Medical imaging procedure using X-rays to produce cross-sectional images
This article is about X-ray computed tomography as used in medicine. For cross-sectional images used in industry, seeIndustrial computed tomography. For means of tomography other than X-ray, seeTomography.
Medical intervention
CT scan
Modern photon-counting CT scanner in 2021 (Siemens NAEOTOM Alpha)
Acomputed tomography scan (CT scan), formerly calledcomputed axial tomography scan (CAT scan), is amedical imaging technique used to obtain detailed internal images of the body.[2] The personnel that perform CT scans are calledradiographers or radiology technologists.[3][4]CT scanners use a rotatingX-ray tube and a row of detectors placed in agantry to measure X-rayattenuations by different tissues inside the body. The multipleX-ray measurements taken from different angles are then processed on a computer usingtomographic reconstruction algorithms to producetomographic (cross-sectional) images (virtual "slices") of a body. CT scans can be used in patients with metallic implants or pacemakers, for whommagnetic resonance imaging (MRI) iscontraindicated.
1977 Dutch newsreel about CT scan
Since its development in the 1970s, CT scanning has proven to be a versatile imaging technique. While CT is most prominently used inmedical diagnosis, it can also be used to form images of non-living objects. The 1979Nobel Prize in Physiology or Medicine was awarded jointly to South African-American physicistAllan MacLeod Cormack and British electrical engineerGodfrey Hounsfield "for the development of computer-assisted tomography".[5][6]
Sequential CT, also known as step-and-shoot CT, is a type of scanning method in which the CT table moves stepwise. The table increments to a particular location and then stops which is followed by theX-ray tube rotation and acquisition of a slice. The table then increments again, and another slice is taken. The table movement stops while taking slices. This results in an increased time of scanning.[7]
Drawing of CT fan beam and patient in a CT imaging systemCT scan of the thorax. The axial slice (right) is the image that corresponds to number 2/33 on the coronal slice (left).
Spinning tube, commonly calledspiral CT, or helical CT, is an imaging technique in which an entireX-ray tube is spun around the central axis of the area being scanned. These are the dominant type of scanners on the market because they have been manufactured longer and offer a lower cost of production and purchase. The main limitation of this type of CT is the bulk and inertia of the equipment (X-ray tube assembly and detector array on the opposite side of the circle) which limits the speed at which the equipment can spin. Some designs use two X-ray sources and detector arrays offset by an angle, as a technique to improve temporal resolution.[8][9]
Electron beam tomography (EBT) is a specific form of CT in which a large enough X-ray tube is constructed so that only the path of theelectrons, travelling between thecathode andanode of the X-ray tube, are spun usingdeflection coils.[10] This type had a major advantage since sweep speeds can be much faster, allowing for less blurry imaging of moving structures, such as the heart and arteries.[11] Fewer scanners of this design have been produced when compared with spinning tube types, mainly due to the higher cost associated with building a much larger X-ray tube and detector array and limited anatomical coverage.[12]
Dual energy CT, also known as spectral CT, is an advancement of computed Ttmography in which two energies are used to create two sets of data.[13] A dual energy CT may employ dual source, single source with dual detector layer, single source with energy switching methods to get two different sets of data.[14]
Dual source CT is an advanced scanner with a two X-ray tube detector system, unlike conventional single tube systems.[15][16] These two detector systems are mounted on a single gantry at 90° in the same plane.[17] Dual source CT scanners allow fast scanning with higher temporal resolution by acquiring a full CT slice in only half a rotation. Fast imaging reduces motion blurring at high heart rates and potentially allowing for shorter breath-hold time. This is particularly useful for ill patients having difficulty holding their breath or unable to take heart-rate lowering medication.[17][18]
Single source with energy switching is another mode of dual energy CT in which a single tube is operated at two different energies by switching the energies frequently.[19][20]
