examined. Different body types and organs require different amounts of radiation.<ref>{{Cite web |title=Radiation – Effects on organs of the body (somatic effects) |url=https://www.britannica.com/science/radiation |access-date=2021-03-21 |website=Encyclopedia Britannica |language=en}}</ref>

;Partial volume effect: This appears as "blurring" of edges. It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g., bone) and a larger amount of lower density (e.g., cartilage).<ref>{{Cite journal |last1=González Ballester |first1=Miguel Angel |last2=Zisserman |first2=Andrew P. |last3=Brady |first3=Michael |date=December 2002 |title=Estimation of the partial volume effect in MRI |journal=Medical Image Analysis |volume=6 |issue=4 |pages=389–405 |doi=10.1016/s1361-8415(02)00061-0 |issn=1361-8415 |pmid=12494949}}</ref> The reconstruction assumes that the X-ray attenuation within each voxel is homogeneous; this may not be the case at sharp edges. This is most commonly seen in the z-direction (craniocaudal direction), due to the conventional use of highly [[isotropic|anisotropic]] voxels, which have a much lower out-of-plane resolution, than in-plane resolution. This can be partially overcome by scanning using thinner slices, or an isotropic acquisition on a modern scanner.<ref>{{Cite journal |last1=Goldszal |first1=Alberto F. |last2=Pham |first2=Dzung L. |date=2000-01-01 |title=Volumetric Segmentation |journal=Handbook of Medical Imaging |language=en |pages=185–194 |doi=10.1016/B978-012077790-7/50016-3 |isbn=978-0-12-077790-7}}</ref>

;Noise: This appears as grain on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy.<ref>{{Cite book |last1=Newton |first1=Thomas H. |url=https://books.google.com/books?id=2mxsAAAAMAAJ&q=noise+in+computed+tomography |title=Radiology of the Skull and Brain: Technical aspects of computed tomography |last2=Potts |first2=D. Gordon |date=1971 |publisher=Mosby |isbn=978-0-8016-3662-2 |pages=3941–3950 |language=en}}</ref>

;Windmill: Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.<ref>{{Cite book |last1=Brüning |first1=R. |url=https://books.google.com/books?id=ImOlZNOk25sC&q=windmill+artifact+ct&pg=PA44 |title=Protocols for Multislice CT |last2=Küttner |first2=A. |last3=Flohr |first3=T. |date=2006-01-16 |publisher=Springer Science & Business Media |isbn=978-3-540-27273-1 |language=en}}</ref><ref>{{Cite book |last=Peh |first=Wilfred C. G. |url=https://books.google.com/books?id=sZswDwAAQBAJ&q=windmill+artifact+ct&pg=PA49 |title=Pitfalls in Musculoskeletal Radiology |date=2017-08-11 |publisher=Springer |isbn=978-3-319-53496-1 |language=en}}</ref>

CT scanning has several advantages over traditional [[Plane (mathematics)|two-dimensional]] medical [[radiography]]. First, CT eliminates the superimposition of images of structures outside the area of interest.<ref>{{Cite book |last1=Mikla |first1=Victor I. |url=https://books.google.com/books?id=Y81JrnVA_5sC&q=ct+scan+removes+superimposition&pg=PA37 |title=Medical Imaging Technology |last2=Mikla |first2=Victor V. |date=2013-08-23 |publisher=Elsevier |isbn=978-0-12-417036-0 |page=37 |language=en}}</ref> Second, CT scans have greater [[image resolution]], enabling examination of finer details. CT can distinguish between [[tissue (biology)|tissues]] that differ in radiographic [[density]] by 1% or less.<ref>{{Cite book |url=https://books.google.com/books?id=rOppAAAAMAAJ&q=CT+can+distinguish+between+tissue |title=Radiology for the Dental Professional |publisher=Elsevier Mosby |year=2008 |isbn=978-0-323-03071-7 |pages=337}}</ref> Third, CT scanning enables multiplanar reformatted imaging: scan data can be visualized in the [[transverse plane|transverse (or axial)]], [[Coronal plane|coronal]], or [[Sagittal plane|sagittal]] plane, depending on the diagnostic task.<ref>{{Cite book |last=Pasipoularides |first=Ares |url=https://books.google.com/books?id=eMKqdIvxEmQC&q=ct+scan+enables+multiple+plane+reformatting&pg=PA595 |title=Heart's Vortex: Intracardiac Blood Flow Phenomena |date=November 2009 |publisher=PMPH-USA |isbn=978-1-60795-033-2 |pages=595 |language=en}}</ref>

The improved resolution of CT has permitted the development of new investigations. For example, CT [[angiography]] avoids the invasive insertion of a [[catheter]]. CT scanning can perform a [[virtual colonoscopy]] with greater accuracy and less discomfort for the patient than a traditional [[colonoscopy]].<ref name="Heiken">{{Cite journal |last1=Heiken |first1=JP |last2=Peterson CM |last3=Menias CO |date=November 2005 |title=Virtual colonoscopy for colorectal cancer screening: current status: Wednesday 5 October 2005, 14:00–16:00 |journal=Cancer Imaging |publisher=International Cancer Imaging Society |volume=5 |issue=Spec No A |pages=S133–S139 |doi=10.1102/1470-7330.2005.0108 |pmc=1665314 |pmid=16361129}}</ref><ref name="pmid16106357">{{Cite journal |last1=Bielen DJ |last2=Bosmans HT |last3=De Wever LL |last4=Maes |first4=Frederik |last5=Tejpar |first5=Sabine |last6=Vanbeckevoort |first6=Dirk |last7=Marchal |first7=Guy J.F. |display-authors=3 |name-list-style=vanc |date=September 2005 |title=Clinical validation of high-resolution fast spin-echo MR colonography after colon distention with air |journal=J Magn Reson Imaging |volume=22 |issue=3 |pages=400–5 |doi=10.1002/jmri.20397 |pmid=16106357 |doi-access=free |s2cid=22167728}}</ref> Virtual colonography is far more accurate than a [[barium enema]] for detection of tumors and uses a lower radiation dose.<ref>{{Cite web |title=CT Colonography |url=https://www.radiologyinfo.org/en/info.cfm?pg=ct_colo |website=Radiologyinfo.org}}</ref>

