Editing CT scan (section)

== Adverse effects ==

=== Cancer ===
{{Main|Radiation-induced cancer}}
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 X-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 X-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 X-ray techniques (chest X-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 |last1=Radiological Society of North America |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 Australian cohort it was found that up to 3.7% 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 }}</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 one to five 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 |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}}</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 |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}}</ref>
One study estimated that as many as 0.4% of cancers in the United States resulted from CT scans, and that this may have increased to as much as 1.5 to 2% based on the rate of CT use in 2007.<ref name="Brenner2007" /> Others dispute this estimate,<ref>{{Cite journal |last1=Kalra |first1=Mannudeep K. |last2=Maher |first2=Michael M. |last3=Rizzo |first3=Stefania |last4=Kanarek |first4=David |last5=Shephard |first5=Jo-Anne O. |date=April 2004 |title=Radiation exposure from Chest CT: Issues and Strategies |journal=Journal of Korean Medical Science |volume=19 |issue=2 |pages=159–166 |doi=10.3346/jkms.2004.19.2.159 |issn=1011-8934 |pmc=2822293 |pmid=15082885}}</ref> as there is no consensus that the low levels of radiation used in CT scans cause damage. Lower radiation doses are used in many cases, such as in the investigation of renal colic.<ref>{{Cite journal |last1=Rob |first1=S. |last2=Bryant |first2=T. |last3=Wilson |first3=I. |last4=Somani |first4=B.K. |year=2017 |title=Ultra-low-dose, low-dose, and standard-dose CT of the kidney, ureters, and bladder: is there a difference? Results from a systematic review of the literature |journal=Clinical Radiology |volume=72 |issue=1 |pages=11–15 |doi=10.1016/j.crad.2016.10.005 |pmid=27810168}}</ref>

<!--Effect of age  -->
A person's age plays a significant role in the subsequent risk of cancer.<ref name="Furlow2010" /> Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1%, or 1:1000 scans.<ref name="Furlow2010" /> The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly.<ref name="Furlow2010" /> The [[International Commission on Radiological Protection]] estimates that the risk to a fetus being exposed to 10 [[mGy]] (a unit of radiation exposure) increases the rate of cancer before 20 years of age from 0.03% to 0.04% (for reference a CT pulmonary angiogram exposes a fetus to 4&nbsp;mGy).<ref name="Risk2011" /> A 2012 review did not find an association between medical radiation and cancer risk in children noting however the existence of limitations in the evidences over which the review is based.<ref>{{Cite journal |vauthors=Baysson H, Etard C, Brisse HJ, Bernier MO |date=January 2012 |title=[Diagnostic radiation exposure in children and cancer risk: current knowledge and perspectives] |journal=Archives de Pédiatrie |volume=19 |issue=1 |pages=64–73 |doi=10.1016/j.arcped.2011.10.023 |pmid=22130615}}</ref> CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of 2007 having this function built in.<ref name="Semelka2007" /> Furthermore, certain conditions can require children to be exposed to multiple CT scans.<ref name="Brenner2007" /> 

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 |last1=Emmerson |first1=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>

=== Contrast reactions ===
{{Further|Iodinated contrast#Adverse effects}}
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>
There is a higher risk of mortality in those who are female, elderly or in poor health, usually secondary to either anaphylaxis or [[acute kidney injury]].<ref name="Nam2006">{{Cite journal |vauthors=Namasivayam S, Kalra MK, Torres WE, Small WC |date=Jul 2006 |title=Adverse reactions to intravenous iodinated contrast media: a primer for radiologists |journal=Emergency Radiology |volume=12 |issue=5 |pages=210–5 |doi=10.1007/s10140-006-0488-6 |pmid=16688432 |s2cid=28223134}}</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}}</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" />