CT perfusion imaging is a specific form of CT to assess flow throughblood vessels whilst injecting acontrast agent.[21] Blood flow, blood transit time, and organ blood volume, can all be calculated with reasonablesensitivity and specificity.[21] This type of CT may be used on theheart, although sensitivity and specificity for detecting abnormalities are still lower than for other forms of CT.[22] This may also be used on thebrain, where CT perfusion imaging can often detect poor brain perfusion well before it is detected using a conventional spiral CT scan.[21][23] This is better forstroke diagnosis than other CT types.[23]
Positron emission tomography–computed tomography is a hybrid CT modality which combines, in a single gantry, apositron emission tomography (PET) scanner and an X-ray computed tomography (CT) scanner, to acquire sequential images from both devices in the same session, which are combined into a single superposed (co-registered) image. Thus,functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning.[24]
PET-CT gives both anatomical and functional details of an organ under examination and is helpful in detecting different type of cancers.[25][26]
Since its introduction in the 1970s,[27] CT has become an important tool inmedical imaging to supplement conventionalX-ray imaging andmedical ultrasonography. It has more recently been used forpreventive medicine orscreening for disease, for example,CT colonography for people with a high risk ofcolon cancer, or full-motion heart scans for people with a high risk of heart disease. Several institutions offerfull-body scans for the general population although this practice goes against the advice and official position of many professional organizations in the field primarily due to theradiation dose applied.[28]
The use of CT scans has increased dramatically over the last two decades in many countries.[29] An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015.[30][31]
CT scanning of the head is typically used to detectinfarction (stroke),tumors,calcifications,haemorrhage, and bonetrauma.[32] Of the above,hypodense (dark) structures can indicateedema and infarction, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. CT scanning of the head is also used in CT-guidedstereotactic surgery andradiosurgery for treatment of intracranial tumors,arteriovenous malformations, and other surgically treatable conditions using a device known as theN-localizer.[33][34][35][36][37][38]
Contrast CT is generally the initial study of choice forneck masses in adults.[39]CT of the thyroid plays an important role in the evaluation ofthyroid cancer.[40] CT scan often incidentally finds thyroid abnormalities, and so is often the preferred investigation modality for thyroid abnormalities.[40]
A CT scan can be used for detecting both acute and chronic changes in thelung parenchyma, the tissue of thelungs.[41] It is particularly relevant here because normal two-dimensional X-rays do not show such defects. A variety of techniques are used, depending on the suspected abnormality. For evaluation of chronic interstitial processes such asemphysema, andfibrosis,[42] thin sections with high spatial frequency reconstructions are used; often scans are performed both on inspiration and expiration. This special technique is calledhigh resolution CT that produces a sampling of the lung, and not continuous images.[43]
Anincidentally found nodule in the absence of symptoms (sometimes referred to as anincidentaloma) may raise concerns that it might represent a tumor, eitherbenign ormalignant.[45] Perhaps persuaded by fear, patients and doctors sometimes agree to an intensive schedule of CT scans, sometimes up to every three months and beyond the recommended guidelines, in an attempt to do surveillance on the nodules.[46] However, established guidelines advise that patients without a prior history of cancer and whose solid nodules have not grown over a two-year period are unlikely to have any malignant cancer.[46] For this reason, and because no research provides supporting evidence that intensive surveillance gives better outcomes, and because of risks associated with having CT scans, patients should not receive CT screening in excess of those recommended by established guidelines.[46]
A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy.[53] Traditionally, cardiac CT scans are used to detect, diagnose, or follow upcoronary artery disease.[54] More recently CT has played a key role in the fast-evolving field oftranscatheter structural heart interventions, more specifically in the transcatheter repair and replacement of heart valves.[55][56][57]
Coronary CT calcium scan: also used for the assessment of severity of coronary artery disease. Specifically, it looks for calcium deposits in the coronary arteries that can narrow arteries and increase the risk of a heart attack.[60] A typical coronary CT calcium scan is done without the use of radiocontrast, but it can possibly be done from contrast-enhanced images as well.[61]