CT is a moderate-to-high [[radiation]] diagnostic technique. The radiation dose for a particular examination depends on multiple factors: volume scanned, patient build, number and type of scan protocol, and desired resolution and image quality.<ref>{{Cite journal |vauthors=Žabić S, Wang Q, Morton T, Brown KM |date=March 2013 |title=A low dose simulation tool for CT systems with energy integrating detectors |journal=Medical Physics |volume=40 |issue=3 |pages=031102 |bibcode=2013MedPh..40c1102Z |doi=10.1118/1.4789628 |pmid=23464282}}</ref> Two helical CT scanning parameters, tube current and pitch, can be adjusted easily and have a profound effect on radiation. CT scanning is more accurate than two-dimensional radiographs in evaluating anterior interbody fusion, although they may still over-read the extent of fusion.<ref>Brian R. Subach M.D., F.A.C.S et al.[http://www.spinemd.com/publications/articles/reliability-and-accuracy-of-fine-cut-computed-tomography-scans-to-determine-the-status-of-anterior-interbody-usions-with-metallic-cages "Reliability and accuracy of fine-cut computed tomography scans to determine the status of anterior interbody fusions with metallic cages"] {{webarchive|url=https://web.archive.org/web/20121208184918/http://www.spinemd.com/publications/articles/reliability-and-accuracy-of-fine-cut-computed-tomography-scans-to-determine-the-status-of-anterior-interbody-usions-with-metallic-cages |date=2012-12-08 }}</ref>

The [[ionizing radiation|radiation]] used in CT scans can damage body cells, including [[DNA molecule]]s, which can lead to [[radiation-induced cancer]].<ref name="Brenner2007">{{Cite journal |vauthors=Brenner DJ, Hall EJ |date=November 2007 |title=Computed tomography – an increasing source of radiation exposure |url=http://www.columbia.edu/~djb3/papers/nejm1.pdf |url-status=live |journal=N. Engl. J. Med. |volume=357 |issue=22 |pages=2277–84 |doi=10.1056/NEJMra072149 |pmid=18046031 |archive-url=https://web.archive.org/web/20160304060542/http://www.columbia.edu/~djb3/papers/nejm1.pdf |archive-date=2016-03-04 |s2cid=2760372}}</ref> The radiation doses received from CT scans is variable. Compared to the lowest dose xX-ray techniques, CT scans can have 100 to 1,000 times higher dose than conventional X-rays.<ref name="Redberg">Redberg, Rita F., and Smith-Bindman, Rebecca. [https://www.nytimes.com/2014/01/31/opinion/we-are-giving-ourselves-cancer.html "We Are Giving Ourselves Cancer"] {{webarchive|url=https://web.archive.org/web/20170706163542/https://www.nytimes.com/2014/01/31/opinion/we-are-giving-ourselves-cancer.html?nl=opinion&emc=edit_ty_20140131&_r=0 |date=2017-07-06 }}, ''New York Times'', January 30, 2014</ref> However, a lumbar spine xX-ray has a similar dose as a head CT.<ref>{{Cite web |last=Health |first=Center for Devices and Radiological |title=Medical X-ray Imaging – What are the Radiation Risks from CT? |url=https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115329.htm |url-status=live |archive-url=https://web.archive.org/web/20131105050317/https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115329.htm |archive-date=5 November 2013 |access-date=1 May 2018 |website=www.fda.gov}}</ref> Articles in the media often exaggerate the relative dose of CT by comparing the lowest-dose xX-ray techniques (chest xX-ray) with the highest-dose CT techniques. In general, a routine abdominal CT has a radiation dose similar to three years of average [[background radiation]].<ref>{{Cite web |lastlast1=(ACR) |first=[[Radiological Society of North America]] (RSNA) and [[|last2=American College of Radiology]] |date=February 2021 |title=Patient Safety – Radiation Dose in X-Ray and CT Exams |url=https://www.acr.org/-/media/ACR/Files/Radiology-Safety/Radiation-Safety/Dose-Reference-Card.pdf |url-status=dead |archive-url=https://web.archive.org/web/20210101161039/https://www.acr.org/-/media/ACR/Files/Radiology-Safety/Radiation-Safety/Dose-Reference-Card.pdf |archive-date=1 January 2021 |access-date=6 April 2021 |website=acr.org |author1-link=Radiological Society of North America |author2-link=American College of Radiology }}</ref>