=== Scan dose ===
{|  class="sortable wikitable" style="float: right; margin-left:15px; text-align:center"
|-
!Examination
!Typical [[Effective dose (radiation safety)|effective <br /> dose]] ([[Sievert|mSv]])<br /> to the whole body
!Typical [[Absorbed dose|absorbed <br /> dose]] ([[Gray (unit)|mGy]])<br /> to the organ in question
|-
|Annual background radiation
|2.4<ref name="background" />
|2.4<ref name="background" />
|-
|Chest X-ray
|0.02<ref name="FDADose">{{Cite web |year=2009 |title=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=2013-11-05 |website=Food and Drug Administration}}</ref>
|0.01–0.15<ref name="crfdr" />
|-
|Head CT
|1–2<ref name="Furlow2010" />
|56<ref name="nrpb2005">Shrimpton, P.C; Miller, H.C; Lewis, M.A; Dunn, M.  [http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947420292 Doses from Computed Tomography (CT) examinations in the UK – 2003 Review] {{webarchive|url=https://web.archive.org/web/20110922122151/http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947420292 |date=2011-09-22 }}</ref>
|-
|Screening [[mammography]]
|0.4<ref name="Risk2011">{{Cite journal |last1=Davies |first1=H. E. |last2=Wathen, C. G. |last3=Gleeson, F. V. |date=25 February 2011 |title=The risks of radiation exposure related to diagnostic imaging and how to minimise them |journal=BMJ |volume=342 |issue=feb25 1 |pages=d947 |doi=10.1136/bmj.d947 |pmid=21355025 |s2cid=206894472}}</ref>
|3<ref name="Brenner2007" /><ref name="crfdr" />
|-
|Abdominal CT
|8<ref name="FDADose" />
|14<ref name="nrpb2005" />
|-
|Chest CT
|5–7<ref name="Furlow2010" />
|13<ref name="nrpb2005" />
|-
|[[Virtual colonoscopy|CT colonography]]
|6–11<ref name="Furlow2010" />
|
|-
|Chest, abdomen and pelvis CT
|9.9<ref name="nrpb2005" />
|12<ref name="nrpb2005" />
|-
|Cardiac CT angiogram
|9–12<ref name="Furlow2010" />
|40–100<ref name="crfdr" />
|-
|[[Barium enema]]
|15<ref name="Brenner2007" />
|15<ref name="crfdr" />
|-
|Neonatal abdominal CT
|20<ref name="Brenner2007" />
|20<ref name="crfdr" />
|-
|colspan=3| {{Further|Template:Effective dose by medical imaging type}}
|}

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>

For purposes of comparison, the world average dose rate from naturally occurring sources of [[background radiation]] is 2.4&nbsp;[[mSv]] per year, equal for practical purposes in this application to 2.4&nbsp;mGy per year.<ref name="background">{{Cite journal |vauthors=Cuttler JM, Pollycove M |year=2009 |title=Nuclear energy and health: and the benefits of low-dose radiation hormesis |journal=Dose-Response |volume=7 |issue=1 |pages=52–89 |doi=10.2203/dose-response.08-024.Cuttler |pmc=2664640 |pmid=19343116}}</ref> While there is some variation, most people (99%) received less than 7&nbsp;mSv per year as background radiation.<ref>{{Cite book |url=https://books.google.com/books?id=qCebxPjdSBUC&pg=PA164 |title=A half century of health physics |publisher=Lippincott Williams & Wilkins |year=2005 |isbn=978-0-7817-6934-1 |editor-last=Michael T. Ryan |location=Baltimore, Md. |page=164 |editor-last2=Poston, John W.}}</ref> Medical imaging as of 2007 accounted for half of the radiation exposure of those in the United States with CT scans making up two thirds of this amount.<ref name="Furlow2010" /> In the United Kingdom it accounts for 15% of radiation exposure.<ref name="Risk2011" /> The average radiation dose from medical sources is ≈0.6&nbsp;mSv per person globally as of 2007.<ref name="Furlow2010" /> Those in the nuclear industry in the United States are limited to doses of 50&nbsp;mSv a year and 100&nbsp;mSv every 5 years.<ref name="Furlow2010" />

[[Lead]] is the main material used by radiography personnel for [[shielding (radiography)|shielding]] against scattered X-rays.

