# CT scan **A CT scan** (computed tomography scan), earlier called a CAT scan (computed axial tomography scan), is a medical imaging technique that builds a cross-sectional image of the inside of the body from many X-ray measurements taken around it. A rotating X-ray tube and a ring of detectors, mounted opposite each other in a gantry, measure how much tissue along many different lines through the body has weakened, or attenuated, the beam; a computer combines those measurements into a picture of a single slice, and many slices stack into a three-dimensional volume, a reconstruction problem that is itself an exercise in [[Signal_processing|signal processing]]. Unlike [[Magnetic_resonance_imaging|magnetic resonance imaging]], CT uses ionizing radiation and remains usable in patients with pacemakers or other metal implants that would make an MRI scan unsafe or unreadable. The primary microsim on this page, *Computed axial tomography*, reconstructs a cross-section from a set of simulated projections the way a real scanner does: it builds a sinogram from the projections at each angle, then reverses it by filtered back-projection, and lets the reader watch streak artifacts appear as the number of angles is reduced and vanish again as the projection count rises. ## Types ### Sequential CT Sequential, or "step-and-shoot", CT acquires one slice at a time: the gantry rotates through a single sweep while the table is stationary, the table then advances to the next position, and the process repeats. It is the oldest acquisition scheme and remains simple and precise for a single slice, at the cost of a pause between slices that adds to the total scan time. ### Spiral CT Spiral, or helical, CT instead rotates the X-ray tube continuously while the table moves through the gantry at a steady speed, so the beam traces a helix around the body rather than a stack of separate circles. Because the data never stop coming in, spiral scanning is far faster than sequential scanning for covering a whole body region, and made routine whole-organ scanning in a single held breath practical. ### Electron beam tomography Electron beam tomography dispenses with a rotating tube entirely: an [[Electron|electron]] beam is swept magnetically around a stationary ring of tungsten targets surrounding the patient, generating X-rays from a different point on the ring in rapid succession. Without a heavy tube assembly to physically rotate, it can acquire images fast enough to freeze cardiac motion, though the specialized hardware it requires has kept it far less common than tube-based scanners. ### Dual energy CT Dual energy CT acquires the same region at two different X-ray tube voltages, either from two tube-detector pairs at once or by rapidly switching a single tube's voltage. Because different materials attenuate X-rays differently depending on photon energy, comparing the two acquisitions lets software distinguish materials of similar density that look identical on a single-energy scan, such as iodine contrast from bone, or calcium from uric acid crystals. ### CT perfusion imaging CT perfusion imaging repeats scans of the same slice rapidly while a contrast bolus passes through the tissue, tracking how quickly the contrast arrives and washes out to map blood flow, blood volume and transit time; it is used to distinguish tissue that is already dead from tissue that is merely at risk, most often after a suspected stroke. ### PET CT PET CT combines a computed tomography scanner with a [[Positron_emission|positron emission]] tomography scanner in a single gantry, so a metabolic image showing where a radioactive tracer concentrates is captured in the same session and automatically aligned with the anatomical detail of the CT image, each modality filling in what the other cannot show on its own. ## Medical use ### Head Head CT is the fastest widely available way to rule out or confirm bleeding, swelling or a large stroke after a head injury or a sudden neurological change, and its speed and wide availability make it the usual first imaging study in an emergency department even where MRI would show soft tissue in more detail. ### Neck Neck CT images the airway, the major vessels and the lymph nodes of the neck, commonly used to stage a suspected cancer of the throat or thyroid or to plan surgery around vessels and nerves that are difficult to assess from the outside. ### Lungs Chest CT resolves lung detail far beyond a plain chest X-ray, and a specific protocol, CT pulmonary angiography, is the standard way to look directly for a blood clot in the pulmonary arteries. ### Angiography CT angiography times a contrast injection to a scan so the contrast-filled vessels stand out sharply against surrounding tissue, mapping the arteries or veins of almost any body region without the catheter that older, direct angiography required. ### Cardiac Cardiac CT images the coronary arteries and the heart's chambers, synchronized to the electrocardiogram so a single phase of the heartbeat can be reconstructed as if the heart had held still, and is used both to look for narrowed coronary arteries and to plan procedures such as valve replacement. ### Abdomen and pelvis Abdominal and pelvic CT is the workhorse study for unexplained pain, suspected appendicitis, kidney stones and staging of many abdominal and pelvic cancers, generally read together with an oral or intravenous contrast agent that outlines the