Large scale population-based studies have consistently demonstrated that low dose radiation from CT scans has impacts on cancer incidence in a variety of cancers.<ref name="MathewsForsythe2013">{{Cite journal |last1=Mathews |first1=J. D. |last2=Forsythe |first2=A. V. |last3=Brady |first3=Z. |last4=Butler |first4=M. W. |last5=Goergen |first5=S. K. |last6=Byrnes |first6=G. B. |last7=Giles |first7=G. G. |last8=Wallace |first8=A. B. |last9=Anderson |first9=P. R. |last10=Guiver |first10=T. A. |last11=McGale |first11=P. |last12=Cain |first12=T. M. |last13=Dowty |first13=J. G. |last14=Bickerstaffe |first14=A. C. |last15=Darby |first15=S. C. |year=2013 |title=Cancer risk in 680 000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians |journal=BMJ |volume=346 |issue=may21 1 |pages=f2360 |doi=10.1136/bmj.f2360 |issn=1756-1833 |pmc=3660619 |pmid=23694687}}</ref><ref name="pearce_ctscans">{{cite journal |last1=Pearce |first1=MS |last2=Salotti |first2=JA |last3=Little |first3=MP |last4=McHugh |first4=K |last5=Lee |first5=C |last6=Kim |first6=KP |last7=Howe |first7=NL |last8=Ronckers |first8=CM |last9=Rajaraman |first9=P |last10=Sir Craft |first10=AW |last11=Parker |first11=L |last12=Berrington de González |first12=A |title=Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. |journal=Lancet |date=4 August 2012 |volume=380 |issue=9840 |pages=499–505 |doi=10.1016/S0140-6736(12)60815-0 |pmid=22681860|pmc=3418594 }}</ref><ref name="meulepas2019">{{cite journal |last1=Meulepas |first1=Johanna M |last2=Ronckers |first2=Cécile M |last3=Smets |first3=Anne M J B |last4=Nievelstein |first4=Rutger A J |last5=Gradowska |first5=Patrycja |last6=Lee |first6=Choonsik |last7=Jahnen |first7=Andreas |last8=van Straten |first8=Marcel |last9=de Wit |first9=Marie-Claire Y |last10=Zonnenberg |first10=Bernard |last11=Klein |first11=Willemijn M |last12=Merks |first12=Johannes H |last13=Visser |first13=Otto |last14=van Leeuwen |first14=Flora E |last15=Hauptmann |first15=Michael |title=Radiation Exposure From Pediatric CT Scans and Subsequent Cancer Risk in the Netherlands |journal=JNCI: Journal of the National Cancer Institute |date=1 March 2019 |volume=111 |issue=3 |pages=256–263 |doi=10.1093/jnci/djy104|pmid=30020493 |pmc=6657440 }}</ref><ref name="berrington2016">{{cite journal |last1=de Gonzalez |first1=Amy Berrington |last2=Salotti |first2=Jane A |last3=McHugh |first3=Kieran |last4=Little |first4=Mark P |last5=Harbron |first5=Richard W |last6=Lee |first6=Choonsik |last7=Ntowe |first7=Estelle |last8=Braganza |first8=Melissa Z |last9=Parker |first9=Louise |last10=Rajaraman |first10=Preetha |last11=Stiller |first11=Charles |last12=Stewart |first12=Douglas R |last13=Craft |first13=Alan W |last14=Pearce |first14=Mark S |title=Relationship between paediatric CT scans and subsequent risk of leukaemia and brain tumours: assessment of the impact of underlying conditions |journal=British Journal of Cancer |date=February 2016 |volume=114 |issue=4 |pages=388–394 |doi=10.1038/bjc.2015.415|pmid=26882064 |pmc=4815765 }}</ref> For example, in a large population-based cohort it was found that up to 4% of brain cancers were caused by CT scan radiation.<ref name="nrsCT1">{{cite journal |last1=Smoll |first1=Nicolas R |last2=Brady |first2=Zoe |last3=Scurrah |first3=Katrina J |last4=Lee |first4=Choonsik |last5=Berrington de González |first5=Amy |last6=Mathews |first6=John D |title=Computed tomography scan radiation and brain cancer incidence |journal=Neuro-Oncology |date=14 January 2023 |volume=25 |issue=7 |pages=1368–1376 |doi=10.1093/neuonc/noad012|pmid=36638155 |pmc=10326490 |pmc-embargo-date=January 14, 2024 }}</ref> Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.<ref name="Redberg" /> An Australian study of 10.9&nbsp;million people reported that the increased incidence of cancer after CT scan exposure in this cohort was mostly due to irradiation. In this group, one in every 1,800 CT scans was followed by an excess cancer. If the lifetime risk of developing cancer is 40% then the absolute risk rises to 40.05% after a CT. The risks of CT scan radiation are especially important in patients undergoing recurrent CT scans within a short time span of 1one to 5five years.<ref name="SasieniShelton2011">{{Cite journal |last1=Sasieni |first1=P D |last2=Shelton |first2=J |last3=Ormiston-Smith |first3=N |last4=Thomson |first4=C S |last5=Silcocks |first5=P B |year=2011 |title=What is the lifetime risk of developing cancer?: the effect of adjusting for multiple primaries |journal=British Journal of Cancer |volume=105 |issue=3 |pages=460–465 |doi=10.1038/bjc.2011.250 |issn=0007-0920 |pmc=3172907 |pmid=21772332}}</ref><ref name="Patients undergoing recurrent CT sc">{{Cite journal |last1=Rehani |first1=Madan M. |last2=Yang |first2=Kai |last3=Melick |first3=Emily R. |last4=Heil |first4=John |last5=Šalát |first5=Dušan |last6=Sensakovic |first6=William F. |last7=Liu |first7=Bob |year=2020 |title=Patients undergoing recurrent CT scans: assessing the magnitude |journal=European Radiology |volume=30 |issue=4 |pages=1828–1836 |doi=10.1007/s00330-019-06523-y |pmid=31792585 |s2cid=208520824}}</ref><ref name="Multinational data on cumulative ra">{{Cite journal |last1=Brambilla |first1=Marco |last2=Vassileva |first2=Jenia |last3=Kuchcinska |first3=Agnieszka |last4=Rehani |first4=Madan M. |year=2020 |title=Multinational data on cumulative radiation exposure of patients from recurrent radiological procedures: call for action |journal=European Radiology |volume=30 |issue=5 |pages=2493–2501 |doi=10.1007/s00330-019-06528-7 |pmid=31792583 |s2cid=208520544}}</ref>

Some experts note that CT scans are known to be "overused," and "there is distressingly little evidence of better health outcomes associated with the current high rate of scans."<ref name="Redberg" /> On the other hand, a recent paper analyzing the data of patients who received high [[cumulative dose]]s showed a high degree of appropriate use.<ref name="Patients undergoing recurrent CT ex">{{Cite journal |last1=Rehani |first1=Madan M. |last2=Melick |first2=Emily R. |last3=Alvi |first3=Raza M. |last4=Doda Khera |first4=Ruhani |last5=Batool-Anwar |first5=Salma |last6=Neilan |first6=Tomas G. |last7=Bettmann |first7=Michael |year=2020 |title=Patients undergoing recurrent CT exams: assessment of patients with non-malignant diseases, reasons for imaging and imaging appropriateness |journal=European Radiology |volume=30 |issue=4 |pages=1839–1846 |doi=10.1007/s00330-019-06551-8 |pmid=31792584 |s2cid=208520463}}</ref> This creates an important issue of cancer risk to these patients. Moreover, a highly significant finding that was previously unreported is that some patients received >100 mSv dose from CT scans in a single day,<ref name="Patients undergoing recurrent CT sc" /> which counteracts existing criticisms some investigators may have on the effects of protracted versus acute exposure.

There are contrarian views and the debate is ongoing. Some studies have shown that publications indicating an increased risk of cancer from typical doses of body CT scans are plagued with serious methodological limitations and several highly improbable results,<ref>{{Cite journal |last1=Eckel |first1=Laurence J. |last2=Fletcher |first2=Joel G. |last3=Bushberg |first3=Jerrold T. |last4=McCollough |first4=Cynthia H. |date=2015-10-01 |title=Answers to Common Questions About the Use and Safety of CT Scans |url=https://www.mayoclinicproceedings.org/article/S0025-6196(15)00591-1/fulltext |journal=Mayo Clinic Proceedings |language=en |volume=90 |issue=10 |pages=1380–1392 |doi=10.1016/j.mayocp.2015.07.011 |issn=0025-6196 |pmid=26434964 |doi-access=free}}</ref> concluding that no evidence indicates such low doses cause any long-term harm.<ref>{{Cite web |title=Expert opinion: Are CT scans safe? |url=https://www.sciencedaily.com/releases/2015/10/151005151507.htm |access-date=2019-03-14 |website=ScienceDaily |language=en}}</ref><ref>{{Cite journal |last1=McCollough |first1=Cynthia H. |last2=Bushberg |first2=Jerrold T. |last3=Fletcher |first3=Joel G. |last4=Eckel |first4=Laurence J. |date=2015-10-01 |title=Answers to Common Questions About the Use and Safety of CT Scans |url=https://www.mayoclinicproceedings.org/article/S0025-6196(15)00591-1/abstract |journal=Mayo Clinic Proceedings |language=English |volume=90 |issue=10 |pages=1380–1392 |doi=10.1016/j.mayocp.2015.07.011 |issn=0025-6196 |pmid=26434964 |doi-access=free}}</ref><ref>{{Cite web |date=4 February 2016 |title=No evidence that CT scans, X-rays cause cancer |url=https://www.medicalnewstoday.com/articles/306067.php |access-date=2019-03-14 |website=Medical News Today |language=en}}</ref>