To better visualize the anatomy, post-processing of the images is common.[54] Most common are multiplanar reconstructions (MPR) andvolume rendering. For more complex anatomies and procedures, such as heart valve interventions, a true3D reconstruction or a 3D print is created based on these CT images to gain a deeper understanding.[62][63][64][65]
CT is an accurate technique for diagnosis ofabdominal diseases likeCrohn's disease,[66] GIT bleeding, and diagnosis and staging of cancer, as well as follow-up after cancer treatment to assess response.[67] It is commonly used to investigateacute abdominal pain.[68]
Non-contrast-enhanced CT scans are the gold standard for diagnosingkidney stone disease.[69] They allow clinicians to estimate the size, volume, and density of stones, helping to guide further treatment; with size being especially important in predicting the time to spontaneous passage of a stone.[70]
For theaxial skeleton andextremities, CT is often used to image complexfractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries, anddislocations can easily be recognized with a 0.2 mm resolution.[71][72] With modern dual-energy CT scanners, new areas of use have been established, such as aiding in the diagnosis ofgout.[73]
Industrial CT scanning (industrial computed tomography) is a process which uses X-ray equipment to produce 3D representations of components both externally and internally. Industrial CT scanning has been used in many areas of industry for internal inspection of components. Some of the key uses for CT scanning have been flaw detection, failure analysis, metrology, assembly analysis, image-based finite element methods[76] and reverse engineering applications. CT scanning is also employed in the imaging and conservation of museum artifacts.[77]
CT scanning has also found an application in transport security (predominantlyairport security) where it is currently used in a materials analysis context for explosives detectionCTX (explosive-detection device)[78][79][80][81] and is also under consideration for automated baggage/parcel security scanning usingcomputer vision based object recognition algorithms that target the detection of specific threat items based on 3D appearance (e.g. guns, knives, liquid containers).[82][83][84] Its usage in airport security pioneered atShannon Airport in March 2022 has ended the ban on liquids over 100 ml there, a move thatHeathrow Airport plans for a full roll-out on 1 December 2022 and the TSA spent $781.2 million on an order for over 1,000 scanners, ready to go live in the summer.
X-ray CT is used in geological studies to quickly reveal materials inside a drill core.[85] Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images.[86]
Traditional methods of studying fossils are often destructive, such as the use of thin sections and physical preparation. X-ray CT is used in paleontology to non-destructively visualize fossils in 3D.[87] This has many advantages. For example, we can look at fragile structures that might never otherwise be able to be studied. In addition, one can freely move around models of fossils in virtual 3D space to inspect it without damaging the fossil.
X-ray CT andmicro-CT can also be used for the conservation and preservation of objects of cultural heritage. For many fragile objects, direct research and observation can be damaging and can degrade the object over time. Using CT scans, conservators and researchers are able to determine the material composition of the objects they are exploring, such as the position of ink along the layers of a scroll, without any additional harm. These scans have been optimal for research focused on the workings of theAntikythera mechanism or the text hidden inside the charred outer layers of theEn-Gedi Scroll. However, they are not optimal for every object subject to these kinds of research questions, as there are certain artifacts like theHerculaneum papyri in which the material composition has very little variation along the inside of the object. After scanning these objects, computational methods can be employed to examine the insides of these objects, as was the case with the virtual unwrapping of theEn-Gedi scroll and theHerculaneum papyri.[88] Micro-CT has also proved useful for analyzing more recent artifacts such as still-sealed historic correspondence that employed the technique ofletterlocking (complex folding and cuts) that provided a "tamper-evident locking mechanism".[89][90] Further examples of use cases in archaeology is imaging the contents of sarcophagi or ceramics.[91]
Recently, CWI in Amsterdam has collaborated with Rijksmuseum to investigate art object inside details in the framework called IntACT.[92]
Varied types of fungus can degrade wood to different degrees, one Belgium research group has been used X-ray CT 3 dimension with sub-micron resolution unveiled fungi can penetrate micropores of 0.6 μm[93] under certain conditions.