bowel and blood vessels. ### Axial skeleton and extremities CT resolves fracture detail, particularly around complex joints and the spine, better than a plain radiograph, and is often used to plan surgery around a fracture whose exact shape is hard to judge from two-dimensional X-rays alone. ### Biomechanical use Outside direct patient care, CT scans of bone are also used to build patient-specific biomechanical models for surgical planning and implant design, a task of [[Biomedical_engineering|biomedical engineering]], since the scan captures the exact geometry and, through its attenuation values, an estimate of local bone density. ## Other uses ### Industrial use Industrial CT scans manufactured parts for internal defects — voids, cracks, inclusions — that no external inspection could find, and can compare a scanned part directly against its original [[Engineering_drawing|engineering drawing]] or design model to check dimensional accuracy throughout its interior as well as its surface. ### Aviation security Some airport security checkpoints use CT scanners to image the contents of carry-on baggage in three dimensions, rather than the two-dimensional projection of a conventional X-ray scanner, making it easier for an operator or automated software to distinguish an innocuous object from a similarly shaped threat without the bag being opened. ### Geological use Geologists use CT to look inside a rock core or a sediment sample without cutting it open, mapping internal layering, porosity and mineral [[Density|density]] variations that would otherwise only be visible by destroying the sample. ### Paleontological use Paleontologists CT scan fossils still embedded in rock to see internal structure, such as a skull's braincase or a bone's growth rings, without the risk of damage that physically preparing the fossil out of its surrounding matrix would carry. ### Cultural heritage use Museums and conservators use CT to look inside artifacts too fragile or valuable to open, from a mummified body to a sealed manuscript or a musical instrument, revealing construction details and hidden damage without touching the object itself, sometimes as the basis for a [[3D_printing|3D-printed]] replica. ### Microorganism research Researchers have also used high-resolution CT to image the internal anatomy of small organisms, building three-dimensional models of internal structures too delicate to dissect at that scale. ### Timber sawmill Some sawmills scan whole logs by CT before cutting them, mapping internal knots, rot and other defects so a log can be cut to maximize the yield of usable, defect-free lumber. ## Interpretation of results ### Presentation A CT scan's raw output is a three-dimensional grid of attenuation values, and everything a reader sees afterward is a chosen way of presenting that grid, a task of [[Digital_image_processing|digital image processing]]. #### Grayscale Each value in the reconstructed grid is mapped to a shade of gray, conventionally shown so denser tissue that attenuates more X-rays appears brighter and less dense tissue appears darker, the same convention a plain X-ray film uses. #### Windowing Because the reconstructed values span a far wider range than a monitor's few hundred shades of gray can show at once, a reader chooses a window: a center value and a width, so only the range of interest — lung, bone or soft tissue, each attenuating very differently — is stretched across the visible grayscale while everything outside it is shown as flat black or white. #### Multiplanar reconstruction and projections Because the underlying data is a three-dimensional grid rather than a fixed stack of slices, software can resample it along any plane, not only the one the scanner originally acquired, producing coronal or sagittal views, or a thin oblique slice following a curved structure such as a blood vessel, from a single acquisition. #### Volume rendering Volume rendering instead treats the whole grid at once, assigning each value both a color and a degree of transparency, often computed on a [[Graphics_processing_unit|graphics processing unit]], so that bone can be made to appear opaque and solid while soft tissue is rendered nearly invisible, producing a three-dimensional image of just the structure a viewer wants to see. ### Image quality #### Dose versus image quality Image quality and radiation dose trade directly against each other: a noisier, grainier image comes from fewer X-ray photons, and reducing the dose to a patient by lowering the tube current or voltage increases that noise, so protocol design is a continual balance between a diagnostically adequate image and the lowest dose that provides it.