Editing CT scan (section)

== Mechanism ==
[[File:ct-internals.jpg|thumb|right|CT scanner with cover removed to show internal components. Legend:
<br />T: X-ray tube
<br />D: X-ray detectors
<br />X: X-ray beam
<br />R: Gantry rotation]]
[[File:Sinogram and sample image of computed tomography of the jaw.jpg|thumb|Left image is a ''sinogram'' which is a graphic representation of the raw data obtained from a CT scan. At right is an image sample derived from the raw data.<ref>{{Cite journal |last1=Jun |first1=Kyungtaek |last2=Yoon |first2=Seokhwan |year=2017 |title=Alignment Solution for CT Image Reconstruction using Fixed Point and Virtual Rotation Axis |journal=Scientific Reports |volume=7 |pages=41218 |arxiv=1605.04833 |bibcode=2017NatSR...741218J |doi=10.1038/srep41218 |issn=2045-2322 |pmc=5264594 |pmid=28120881}}</ref>]]
{{Main|Operation of computed tomography}}
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}}</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}}</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}}</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 name="Cardiovascular Computed Tomography" />

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 or more (os temporale) and can cause [[artifact (error)#Medical imaging|artifacts]]. 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 extinguish 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. 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">[http://fitsweb.uchc.edu/ctanatomy/extrem/index.html Computerized Tomography chapter] {{webarchive|url=https://web.archive.org/web/20160304001946/http://fitsweb.uchc.edu/ctanatomy/extrem/index.html |date=2016-03-04 }} at [[University of Connecticut Health Center]].</ref> 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.

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}}</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 ===
{{Main|Contrast CT}}
[[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}}</ref>