Sawmills use industrial CT scanners to detect round defects, for instance knots, to improve total value of timber productions. Most sawmills are planning to incorporate this robust detection tool to improve productivity in the long run, however initial investment cost is high.[94]
The result of a CT scan is a volume ofvoxels, which may be presented to a human observer by various methods, which broadly fit into the following categories:
Slices (of varying thickness). Thin slice is generally regarded as planes representing a thickness of less than 3mm.[95][96] Thick slice is generally regarded as planes representing a thickness between 3 mm and 5 mm.[96][97]
Technically, all volume renderings become projections when viewed on a2-dimensional display, making the distinction between projections and volume renderings a bit vague. The epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations.[99][100]
Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet.[101] 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.[102]
Pixels in an image obtained by CT scanning are displayed in terms of relativeradiodensity. The pixel itself is displayed according to the meanattenuation of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on theHounsfield scale. Apixel 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 avoxel, which is a three-dimensional unit.[103] Water has an attenuation of 0Hounsfield units (HU), while air is −1,000 HU, cancellous bone is typically +400 HU, and cranial bone can reach 2,000 HU.[104] The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely block 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.[105]
CT data sets have a very highdynamic range which must be reduced for display or printing. This is typically done via a process of "windowing", which maps a range (the "window") of pixel values to a grayscale ramp. For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU. Pixel values of 0 and lower, are displayed as black; values of 80 and higher are displayed as white; values within the window are displayed as a gray intensity proportional to position within the window.[106] The window used for display must be matched to the X-ray density of the object of interest, in order to optimize the visible detail.[107] Window width and window level parameters are used to control the windowing of a scan.[108]
Typical screen layout for diagnostic software, showing one volume rendering (VR) and multiplanar view of three thin slices in theaxial (upper right),sagittal (lower left), andcoronal planes (lower right)Special planes are sometimes useful, such as this oblique longitudinal plane in order to visualize the neuroforamina of the vertebral column, showing narrowing at two levels, causingradiculopathy. The smaller images are axial plane slices.
Multiplanar reconstruction (MPR) is the process of converting data from oneanatomical plane (usuallytransverse) to other planes. It can be used for thin slices as well as projections. Multiplanar reconstruction is possible as present CT scanners provide almostisotropic resolution.[109]
MPR is used almost in every scan. The spine is frequently examined with it.[110] An image of the spine in axial plane can only show one vertebral bone at a time and cannot show its relation with other vertebral bones. By reformatting the data in other planes, visualization of the relative position can be achieved in sagittal and coronal plane.[111]
New software allows the reconstruction of data in non-orthogonal (oblique) planes, which help in the visualization of organs which are not in orthogonal planes.[112][113] It is better suited for visualization of the anatomical structure of the bronchi as they do not lie orthogonal to the direction of the scan.[114]
Curved-plane reconstruction (or curved planar reformation = CPR) is performed mainly for the evaluation of vessels. This type of reconstruction helps to straighten the bends in a vessel, thereby helping to visualize a whole vessel in a single image or in multiple images. After a vessel has been "straightened", measurements such as cross-sectional area and length can be made. This is helpful in preoperative assessment of a surgical procedure.[115]
Examples of different algorithms of thickening multiplanar reconstructions[116]
Type of projection
Schematic illustration
Examples (10 mm slabs)
Description
Uses
Average intensity projection (AIP)
The average attenuation of each voxel is displayed. The image will get smoother as slice thickness increases. It will look more and more similar to conventionalprojectional radiography as slice thickness increases.
Useful for identifying the internal structures of a solid organ or the walls of hollow structures, such as intestines.
The voxel with the highest attenuation is displayed. Therefore, high-attenuating structures such as blood vessels filled with contrast media are enhanced.
Useful for angiographic studies and identification of pulmonary nodules.
