Editing CT scan (section)

==== 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>