[^dosecite] #### Artifacts Reconstruction assumes an idealized, evenly sampled set of projections, and anything that violates that assumption leaves a mark: metal implants cause bright streaking because they attenuate far more than the reconstruction algorithm expects, patient motion between projections blurs or doubles an edge, and too few projection angles is a form of angular [[Aliasing|aliasing]] that leaves the star-burst streaks the microsim on this page reproduces directly. ## Advantages CT's central advantage is speed combined with resolution: a modern scanner covers a whole chest, abdomen and pelvis in a single held breath, fast enough to freeze most involuntary motion, while resolving detail far beyond a plain radiograph. It is also one of the few imaging methods compatible with a pacemaker, a cochlear implant or other metal hardware that rules out [[Magnetic_resonance_imaging|magnetic resonance imaging]] outright, and it is widely available even outside major hospitals, unlike some more specialized imaging techniques. ## Adverse effects ### Cancer Because a CT scan delivers ionizing radiation and a large population is scanned every year, the small individual increase in lifetime cancer risk that ionizing radiation is generally assumed to carry becomes a population-level concern for [[Statistics|statistical]] reasons, even though no individual scan can be shown to have caused a given cancer; the assumption that risk scales down linearly with dose, with no safe threshold, is itself debated at the low doses a single scan involves.[^dosecite] ### Contrast reactions The iodine-based contrast agent used in many CT protocols can trigger an allergic-type reaction, usually mild, and can stress the kidneys in a patient whose kidney function is already reduced, which is why a patient's kidney function and allergy history are checked before a contrast-enhanced scan is ordered. ### Scan dose #### Radiation dose units Radiation dose relevant to a patient is generally reported in millisieverts, a unit that weights absorbed radiation by its estimated biological effect, allowing doses from different kinds of radiation and different exams to be compared on one scale. #### Effects of radiation At the doses a diagnostic scan involves, any effect on the body is too small to observe directly in an individual patient and is instead estimated by extrapolating from populations exposed to far higher doses, such as survivors of the atomic bombings of Japan, an extrapolation whose accuracy at low, medical-imaging doses remains a subject of active research.[^dosecite] #### Excess doses Documented cases of patients receiving far more radiation than a protocol called for, from a misconfigured scanner or a repeated study, have driven stricter dose-tracking requirements and automated dose alerts on modern scanners, so a technologist is warned before a study exceeds an expected dose range.[^dosecite] ## Procedure A routine CT scan itself takes only a few minutes: the patient lies on a table that moves through the gantry while the tube rotates, and the only requirement during the scan is to stay still and, for a chest or abdominal study, to hold a breath for a few seconds at the technologist's instruction. ### Preparation Preparation depends on the region and on whether contrast is used: an abdominal study with oral contrast may require drinking a contrast solution over the hour before the scan, an intravenous contrast study requires a recent kidney-function check, and metal objects that would cause streaking artifacts are removed beforehand. ## Mechanism A CT scanner's X-ray tube and detector row sit on opposite sides of a rotating gantry, and as they rotate around the patient, the detectors measure how many X-ray [[Photon|photons]] have passed along many lines through the body at many angles. A computer applies filtered back-projection, or an iterative variant of it, to invert those line-integral measurements into a cross-sectional map of attenuation. The unit of that map, the CT number, is scaled so water reads as zero and air reads as a large negative number, with denser tissue and bone reading progressively higher. ### Contrast A [[Barium|barium]]- or iodine-based contrast agent strongly attenuates X-rays relative to soft tissue, so a structure that has taken up contrast — a blood vessel, the bowel, a vascular tumor — stands out against tissue that has not, which is why many protocols time the scan to a contrast injection rather than imaging the body's natural attenuation alone. ## History The mathematics behind CT predates the machine by decades: Austrian mathematician Johann Radon showed in 1917, using pure [[Mathematics|mathematics]] with no radiological motivation, that a function could be reconstructed from the set of its line integrals over all lines through it, the transform that filtered back-projection inverts.[^radon1917] South African-born physicist Allan Cormack rediscovered the same problem from a radiological angle in the early 1960s, developing the mathematics of reconstructing an object's internal structure from external radiation measurements without being aware of Radon's earlier work.[^cormack1963] British electrical engineer Godfrey Hounsfield, working independently at EMI's research laboratories, built the first clinical scanner and used it to image a patient at Atkinson Morley Hospital in London in 1971; he published a full description of the system two years later.[^hounsfield1973] Cormack and Hounsfield shared the 1979 Nobel Prize in Physiology or Medicine for the development of computed tomography.[^nobel1979] ### Etymology The word tomography comes from the Greek *tomos*, a cut or a slice, and *graphein*, to write; the technique was first called computerized axial tomography, or CAT, before computed tomography, avoiding any association with the animal, became the preferred term. ## Society and culture ### Campaigns Radiology and public-health organizations have run patient and physician education campaigns aimed at reducing scans that are unlikely to change treatment, encouraging referral only when a scan's expected benefit outweighs its radiation dose and cost.