A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). With the help ofedge detection image processing algorithms a 3D model can be constructed from the initial data and displayed on screen. Various thresholds can be used to get multiple models, each anatomical component such as muscle, bone and cartilage can be differentiated on the basis of different colours given to them. However, this mode of operation cannot show interior structures.[117]
Surface rendering is limited technique as it displays only the surfaces that meet a particular threshold density, and which are towards the viewer. However, In volume rendering, transparency, colours andshading are used which makes it easy to present a volume in a single image. For example, Pelvic bones could be displayed as semi-transparent, so that, even viewing at an oblique angle one part of the image does not hide another.[118]
An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. In general, higher radiation doses result in higher-resolution images,[119] while lower doses lead to increased image noise and unsharp images. However, increased dosage raises the adverse side effects, including the risk ofradiation-induced cancer – a four-phase abdominal CT gives the same radiation dose as 300 chest X-rays.[120] Several methods that can reduce the exposure to ionizing radiation during a CT scan exist.[121]
Individualize the examination and adjust the radiation dose to the body type and body organ examined. Different body types and organs require different amounts of radiation.[123]
Higher resolution is not always suitable, such as detection of small pulmonary masses.[124]
Although images produced by CT are generally faithful representations of the scanned volume, the technique is susceptible to a number ofartifacts, such as the following:[125][126]Chapters 3 and 5
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, andCompton 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.[127] Approaches such as metal artifact reduction (MAR) can also reduce this artifact.[128][129] MAR techniques include spectral imaging, where CT images are taken withphotons of different energy levels, and then synthesized intomonochromatic images with special software such as GSI (Gemstone Spectral Imaging).[130]
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).[131] 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 highlyanisotropic 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.[132]
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.[133] Ring artifacts can largely be reduced by intensity normalization, also referred to as flat field correction.[134] Remaining rings can be suppressed by a transformation to polar space, where they become linear stripes.[133] 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.[135]
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.[136]
Windmill
Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.[137][138]
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 higherphoton 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.[127][139][140]
CT scanning has several advantages over traditionaltwo-dimensional medicalradiography. First, CT eliminates the superimposition of images of structures outside the area of interest.[141] Second, CT scans have greaterimage resolution, enabling examination of finer details. CT can distinguish betweentissues that differ in radiographic density by 1% or less.[142] Third, CT scanning enables multiplanar reformatted imaging: scan data can be visualized in thetransverse (or axial),coronal, orsagittal plane, depending on the diagnostic task.[143]
The improved resolution of CT has permitted the development of new investigations. For example, CTangiography avoids the invasive insertion of acatheter. CT scanning can perform avirtual colonoscopy with greater accuracy and less discomfort for the patient than a traditionalcolonoscopy.[144][145] Virtual colonography is far more accurate than abarium enema for detection of tumors and uses a lower radiation dose.[146]
CT is a moderate-to-highradiation diagnostic technique. The radiation dose for a particular examination depends on multiple factors: volume scanned, patient build, number and type of scan protocol, and desired resolution and image quality.[147] Two helical CT scanning parameters, tube current and pitch, can be adjusted easily and have a profound effect on radiation. CT scanning is more accurate than two-dimensional radiographs in evaluating anterior interbody fusion, although they may still over-read the extent of fusion.[148]
Theradiation used in CT scans can damage body cells, includingDNA molecules, which can lead toradiation-induced cancer.[149] 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.[150] However, a lumbar spine X-ray has a similar dose as a head CT.[151] 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 averagebackground radiation.[152]
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.[153][154][155][156] 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.[157] Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.[150] An Australian study of 10.9 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.[158][159][160]
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."[150] On the other hand, a recent paper analyzing the data of patients who received highcumulative doses showed a high degree of appropriate use.[161] 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,[159] 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,[162] concluding that no evidence indicates such low doses cause any long-term harm.[163][164][165]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.[149] Others dispute this estimate,[166] 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.[167]
A person's age plays a significant role in the subsequent risk of cancer.[168] Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1%, or 1:1000 scans.[168] 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.[168] TheInternational Commission on Radiological Protection estimates that the risk to a fetus being exposed to 10mGy (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 mGy).[169] 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.[170] 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.[171] Furthermore, certain conditions can require children to be exposed to multiple CT scans.[149]