Current recommendations are to inform patients of the risks of CT scanning.<ref name="pmid17646450">{{Cite journal |vauthors=Larson DB, Rader SB, Forman HP, Fenton LZ |date=August 2007 |title=Informing parents about CT radiation exposure in children: it's OK to tell them |journal=Am J Roentgenol |volume=189 |issue=2 |pages=271–5 |doi=10.2214/AJR.07.2248 |pmid=17646450 |s2cid=25020619}}</ref> However, employees of imaging centers tend not to communicate such risks unless patients ask.<ref>{{Citation |lastlast1=Emmerson |firstfirst1=Benjamin |title=Radiology Patient Safety and Communication |date=2023 |url=https://www.ncbi.nlm.nih.gov/books/NBK567713 |work=StatPearls |access-date=2023-11-24 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=33620790 |last2=Young |first2=Michael}}</ref>

In the United States half of CT scans are [[contrast CT]]s using intravenously injected [[radiocontrast agent]]s.<ref name="Nam2006" /> The most common reactions from these agents are mild, including nausea, vomiting, and an itching rash. Severe life-threatening reactions may rarely occur.<ref name="Contrast2005">{{Cite journal |last=Christiansen C |date=2005-04-15 |title=X-ray contrast media – an overview |journal=Toxicology |volume=209 |issue=2 |pages=185–7 |doi=10.1016/j.tox.2004.12.020 |pmid=15767033|bibcode=2005Toxgy.209..185C }}</ref> Overall reactions occur in 1 to 3% with [[nonionic contrast]] and 4 to 12% of people with [[ionic contrast]].<ref name="Wang2011" /> Skin rashes may appear within a week to 3% of people.<ref name="Contrast2005" />

The old [[radiocontrast agent]]s caused [[anaphylaxis]] in 1% of cases while the newer, low-osmolar agents cause reactions in 0.01–0.04% of cases.<ref name="Contrast2005" /><ref name="Drug01">{{Cite journal |vauthors=Drain KL, Volcheck GW |year=2001 |title=Preventing and managing drug-induced anaphylaxis |journal=Drug Safety |volume=24 |issue=11 |pages=843–53 |doi=10.2165/00002018-200124110-00005 |pmid=11665871 |s2cid=24840296}}</ref> Death occurs in about 2 to 30 people per 1,000,000 administrations, with newer agents being safer.<ref name="Wang2011">{{Cite journal |vauthors=Wang H, Wang HS, Liu ZP |date=October 2011 |title=Agents that induce pseudo-allergic reaction |journal=Drug Discov Ther |volume=5 |issue=5 |pages=211–9 |doi=10.5582/ddt.2011.v5.5.211 |pmid=22466368 |s2cid=19001357|doi-access=free }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=bEvnfm7V-LIC&pg=PA187 |title=Anaphylaxis and hypersensitivity reactions |date=2010-12-09 |publisher=Humana Press |isbn=978-1-60327-950-5 |editor-last=Castells |editor-first=Mariana C. |location=New York |page=187}}</ref>

The contrast agent may induce [[contrast-induced nephropathy]].<ref name="Contrast2009">{{Cite journal |vauthors=Hasebroock KM, Serkova NJ |date=April 2009 |title=Toxicity of MRI and CT contrast agents |journal=Expert Opinion on Drug Metabolism & Toxicology |volume=5 |issue=4 |pages=403–16 |doi=10.1517/17425250902873796 |pmid=19368492 |s2cid=72557671}}</ref> This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting [[kidney failure]],<ref name="Contrast2009" /> preexisting [[diabetes mellitus|diabetes]], or reduced intravascular volume. People with mild kidney impairment are usually advised to ensure full hydration for several hours before and after the injection. For moderate kidney failure, the use of [[iodinated contrast]] should be avoided; this may mean using an alternative technique instead of CT. Those with severe [[kidney failure]] requiring [[Kidney dialysis|dialysis]] require less strict precautions, as their kidneys have so little function remaining that any further damage would not be noticeable and the dialysis will remove the contrast agent; it is normally recommended, however, to arrange dialysis as soon as possible following contrast administration to minimize any adverse effects of the contrast.

In addition to the use of intravenous contrast, orally administered contrast agents are frequently used when examining the abdomen.<ref>{{Cite journal |last1=Rawson |first1=James V. |last2=Pelletier |first2=Allen L. |date=2013-09-01 |title=When to Order Contrast-Enhanced CT |url=https://www.aafp.org/afp/2013/0901/p312.html |journal=American Family Physician |volume=88 |issue=5 |pages=312–316 |issn=0002-838X |pmid=24010394}}</ref> These are frequently the same as the intravenous contrast agents, merely diluted to approximately 10% of the concentration. However, oral alternatives to iodinated contrast exist, such as very dilute (0.5–1% w/v) [[barium sulfate]] suspensions. Dilute barium sulfate has the advantage that it does not cause allergic-type reactions or kidney failure, but cannot be used in patients with suspected bowel perforation or suspected bowel injury, as leakage of barium sulfate from damaged bowel can cause fatal [[peritonitis]].<ref>{{Cite book |last1=Thomsen |first1=Henrik S. |url=https://books.google.com/books?id=Bun1CAAAQBAJ&q=intravenous+contrast+in+ct |title=Trends in Contrast Media |last2=Muller |first2=Robert N. |last3=Mattrey |first3=Robert F. |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-3-642-59814-2 |language=en}}</ref>

Side effects from [[contrast agent]]s, administered [[Intravenous therapy|intravenously]] in some CT scans, might impair [[kidney]] performance in patients with [[kidney disease]], although this risk is now believed to be lower than previously thought.<ref>{{Cite journal |last=Davenport |first=Matthew |year=2020 |title=Use of Intravenous Iodinated Contrast Media in Patients with Kidney Disease: Consensus Statements from the American College of Radiology and the National Kidney Foundation |journal=Radiology |volume=294 |issue=3 |pages=660–668 |doi=10.1148/radiol.2019192094 |pmid=31961246 |doi-access=free}}</ref><ref name="Contrast2009" />