==== Radiation dose units ====
The radiation dose reported in the [[Gray (unit)|gray or mGy]] unit is proportional to the amount of energy that the irradiated body part is expected to absorb, and the physical effect (such as DNA [[double strand breaks]]) on the cells' chemical bonds by X-ray radiation is proportional to that energy.<ref>{{Cite journal |vauthors=Polo SE, Jackson SP |date=March 2011 |title=Dynamics of DNA damage response proteins at DNA breaks: a focus on protein modifications |journal=Genes Dev. |volume=25 |issue=5 |pages=409–33 |doi=10.1101/gad.2021311 |pmc=3049283 |pmid=21363960}}</ref>

The [[sievert]] unit is used in the report of the [[effective dose (radiation)|effective dose]]. The sievert unit, in the context of CT scans, does not correspond to the actual radiation dose that the scanned body part absorbs but to another radiation dose of another scenario, the whole body absorbing the other radiation dose and the other radiation dose being of a magnitude, estimated to have the same probability to induce cancer as the CT scan.<ref>[http://www.aapm.org/pubs/reports/RPT_96.pdf The Measurement, Reporting, and Management of Radiation Dose in CT] {{webarchive|url=https://web.archive.org/web/20170623014823/https://www.aapm.org/pubs/reports/rpt_96.pdf |date=2017-06-23 }} "It is a single dose parameter that reflects the risk of a nonuniform exposure in terms of an equivalent whole-body exposure."</ref> Thus, as is shown in the table above, the actual radiation that is absorbed by a scanned body part is often much larger than the effective dose suggests. A specific measure, termed the [[computed tomography dose index]] (CTDI), is commonly used as an estimate of the radiation absorbed dose for tissue within the scan region, and is automatically computed by medical CT scanners.<ref>{{Cite journal |vauthors=Hill B, Venning AJ, Baldock C |year=2005 |title=A preliminary study of the novel application of normoxic polymer gel dosimeters for the measurement of CTDI on diagnostic X-ray CT scanners |journal=Medical Physics |volume=32 |issue=6 |pages=1589–1597 |bibcode=2005MedPh..32.1589H |doi=10.1118/1.1925181 |pmid=16013718}}</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 |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)}}</ref>

==== Effects of radiation ====
{{Further|Radiobiology}}
Most adverse health effects of radiation exposure may be grouped in two general categories:
*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}}</ref>
*stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells.<ref>Paragraph 55 in: {{Cite web |title=The 2007 Recommendations of the International Commission on Radiological Protection |url=http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103 |url-status=live |archive-url=https://web.archive.org/web/20121116084754/http://www.icrp.org/publication.asp?id=ICRP+Publication+103 |archive-date=2012-11-16 |website=[[International Commission on Radiological Protection]]}} Ann. ICRP 37 (2-4)</ref>

The added lifetime risk of developing cancer by a single abdominal CT of 8 mSv is estimated to be 0.05%, or 1 one in 2,000.<ref>{{Cite web |date=March 2013 |title=Do CT scans cause cancer? |url=https://www.health.harvard.edu/staying-healthy/do-ct-scans-cause-cancer |url-status=dead |archive-url=https://web.archive.org/web/20171209152338/https://www.health.harvard.edu/staying-healthy/do-ct-scans-cause-cancer |archive-date=2017-12-09 |access-date=2017-12-09 |website=[[Harvard Medical School]]}}</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-US}}</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>

==== Excess doses ====
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 200 patients were exposed to radiation at approximately eight times the expected dose for an 18-month period; over 40% of them lost patches of hair. This event prompted a 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}}</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" />