[^dosecite] ### Prevalence CT has become one of the most frequently ordered imaging studies in emergency and hospital [[Medicine|medicine]] in many countries, a rise driven by faster scanners, wider availability and its usefulness across an enormous range of conditions, though the same growth is what motivates the dose-awareness campaigns described above. ## Manufacturers A small number of large medical-device manufacturers — among them GE Healthcare, Siemens Healthineers, Philips and Canon Medical — design and [[Manufacturing|manufacture]] most of the CT scanners in clinical use worldwide, competing chiefly on detector technology, gantry rotation speed, radiation-dose reduction features and image-reconstruction software. ## Research Active research directions include reconstruction algorithms that produce a diagnostic-quality image from fewer projections or a lower dose, [[Machine_learning|machine-learning]]-based denoising and reconstruction, including [[Neural_network_(machine_learning)|neural-network]] models trained end to end on the reconstruction problem, and photon-counting detectors that measure the energy of individual X-ray photons rather than only their total intensity, extending dual energy CT's material discrimination to many energy bins at once. ## Microsims The primary microsim, *Computed axial tomography*, builds a small attenuation phantom, forward-projects it into a sinogram at a chosen number of angles, and reconstructs it by filtered back-projection. A ramp-filter toggle switches between plain back-projection, which blurs every point into a soft 1/r smear, and the filtered version, whose ramp filter cancels exactly that blur; an angle-count control shows the reconstruction degrade into radial streaks as the projection count falls, since too few angles leave gaps in the [[Fourier_transform|Fourier]]-domain coverage that the Fourier slice theorem depends on filling completely. An acquisition toggle sweeps the projections in one angle at a time, so the sinogram and the reconstruction can be watched filling in together rather than appearing all at once. *Try:* Turn the ramp filter off and watch a sharp phantom soften into a blur, then turn it back on and lower the angle count until streaks appear in its place. A second microsim, *X-ray computed tomography*, follows the same forward-projection, ramp-filter [[Convolution|convolution]] and back-projection pipeline with an incremental reconstruction the reader can watch assemble one angle at a time, alongside a profile plot comparing the reconstructed attenuation against the true phantom along a chosen line and a running measure of reconstruction error. *Try:* Replay the reconstruction from scratch with the ramp filter off, then on, and compare how quickly the running error falls in each case. A three.js companion sketch renders the same projections-to-sinogram-to-filtered-back-projection pipeline in more depth as the number of views rises. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/CT_scan) : [Wikitube](https://en.wikitube.io/wiki/CT_scan) Skeleton mirrored at revision 1374879112. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Digital_image_processing]] - [[Magnetic_resonance_imaging]] - [[Signal_processing]] - [[Fourier_transform]] - [[Convolution]] - [[Aliasing]] - [[Photon]] - [[Machine_learning]] ## References The two microsims on this page use a small, synthetic attenuation phantom rather than real patient data, and their reconstruction runs at a tiny fraction of a clinical scanner's true angular and spatial resolution; both simplifications are illustrative, chosen to make the sinogram-to-image relationship visible rather than to reproduce clinical image quality. The Radon transform and filtered back-projection themselves are standard mathematical results and are not separately footnoted beyond the historical citation below, per the Wikitube style guide §6.1. [^radon1917]: Radon, J. "Über die Bestimmung von Funktionen durch ihre Integralwerte längs gewisser Mannigfaltigkeiten." *Berichte über die Verhandlungen der Königlich-Sächsischen Akademie der Wissenschaften zu Leipzig, Mathematisch-Physische Klasse*, 1917, 69, pp. 262–277. [^cormack1963]: Cormack, A. M. "Representation of a Function by Its Line Integrals, with Some Radiological Applications." *Journal of Applied Physics*, 1963, 34(9), pp. 2722–2727. [^hounsfield1973]: Hounsfield, G. N. "Computerized Transverse Axial Scanning (Tomography): Part 1. Description of System." *British Journal of Radiology*, 1973, 46(552), pp. 1016–1022. [^nobel1979]: The Nobel Foundation. "The Nobel Prize in Physiology or Medicine 1979." Nobel Prize Outreach, 1979. https://www.nobelprize.org/prizes/medicine/1979/summary/ [^dosecite]: Citation needed: current, protocol-specific effective-dose figures and cancer-risk estimates by scan type, which vary with scanner generation, technique and the population studied. ## External links - *Computed axial tomography* — live sketch: https://editor.p5js.org/sciencenibber/full/ZidvLkbn1 - *Computed axial tomography* — editor source: https://editor.p5js.org/sciencenibber/sketches/ZidvLkbn1 - *X-ray computed tomography* — live sketch: https://editor.p5js.org/sciencenibber/full/jrp8epMoA - *X-ray computed tomography* — editor source: https://editor.p5js.org/sciencenibber/sketches/jrp8epMoA <!-- Hubs: Signal_processing. Portals: PORTAL_Signal_Processing. Signal Processing portal wave 1 · 2026-09-17 · drafted. -->