The table reports average radiation exposures; however, there can be a wide variation in radiation doses between similar scan types, where the highest dose could be as much as 22 times higher than the lowest dose.<ref name="Furlow2010" /> A typical plain film X-ray involves radiation dose of 0.01 to 0.15&nbsp;mGy, while a typical CT can involve 10–20&nbsp;mGy for specific organs, and can go up to 80&nbsp;mGy for certain specialized CT scans.<ref name="crfdr">{{Cite journal |vauthors=Hall EJ, Brenner DJ |date=May 2008 |title=Cancer risks from diagnostic radiology |journal=The British Journal of Radiology |volume=81 |issue=965 |pages=362–78 |doi=10.1259/bjr/01948454 |pmid=18440940 |s2cid=23348032}}</ref>

The [[equivalent dose]] is the effective dose of a case, in which the whole body would actually absorb the same radiation dose, and the sievert unit is used in its report. In the case of non-uniform radiation, or radiation given to only part of the body, which is common for CT examinations, using the local equivalent dose alone would overstate the biological risks to the entire organism.<ref>{{Cite book |last1=Issa |first1=Ziad F. |title=Clinical Arrhythmology and Electrophysiology |last2=Miller |first2=John M. |last3=Zipes |first3=Douglas P. |date=2019-01-01 |publisher=Elsevier |isbn=978-0-323-52356-1 |pages=1042–1067 |language=en |chapter=Complications of Catheter Ablation of Cardiac Arrhythmias |doi=10.1016/b978-0-323-52356-1.00032-3}}</ref><ref>{{Cite web |title=Absorbed, Equivalent, and Effective Dose – ICRPaedia |url=http://icrpaedia.org/Absorbed,_Equivalent,_and_Effective_Dose |access-date=2021-03-21 |website=icrpaedia.org}}</ref><ref>{{Cite book |last=Materials |first=National Research Council (US) Committee on Evaluation of EPA Guidelines for Exposure to Naturally Occurring Radioactive |url=https://www.ncbi.nlm.nih.gov/books/NBK230653/ |title=Radiation Quantities and Units, Definitions, Acronyms |date=1999 |publisher=National Academies Press (US) |language=en}}</ref>

*deterministic effects (harmful tissue reactions) due in large part to the killing/malfunction of cells following high doses;<ref>{{Cite book |last1=Pua |first1=Bradley B. |url=https://books.google.com/books?id=7fpyDwAAQBAJ&q=deterministic+effects&pg=PA53 |title=Interventional Radiology: Fundamentals of Clinical Practice |last2=Covey |first2=Anne M. |last3=Madoff |first3=David C. |date=2018-12-03 |publisher=Oxford University Press |isbn=978-0-19-027624-9 |language=en}}</ref>

Because of increased susceptibility of fetuses to radiation exposure, the radiation dosage of a CT scan is an important consideration in the choice of [[medical imaging in pregnancy]].<ref>{{Cite web |last=CDC |date=2020-06-05 |title=Radiation and pregnancy: A fact sheet for clinicians |url=https://www.cdc.gov/nceh/radiation/emergencies/prenatalphysician.htm |access-date=2021-03-21 |website=Centers for Disease Control and Prevention |language=en-usUS}}</ref><ref>{{Citation |last1=Yoon |first1=Ilsup |title=Radiation Exposure In Pregnancy |date=2021 |url=http://www.ncbi.nlm.nih.gov/books/NBK551690/ |work=StatPearls |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31869154 |access-date=2021-03-21 |last2=Slesinger |first2=Todd L.}}</ref>

In October, 2009, the US [[Food and Drug Administration]] (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based on [[radiation burn]]s caused by incorrect settings at one particular facility for this particular type of CT scan. Over 256200 patients were exposed to radiationsradiation at approximately eight times the expected dose for overan 18-month period.; Overover 40% of them lost patches of hair,. andThis event prompted the editorial toa call for increased CT quality assurance programs. It was noted that "while unnecessary radiation exposure should be avoided, a medically needed CT scan obtained with appropriate acquisition parameter has benefits that outweigh the radiation risks."<ref name="Furlow2010">{{Cite book |last=Whaites |first=Eric |url=https://books.google.com/books?id=qdOSDdETuxcC&q=Typical+effective+dose&pg=PA27 |title=Radiography and Radiology for Dental Care Professionals E-Book |date=2008-10-10 |publisher=Elsevier Health Sciences |isbn=978-0-7020-4799-2 |pages=25 |language=en}}</ref><ref>{{Cite journal |vauthors=Wintermark M, Lev MH |date=January 2010 |title=FDA investigates the safety of brain perfusion CT |journal=AJNR Am J Neuroradiol |volume=31 |issue=1 |pages=2–3 |doi=10.3174/ajnr.A1967 |pmc=7964089 |pmid=19892810 |doi-access=free}}</ref> Similar problems have been reported at other centers.<ref name="Furlow2010" /> These incidents are believed to be due to [[human error]].<ref name="Furlow2010" />

CT scan procedure varies according to the type of the study and the organ being imaged. The patient is made to lie on the CT table and the centering of the table is done according to the body part. The IV line is established in case of Contrast contrast-enhanced CT. After selecting proper{{Clarify|date=December 2023}} and rate of contrast from the pressure injector, the scout is taken to localize and plan the scan. Once the plan is selected, Thethe contrast is given. The raw data is processed according to the study and proper windowing is done to make scans easy to diagnose.<ref name="auto2">{{Cite book |last1=Whitley |first1=Stewart A. |url=https://books.google.com/books?id=95DUDwAAQBAJ&q=ct+scan+preparation |title=Clark's Procedures in Diagnostic Imaging: A System-Based Approach |last2=Dodgeon |first2=Jan |last3=Meadows |first3=Angela |last4=Cullingworth |first4=Jane |last5=Holmes |first5=Ken |last6=Jackson |first6=Marcus |last7=Hoadley |first7=Graham |last8=Kulshrestha |first8=Randeep |date=2020-01-06 |publisher=CRC Press |isbn=978-1-4987-1552-2 |language=en}}</ref>

# [[Assessment of kidney function|Checking of KFTkidney function]], especially [[creatinine]] and [[urea]] ratelevels (in case of [[Contrast CT|CECT]]).<ref>{{Cite journal |last1=Tippins |first1=R. B. |last2=Torres |first2=W. E. |last3=Baumgartner |first3=B. R. |last4=Baumgarten |first4=D. A. |date=August 2000 |title=Are screening serum creatinine levels necessary prior to outpatient CT examinations? |url=https://pubmed.ncbi.nlm.nih.gov/10924574/ |journal=Radiology |volume=216 |issue=2 |pages=481–484 |doi=10.1148/radiology.216.2.r00au23481 |issn=0033-8419 |pmid=10924574}}</ref>