Current recommendations are to inform patients of the risks of CT scanning.[172] However, employees of imaging centers tend not to communicate such risks unless patients ask.[173]
In the United States half of CT scans arecontrast CTs using intravenously injectedradiocontrast agents.[174] The most common reactions from these agents are mild, including nausea, vomiting, and an itching rash. Severe life-threatening reactions may rarely occur.[175] Overall reactions occur in 1 to 3% withnonionic contrast and 4 to 12% of people withionic contrast.[176] Skin rashes may appear within a week to 3% of people.[175]
The oldradiocontrast agents causedanaphylaxis in 1% of cases while the newer, low-osmolar agents cause reactions in 0.01–0.04% of cases.[175][177] Death occurs in about 2 to 30 people per 1,000,000 administrations, with newer agents being safer.[176][178]There is a higher risk of mortality in those who are female, elderly or in poor health, usually secondary to either anaphylaxis oracute kidney injury.[174]
The contrast agent may inducecontrast-induced nephropathy.[179] This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexistingkidney failure,[179] preexistingdiabetes, 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 ofiodinated contrast should be avoided; this may mean using an alternative technique instead of CT. Those with severekidney failure requiringdialysis 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.[180] 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 fatalperitonitis.[181]
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.[168] A typical plain film X-ray involves radiation dose of 0.01 to 0.15 mGy, while a typical CT can involve 10–20 mGy for specific organs, and can go up to 80 mGy for certain specialized CT scans.[185]
For purposes of comparison, the world average dose rate from naturally occurring sources ofbackground radiation is 2.4 mSv per year, equal for practical purposes in this application to 2.4 mGy per year.[183] While there is some variation, most people (99%) received less than 7 mSv per year as background radiation.[187] 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.[168] In the United Kingdom it accounts for 15% of radiation exposure.[169] The average radiation dose from medical sources is ≈0.6 mSv per person globally as of 2007.[168] Those in the nuclear industry in the United States are limited to doses of 50 mSv a year and 100 mSv every 5 years.[168]
Lead is the main material used by radiography personnel forshielding against scattered X-rays.
The radiation dose reported in thegray 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 DNAdouble strand breaks) on the cells' chemical bonds by X-ray radiation is proportional to that energy.[188]
Thesievert unit is used in the report of theeffective 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.[189] 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 thecomputed 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.[190]
Theequivalent 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.[191][192][193]
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;[194]
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.[195]
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.[196]
Because of increased susceptibility of fetuses to radiation exposure, the radiation dosage of a CT scan is an important consideration in the choice ofmedical imaging in pregnancy.[197][198]
In October, 2009, the USFood and Drug Administration (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based onradiation burns 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."[168][199] Similar problems have been reported at other centers.[168] These incidents are believed to be due tohuman error.[168]
CT scan procedure varies according to the type of the study and the organ being imaged. The patient lies on the CT table and the centering of the table is done according to the body part. The IV line is established in case of contrast-enhanced CT. After selecting proper[clarification needed] and rate of contrast from the pressure injector, the scout is taken to localize and plan the scan. Once the plan is selected, the contrast is given. The raw data is processed according to the study and proper windowing is done to make scans easy to diagnose.[200]
CT scanner with cover removed to show internal components. Legend: T: X-ray tube D: X-ray detectors X: X-ray beam R: Gantry rotationLeft image is asinogram 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.[202]
Computed tomography operates by using anX-ray generator that rotates around the object;X-ray detectors are positioned on the opposite side of the circle from the X-ray source.[203] As the X-rays pass through the patient, they are attenuated differently by various tissues according to the tissue density.[204] A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation.[205] Once the scan data has been acquired, the data must be processed using a form oftomographic reconstruction, which produces a series of cross-sectional images.[206] These cross-sectional images are made up of small units of pixels or voxels.[207]
Pixels in an image obtained by CT scanning are displayed in terms of relativeradiodensity. The pixel itself is displayed according to the meanattenuation of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on theHounsfield scale. Apixel 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 avoxel, which is a three-dimensional unit.[207]
Water has an attenuation of 0Hounsfield units (HU), while air is −1,000 HU, cancellous bone is typically +400 HU, and cranial bone can reach 2,000 HU or more (os temporale) and can causeartifacts. The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of +1000 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.[101] 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.
