Editing CT scan (section)

==== Artifacts ====
Although images produced by CT are generally faithful representations of the scanned volume, the technique is susceptible to a number of [[artifact (error)#Medical imaging|artifacts]], such as the following:<ref name="ref1" /><ref>{{Cite journal |last1=Bhowmik |first1=Ujjal Kumar |last2=Zafar Iqbal, M. |last3=Adhami, Reza R. |date=28 May 2012 |title=Mitigating motion artifacts in FDK based 3D Cone-beam Brain Imaging System using markers |journal=Central European Journal of Engineering |volume=2 |issue=3 |pages=369–382 |bibcode=2012CEJE....2..369B |doi=10.2478/s13531-012-0011-7 |doi-access=free}}</ref><sup>Chapters 3 and 5</sup>

;{{Visible anchor|Streak artifact}}: Streaks are often seen around materials that block most X-rays, such as metal or bone. Numerous factors contribute to these streaks: under sampling, photon starvation, motion, beam hardening, and [[Compton scatter]]. This type of artifact commonly occurs in the posterior fossa of the brain, or if there are metal implants. The streaks can be reduced using newer reconstruction techniques.<ref name="P. Jin and C. A. Bouman and K. D. Sauer 2013">{{Cite journal |last1=P. Jin |last2=C. A. Bouman |last3=K. D. Sauer |year=2013 |title=A Method for Simultaneous Image Reconstruction and Beam Hardening Correction |url=https://engineering.purdue.edu/~bouman/publications/pdf/mic2013.pdf |url-status=dead |journal=IEEE Nuclear Science Symp. & Medical Imaging Conf., Seoul, Korea, 2013 |archive-url=https://web.archive.org/web/20140606234132/https://engineering.purdue.edu/~bouman/publications/pdf/mic2013.pdf |archive-date=2014-06-06 |access-date=2014-04-23}}</ref> Approaches such as metal artifact reduction (MAR) can also reduce this artifact.<ref>{{Cite journal |vauthors=Boas FE, Fleischmann D |year=2011 |title=Evaluation of Two Iterative Techniques for Reducing Metal Artifacts in Computed Tomography |journal=Radiology |volume=259 |issue=3 |pages=894–902 |doi=10.1148/radiol.11101782 |pmid=21357521}}</ref><ref name="mouton13survey">{{Cite journal |last1=Mouton, A. |last2=Megherbi, N. |last3=Van Slambrouck, K. |last4=Nuyts, J. |last5=Breckon, T.P. |year=2013 |title=An Experimental Survey of Metal Artefact Reduction in Computed Tomography |url=http://www.durham.ac.uk/toby.breckon/publications/papers/mouton13survey.pdf |journal=Journal of X-Ray Science and Technology |volume=21 |issue=2 |pages=193–226 |doi=10.3233/XST-130372 |pmid=23694911 |hdl=1826/8204 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> MAR techniques include spectral imaging, where CT images are taken with [[photons]] of different energy levels, and then synthesized into [[monochromatic]] images with special software such as GSI (Gemstone Spectral Imaging).<ref name="PessisCampagna2013">{{Cite journal |last1=Pessis |first1=Eric |last2=Campagna |first2=Raphaël |last3=Sverzut |first3=Jean-Michel |last4=Bach |first4=Fabienne |last5=Rodallec |first5=Mathieu |last6=Guerini |first6=Henri |last7=Feydy |first7=Antoine |last8=Drapé |first8=Jean-Luc |year=2013 |title=Virtual Monochromatic Spectral Imaging with Fast Kilovoltage Switching: Reduction of Metal Artifacts at CT |journal=RadioGraphics |volume=33 |issue=2 |pages=573–583 |doi=10.1148/rg.332125124 |issn=0271-5333 |pmid=23479714 |doi-access=free}}</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 |pages=185–194 |doi=10.1016/B978-012077790-7/50016-3 |isbn=978-0-12-077790-7}}</ref>

;Ring artifact: Probably the most common mechanical artifact, the image of one or many "rings" appears within an image. They are usually caused by the variations in the response from individual elements in a two dimensional X-ray detector due to defect or miscalibration.<ref name="Jha">{{Cite journal |last=Jha |first=Diwaker |date=2014 |title=Adaptive center determination for effective suppression of ring artifacts in tomography images |journal=Applied Physics Letters |volume=105 |issue=14 |pages=143107 |bibcode=2014ApPhL.105n3107J |doi=10.1063/1.4897441}}</ref> Ring artifacts can largely be reduced by intensity normalization, also referred to as flat field correction.<ref name="vvn15">{{Cite journal |last1=Van Nieuwenhove |first1=V |last2=De Beenhouwer |first2=J |last3=De Carlo |first3=F |last4=Mancini |first4=L |last5=Marone |first5=F |last6=Sijbers |first6=J |date=2015 |title=Dynamic intensity normalization using eigen flat fields in X-ray imaging |url=http://www.zora.uzh.ch/id/eprint/120683/1/oe-23-21-27975.pdf |journal=Optics Express |volume=23 |issue=21 |pages=27975–27989 |bibcode=2015OExpr..2327975V |doi=10.1364/oe.23.027975 |pmid=26480456 |doi-access=free |hdl=10067/1302930151162165141}}</ref> Remaining rings can be suppressed by a transformation to polar space, where they become linear stripes.<ref name="Jha" /> A comparative evaluation of ring artefact reduction on X-ray tomography images showed that the method of Sijbers and Postnov can effectively suppress ring artefacts.<ref name="jsap">{{Cite journal |vauthors=Sijbers J, Postnov A |date=2004 |title=Reduction of ring artefacts in high resolution micro-CT reconstructions |journal=Phys Med Biol |volume=49 |issue=14 |pages=N247–53 |doi=10.1088/0031-9155/49/14/N06 |pmid=15357205 |s2cid=12744174}}</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}}</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}}</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}}</ref>

;Beam hardening: This can give a "cupped appearance" when grayscale is visualized as height. It occurs because conventional sources, like X-ray tubes emit a polychromatic spectrum. Photons of higher [[photon energy]] levels are typically attenuated less. Because of this, the mean energy of the spectrum increases when passing the object, often described as getting "harder". This leads to an effect increasingly underestimating material thickness, if not corrected. Many algorithms exist to correct for this artifact. They can be divided into mono- and multi-material methods.<ref name="P. Jin and C. A. Bouman and K. D. Sauer 2013" /><ref>{{Cite journal |vauthors=Van de Casteele E, Van Dyck D, Sijbers J, Raman E |year=2004 |title=A model-based correction method for beam hardening artefacts in X-ray microtomography |journal=Journal of X-ray Science and Technology |volume=12 |issue=1 |pages=43–57 |citeseerx=10.1.1.460.6487}}</ref><ref>{{Cite journal |vauthors=Van Gompel G, Van Slambrouck K, Defrise M, Batenburg KJ, Sijbers J, Nuyts J |year=2011 |title=Iterative correction of beam hardening artifacts in CT |journal=Medical Physics |volume=38 |issue=1 |pages=36–49 |bibcode=2011MedPh..38S..36V |citeseerx=10.1.1.464.3547 |doi=10.1118/1.3577758 |pmid=21978116}}</ref>