Computed tomography operates by using an [[X-ray generator]] that rotates around the object; [[X-ray detector]]s are positioned on the opposite side of the circle from the X-ray source.<ref>{{Cite web |title=Computed Tomography (CT) |url=https://www.nibib.nih.gov/science-education/science-topics/computed-tomography-ct |access-date=2021-03-20 |website=www.nibib.nih.gov}}</ref> As the X-rays pass through the patient, they are attenuated differently by various tissues according to the tissue density.<ref>{{Cite book |last1=Aichinger |first1=Horst |url=https://books.google.com/books?id=nPisjRy4LNAC&pg=PA3 |title=Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology: Physical Principles and Clinical Applications |last2=Dierker |first2=Joachim |last3=Joite-Barfuß |first3=Sigrid |last4=Säbel |first4=Manfred |date=2011-10-25 |publisher=Springer Science & Business Media |isbn=978-3-642-11241-6 |pages=5 |language=en}}</ref> A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation.<ref>{{Cite book |last=Erdoğan |first=Hakan |url=https://books.google.com/books?id=ylcfAQAAMAAJ&q=A+set+of+many+such+projections+under+different+angles+organized+in+2D+is+called+sinogram |title=Statistical Image Reconstruction Algorithms Using Paraboloidal Surrogates for PET Transmission Scans |date=1999 |publisher=University of Michigan |isbn=978-0-599-63374-2 |language=en}}</ref> Once the scan data has been acquired, the data must be processed using a form of [[tomographic reconstruction]], which produces a series of cross-sectional images.<ref>{{Cite web |last=Themes |first=U. F. O. |date=2018-10-07 |title=CT Image Reconstruction Basics |url=https://radiologykey.com/ct-image-reconstruction-basics/ |access-date=2021-03-20 |website=Radiology Key |language=en-US}}</ref> These cross-sectional images are made up of small units of pixels or voxels.<ref name="Cardiovascular Computed Tomography">{{Cite book |last=Stirrup |first=James |url=https://books.google.com/books?id=SarDDwAAQBAJ&q=ct+images+are+made+of+pixels&pg=PA134 |title=Cardiovascular Computed Tomography |date=2020-01-02 |publisher=Oxford University Press |isbn=978-0-19-880927-2 |language=en}}</ref>

Initially, the images generated in CT scans were in the [[transverse plane|transverse]] (axial) [[anatomical plane]], perpendicular to the long axis of the body. Modern scanners allow the scan data to be reformatted as images in other [[Plane (geometry)|planes]]. [[Geometry processing|Digital geometry processing]] can generate a [[three-dimensional space|three-dimensional]] image of an object inside the body from a series of two-dimensional [[radiography|radiographic]] images taken by [[rotation around a fixed axis]].<ref name="ref1">{{Cite book |last=Hsieh |first=Jiang |url=https://books.google.com/books?id=JX__lLLXFHkC&q=ct+can+have+a+number+of+artifacts&pg=PA167 |title=Computed Tomography: Principles, Design, Artifacts, and Recent Advances |date=2003 |publisher=SPIE Press |isbn=978-0-8194-4425-7 |pages=167 |language=en}}</ref> These cross-sectional images are widely used for medical [[diagnosis]] and [[therapy]].<ref name="urlcomputed tomography – Definition from the Merriam-Webster Online Dictionary">{{Cite web |title=computed tomography – Definition from the Merriam-Webster Online Dictionary |url=http://www.merriam-webster.com/dictionary/computed+tomography |url-status=live |archive-url=https://web.archive.org/web/20110919202302/http://www.merriam-webster.com/dictionary/computed+tomography |archive-date=19 September 2011 |access-date=18 August 2009}}</ref>

[[Contrast media]] used for X-ray CT, as well as for [[radiography|plain film X-ray]], are called [[radiocontrast]]s. Radiocontrasts for CT are, in general, iodine-based.<ref>{{Cite book |last1=Webb |first1=W. Richard |url=https://books.google.com/books?id=lcjsAwAAQBAJ&pg=PA152 |title=Fundamentals of Body CT |last2=Brant |first2=William E. |last3=Major |first3=Nancy M. |date=2014 |publisher=Elsevier Health Sciences |isbn=978-0-323-26358-0 |page=152}}</ref> This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast.<ref>{{Cite book |last1=Webb |first1=Wayne Richard |url=https://books.google.com/books?id=xb-xLHTqOi0C&q=contrast+in+ct |title=Fundamentals of Body CT |last2=Brant |first2=William E. |last3=Major |first3=Nancy M. |date=2006-01-01 |publisher=Elsevier Health Sciences |isbn=978-1-4160-0030-3 |page=168 |language=en}}</ref>

The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of the [[Radon transform]].<ref name="Radon1917">{{Cite book |last1=Thomas |first1=Adrian M. K. |url=https://books.google.com/books?id=zgezC3Osm8QC&q=info:6gaBWGuVV0UJ:scholar.google.com/&pg=PA5 |title=Classic Papers in Modern Diagnostic Radiology |last2=Banerjee |first2=Arpan K. |last3=Busch |first3=Uwe |date=2005-12-05 |publisher=Springer Science & Business Media |isbn=978-3-540-26988-5 |language=en}}</ref><ref name="pmid 18244009">{{Cite journal |last=Radon J |date=1 December 1986 |title=On the determination of functions from their integral values along certain manifolds |journal=IEEE Transactions on Medical Imaging |volume=5 |issue=4 |pages=170–176 |doi=10.1109/TMI.1986.4307775 |pmid=18244009 |s2cid=26553287}}</ref> In October 1963, [[William H. Oldendorf]] received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material".<ref name="Oldendorf1978">{{Cite journal |last=Oldendorf WH |year=1978 |title=The quest for an image of brain: a brief historical and technical review of brain imaging techniques |journal=Neurology |volume=28 |issue=6 |pages=517–33 |doi=10.1212/wnl.28.6.517 |pmid=306588 |s2cid=42007208}}</ref> The first commercially viable CT scanner was invented by [[Godfrey Hounsfield]] in 1972.<ref name="Richmond2004">{{Cite journal |last=Richmond |first=Caroline |year=2004 |title=Obituary – Sir Godfrey Hounsfield |journal=BMJ |volume=329 |issue=7467 |pages=687 |doi=10.1136/bmj.329.7467.687 |pmc=517662}}</ref>