Contrast media used for X-ray CT, as well as forplain film X-ray, are calledradiocontrasts. Radiocontrasts for CT are, in general, iodine-based.[209] 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.[210]
The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of theRadon transform.[211][212] In October 1963,William H. Oldendorf received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material".[213] The first commercially viable CT scanner was invented byGodfrey Hounsfield in 1972.[214]
It is often claimed that revenues from the sales of The Beatles' records in the 1960s helped fund the development of the first CT scanner at EMI. The first production X-ray CT machines were in fact called EMI scanners.[215]
The wordtomography is derived from theGreektome 'slice' andgraphein 'to write'.[216] Computed tomography was originally known as the "EMI scan" as it was developed in the early 1970s at a research branch ofEMI, a company best known today for its music and recording business.[217] It was later known ascomputed axial tomography (CAT orCT scan) andbody section röntgenography.[218]
The termCAT scan is no longer in technical use because current CT scans enable for multiplanar reconstructions. This makesCT scan the most appropriate term, which is used byradiologists in common vernacular as well as in textbooks and scientific papers.[219][220][221]
InMedical Subject Headings (MeSH),computed axial tomography was used from 1977 to 1979, but the current indexing explicitly includesX-ray in the title.[222]
The termsinogram was introduced by Paul Edholm and Bertil Jacobson in 1975.[223]
In response to increased concern by the public and the ongoing progress of best practices, the Alliance for Radiation Safety in Pediatric Imaging was formed within theSociety for Pediatric Radiology. In concert with theAmerican Society of Radiologic Technologists, theAmerican College of Radiology and theAmerican Association of Physicists in Medicine, the Society for Pediatric Radiology developed and launched the Image Gently Campaign which is designed to maintain high-quality imaging studies while using the lowest doses and best radiation safety practices available on pediatric patients.[225] This initiative has been endorsed and applied by a growing list of various professional medical organizations around the world and has received support and assistance from companies that manufacture equipment used in Radiology.
Following upon the success of theImage Gently campaign, the American College of Radiology, the Radiological Society of North America, the American Association of Physicists in Medicine and the American Society of Radiologic Technologists have launched a similar campaign to address this issue in the adult population calledImage Wisely.[226]
Use of CT has increased dramatically over the last two decades.[29] An estimated 72 million scans were performed in the United States in 2007,[30] accounting for close to half of the total per-capita dose rate from radiologic and nuclear medicine procedures.[229] Of the CT scans, six to eleven percent are done in children,[169] an increase of seven to eightfold from 1980.[168] Similar increases have been seen in Europe and Asia.[168] In Calgary, Canada, 12.1% of people who present to the emergency with an urgent complaint received a CT scan, most commonly either of the head or of the abdomen. The percentage who received CT, however, varied markedly by theemergency physician who saw them from 1.8% to 25%.[230] In the emergency department in the United States, CT orMRI imaging is done in 15% of people who present withinjuries as of 2007 (up from 6% in 1998).[231]