It is often claimed that revenues from the sales of The Beatles' records in the 1960s helped fund the development of the first CT scanner at EMI. The first production X-ray CT machinemachines were in fact called "EMI-Scanner" scanners.<ref>{{cite web |last1=Pietzsch |first1=Joachim |title=The Nobel Prize in Physiology or Medicine 1979 |url=https://www.nobelprize.org/prizes/medicine/1979/perspectives/ |website=NobelPrize.org |publisher=Nobelprize.org}}</ref>

The word "''tomography"'' is derived from the [[Ancient Greek|Greek]] ''{{Lang|grc-latn|tome}} 'slice' (slice) and ''{{Lang|grc-latn|graphein''}} ('to write)'.<ref>{{Cite book |last=[[Frank Natterer]] |title=The Mathematics of Computerized Tomography (Classics in Applied Mathematics) |publisher=Society for Industrial and Applied Mathematics |year=2001 |isbn=978-0-89871-493-7 |pages=8}}</ref> Computed tomography was originally known as the "EMI scan" as it was developed in the early 1970s at a research branch of [[EMI]], a company best known today for its music and recording business.<ref>{{Cite book |last=Sperry |first=Len |url=https://books.google.com/books?id=NzgVCwAAQBAJ&q=Computed+tomography+was+originally+known+as+the+%22EMI+scan%22&pg=PA259 |title=Mental Health and Mental Disorders: An Encyclopedia of Conditions, Treatments, and Well-Being [3 volumes]: An Encyclopedia of Conditions, Treatments, and Well-Being |date=2015-12-14 |publisher=ABC-CLIO |isbn=978-1-4408-0383-3 |page=259 |language=en}}</ref> It was later known as ''computed axial tomography'' (''CAT'' or ''CT scan'') and ''body section röntgenography''.<ref>{{Cite journal |last=Hounsfield |first=G. N. |date=1977 |title=The E.M.I. Scanner |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=195 |issue=1119 |pages=281–289 |bibcode=1977RSPSB.195..281H |doi=10.1098/rspb.1977.0008 |issn=0080-4649 |jstor=77187 |pmid=13396 |s2cid=34734270}}</ref>

The term "''CAT scan"'' is no longer usedin technical use because current CT scans enable for multiplanar reconstructions. This makes "''CT scan"'' the most appropriate term, which is used by [[radiologist]]s in common vernacular as well as in textbooks and scientific papers.<ref>{{Cite web |last=Miñano |first=Glenn |title=What's the difference between a CAT-Scan and a CT-Scan? - Cincinnati Children's Blog |url=https://blog.cincinnatichildrens.org//radiology/whats-the-difference-between-a-cat-scan-and-a-ct-scan |access-date=2021-03-19 |website=blog.cincinnatichildrens.org |date=3 November 2015 |languagearchive-date=en2022-06-17 |archive-url=https://web.archive.org/web/20220617035344/https://blog.cincinnatichildrens.org/radiology/whats-the-difference-between-a-cat-scan-and-a-ct-scan/ |url-status=dead }}</ref><ref>{{Cite web |title=Difference Between CT Scan and CAT Scan {{!}} Difference Between |date=28 January 2010 |url=http://www.differencebetween.net/science/health/difference-between-ct-scan-and-cat-scan/ |access-date=2021-03-19 |language=en-US}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=FqDUtcmUG-UC&q=cat+scanner+term+was+used+earlier |title=Conquer Your Headaches |publisher=International Headache Management |year=1994 |isbn=978-0-9636292-5-8 |pages=115}}</ref>

In [[Medical Subject Headings]] (MeSH), "''computed axial tomography"'' was used from 1977 to 1979, but the current indexing explicitly includes "''X-ray"'' in the title.<ref>{{Cite web |title=MeSH Browser |url=https://meshb.nlm.nih.gov/record/ui?ui=D014057 |website=meshb.nlm.nih.gov}}</ref>

The term [[wikt:sinogram|''sinogram'']] was introduced by Paul Edholm and Bertil Jacobson in 1975.<ref>{{Cite journal |last1=Edholm |first1=Paul |last2=Gabor |first2=Herman |date=December 1987 |title=Linograms in Image Reconstruction from Projections |journal=IEEE Transactions on Medical Imaging |volume=MI-6 |issue=4 |pages=301–7 |doi=10.1109/tmi.1987.4307847 |pmid=18244038 |s2cid=20832295}}</ref>

Use of CT has increased dramatically over the last two decades.<ref name="Smith2009" /> An estimated 72&nbsp;million scans were performed in the United States in 2007,<ref name="Berrington2009" /> accounting for close to half of the total per-capita dose rate from radiologic and nuclear medicine procedures.<ref>{{Cite journal |last1=Fred A. Mettler Jr |last2=Mythreyi Bhargavan |last3=Keith Faulkner |last4=Debbie B. Gilley |last5=Joel E. Gray |last6=Geoffrey S. Ibbott |last7=Jill A. Lipoti |last8=Mahadevappa Mahesh |last9=John L. McCrohan |last10=Michael G. Stabin |last11=Bruce R. Thomadsen |last12=Terry T. Yoshizumi |year=2009 |title=Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison with Other Radiation Sources — 1950-2007 |journal=Radiology |volume=253 |pages=520–531 |doi=10.1148/radiol.2532082010 |pmid=19789227 |number=2}}</ref> Of the CT scans, six to eleven percent are done in children,<ref name="Risk2011" /> an increase of seven to eightfold from 1980.<ref name="Furlow2010" /> Similar increases have been seen in Europe and Asia.<ref name="Furlow2010" /> In Calgary, Canada, 12.1% of people who present to the emergency with an urgent complaint received a CT scan, most commonly either of the head or of the abdomen. The percentage who received CT, however, varied markedly by the [[emergency physician]] who saw them from 1.8% to 25%.<ref>{{Cite journal |last=Andrew Skelly |date=Aug 3, 2010 |title=CT ordering all over the map |journal=The Medical Post}}</ref> In the emergency department in the United States, CT or [[Magnetic resonance imaging|MRI]] imaging is done in 15% of people who present with [[injuries]] as of 2007 (up from 6% in 1998).<ref>{{Cite journal |vauthors=Korley FK, Pham JC, Kirsch TD |date=October 2010 |title=Use of advanced radiology during visits to US emergency departments for injury-related conditions, 1998–2007 |journal=JAMA |volume=304 |issue=13 |pages=1465–71 |doi=10.1001/jama.2010.1408 |pmid=20924012 |doi-access=free}}</ref>

The increased use of CT scans has been the greatest in two fields: screening of adults (screening CT of the lung in smokers, virtual colonoscopy, CT cardiac screening, and whole-body CT in asymptomatic patients) and CT imaging of children. Shortening of the scanning time to around 1 second, eliminating the strict need for the subject to remain still or be sedated, is one of the main reasons for the large increase in the pediatric population (especially for the diagnosis of [[appendicitis]]).<ref name="Brenner2007" /> As of 2007, in the United States a proportion of CT scans are performed unnecessarily.<ref name="Semelka2007">{{Cite journal |vauthors=Semelka RC, Armao DM, Elias J, Huda W |date=May 2007 |title=Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI |journal=J Magn Reson Imaging |volume=25 |issue=5 |pages=900–9 |doi=10.1002/jmri.20895 |pmid=17457809 |s2cid=5788891|doi-access=free }}</ref> Some estimates place this number at 30%.<ref name="Risk2011" /> There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public.<ref name="Semelka2007" /> For example, some healthy people avidly pay to receive full-body CT scans as [[screening (medicine)|screening]]. In that case, it is not at all clear that the benefits outweigh the risks and costs. Deciding whether and how to treat [[incidentaloma]]s is complex, radiation exposure is not negligible, and the money for the scans involves [[opportunity cost]].<ref name="Semelka2007" />

Major manufacturers of CT Scannersscanning Devicesdevices and Equipmentequipment are:<ref>{{Cite web |date=November 7, 2019 |title=Global Computed Tomography (CT) Scanners Devices and Equipment Market Report 2020: Major Players are GE Healthcare, Koninklijke Philips, Hitachi, Siemens and Canon Medical Systems – ResearchAndMarkets.com |url=https://www.businesswire.com/news/home/20191107005949/en/Global-Computed-Tomography-CT-Scanners-Devices-Equipment#:~:text=Major%20players%20in%20the%20market,and%20Canon%20Medical%20Systems%20Corporation. |publisher=Business Wire}}</ref>

*{{flagicon|USA}} [[GE Healthcare|GE HealthCare]]

*{{flagicon|China}} [[United Imaging Healthcare]]

[[Photon-counting computed tomography]] is a CT technique currently under development.{{As of?|date=December 2023}} Typical CT scanners use energy integrating detectors; photons are measured as a voltage on a capacitor which is proportional to the xX-rays detected. However, this technique is susceptible to noise and other factors which can affect the linearity of the voltage to xX-ray intensity relationship.<ref>{{Cite book |last1=Jenkins |first1=Ron |url=https://archive.org/details/quantitativexray00jenk |title=Quantitative x-ray spectrometry |last2=Gould |first2=R W |last3=Gedcke |first3=Dale |date=1995 |publisher=Dekker |isbn=978-0-8247-9554-2 |edition=2nd |location=New York |page=[https://archive.org/details/quantitativexray00jenk/page/n89 90] |chapter=Instrumentation |url-access=limited}}</ref> Photon counting detectors (PCDs) are still affected by noise but it does not change the measured counts of photons. PCDs have several potential advantages, including improving signal (and contrast) to noise ratios, reducing doses, improving spatial resolution, and through use of several energies, distinguishing multiple contrast agents.<ref>{{Cite journal |last1=Shikhaliev |first1=Polad M. |last2=Xu |first2=Tong |last3=Molloi |first3=Sabee |date=2005 |title=Photon counting computed tomography: Concept and initial results |journal=Medical Physics |volume=32 |issue=2 |pages=427–36 |bibcode=2005MedPh..32..427S |doi=10.1118/1.1854779 |pmid=15789589}}</ref><ref>{{Cite journal |last1=Taguchi |first1=Katsuyuki |last2=Iwanczyk |first2=Jan S. |date=2013 |title=Vision 20/20: Single photon counting x-ray detectors in medical imaging |journal=Medical Physics |volume=40 |issue=10 |pages=100901 |bibcode=2013MedPh..40j0901T |doi=10.1118/1.4820371 |pmc=3786515 |pmid=24089889}}</ref> PCDs have only recently become feasible in CT scanners due to improvements in detector technologies that can cope with the volume and rate of data required. As of February 2016, photon counting CT is in use at three sites.<ref>{{Cite web |date=24 February 2016 |title=NIH uses photon-counting CT scanner in patients for the first time |url=https://www.nih.gov/news-events/news-releases/nih-uses-photon-counting-ct-scanner-patients-first-time |url-status=live |archive-url=https://web.archive.org/web/20160818032611/https://www.nih.gov/news-events/news-releases/nih-uses-photon-counting-ct-scanner-patients-first-time |archive-date=18 August 2016 |access-date=28 July 2016 |website=National Institutes of Health}}</ref> Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising.<ref>{{Cite web |title=Photon-counting breast CT measures up |url=http://medicalphysicsweb.org/cws/article/research/65633 |url-status=dead |archive-url=https://web.archive.org/web/20160727052920/http://medicalphysicsweb.org/cws/article/research/65633 |archive-date=2016-07-27 |access-date=28 July 2016 |website=medicalphysicsweb}}</ref> In view of recent findings of high cumulative doses to patients from recurrent CT scans, there has been a push for scanning technologies and techniques that reduce ionising radiation doses to patients to sub-[[Sievert|milliSievert]] (sub-mSv in the literature) levels during the CT scan process, a goal that has been lingering.<ref>{{Cite journal |last1=Kachelrieß |first1=Marc |last2=Rehani |first2=Madan M. |date=March 1, 2020 |title=Is it possible to kill the radiation risk issue in computed tomography? |url=https://www.physicamedica.com/article/S1120-1797(20)30051-X/abstract |journal=Physica Medica: European Journal of Medical Physics |volume=71 |pages=176–177 |doi=10.1016/j.ejmp.2020.02.017 |pmid=32163886 |via=www.physicamedica.com |s2cid=212692606}}</ref><ref name="Patients undergoing recurrent CT sc" /><ref name="Multinational data on cumulative ra" /><ref name="Patients undergoing recurrent CT ex" />

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* {{Cite journal |last=Filler |first=Aaron |date=2009-06-30 |title=The History, Development and Impact of Computed Imaging in Neurological Diagnosis and Neurosurgery: CT, MRI, and DTI |url=https://www.nature.com/articles/npre.2009.3267.3 |journal=Nature Precedings |language=en |pages=1 |doi=10.1038/npre.2009.3267.34 |issn=1756-0357 |doi-access=free}}

* {{Cite journal |last1=Boone |first1=John M. |last2=McCollough |first2=Cynthia H. |year=2021 |title=Computed tomography turns 50 |url=https://physicstoday.scitation.org/doi/full/10.1063/PT.3.4834 |journal=Physics Today |volume=74 |issue=9 |pages=34–40 |bibcode=2021PhT....74i..34B |doi=10.1063/PT.3.4834 |issn=0031-9228 |s2cid=239718717 |doi-access=free}}