The increased use of CT scans has been the greatest in two fields: screening of adults (screening CT of the lung in smokers, virtual colonoscopy, CT cardiac screening, and whole-body CT in asymptomatic patients) and CT imaging of children. Shortening of the scanning time to around 1 second, eliminating the strict need for the subject to remain still or be sedated, is one of the main reasons for the large increase in the pediatric population (especially for the diagnosis ofappendicitis).[149] As of 2007, in the United States a proportion of CT scans are performed unnecessarily.[171] Some estimates place this number at 30%.[169] There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public.[171] For example, some healthy people avidly pay to receive full-body CT scans asscreening. In that case, it is not at all clear that the benefits outweigh the risks and costs. Deciding whether and how to treatincidentalomas is complex, radiation exposure is not negligible, and the money for the scans involvesopportunity cost.[171]
Photon-counting computed tomography is a CT technique currently under development.[as of?] Typical CT scanners use energy integrating detectors; photons are measured as a voltage on a capacitor which is proportional to the X-rays detected. However, this technique is susceptible to noise and other factors which can affect the linearity of the voltage to X-ray intensity relationship.[233] Photon counting detectors (PCDs) are still affected by noise but it does not change the measured counts of photons. PCDs have several potential advantages, including improving signal (and contrast) to noise ratios, reducing doses, improving spatial resolution, and through use of several energies, distinguishing multiple contrast agents.[234][235] PCDs have only recently become feasible in CT scanners due to improvements in detector technologies that can cope with the volume and rate of data required. As of February 2016, photon counting CT is in use at three sites.[236] Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising.[237] In view of recent findings of high cumulative doses to patients from recurrent CT scans, there has been a push for scanning technologies and techniques that reduce ionising radiation doses to patients to sub-milliSievert (sub-mSv in the literature) levels during the CT scan process, a goal that has been lingering.[238][159][160][161]
^Talisetti A, Jelnin V, Ruiz C, John E, Benedetti E, Testa G, Holterman AX, Holterman MJ (December 2004). "Electron beam CT scan is a valuable and safe imaging tool for the pediatric surgical patient".Journal of Pediatric Surgery.39 (12):1859–1862.doi:10.1016/j.jpedsurg.2004.08.024.ISSN1531-5037.PMID15616951.
^abcWittsack HJ, Wohlschläger A, Ritzl E, Kleiser R, Cohnen M, Seitz R, Mödder U (2008-01-01). "CT-perfusion imaging of the human brain: Advanced deconvolution analysis using circulant singular value decomposition".Computerized Medical Imaging and Graphics.32 (1):67–77.doi:10.1016/j.compmedimag.2007.09.004.ISSN0895-6111.PMID18029143.
^American Academy of Orthopaedic Surgeons, American College of Emergency Physicians, UMBC (2017-03-20).Critical Care Transport. Jones & Bartlett Learning. p. 389.ISBN978-1-284-04099-9.
^Brown RA, Nelson JA (June 2012). "Invention of the N-localizer for stereotactic neurosurgery and its use in the Brown-Roberts-Wells stereotactic frame".Neurosurgery.70 (2 Supplement Operative):173–176.doi:10.1227/NEU.0b013e318246a4f7.PMID22186842.S2CID36350612.
MacMahon H, Austin JH, Gamsu G, Herold CJ, Jett JR, Naidich DP, Patz EF, Swensen SJ (2005). "Guidelines for Management of Small Pulmonary Nodules Detected on CT Scans: A Statement from the Fleischner Society1".Radiology.237 (2):395–400.doi:10.1148/radiol.2372041887.PMID16244247.S2CID14498160.
Gould MK, Fletcher J, Iannettoni MD, Lynch WR, Midthun DE, Naidich DP, Ost DE (2007). "Evaluation of Patients with Pulmonary Nodules: When is It Lung Cancer?*".Chest.132 (3_suppl):108S –130S.doi:10.1378/chest.07-1353.PMID17873164.S2CID16449420.
^McDermott M, Jacobs T, Morgenstern L (2017-01-01), Wijdicks EF, Kramer AH (eds.), "Chapter 10 – Critical care in acute ischemic stroke",Handbook of Clinical Neurology, Critical Care Neurology Part I,140, Elsevier:153–176,doi:10.1016/b978-0-444-63600-3.00010-6,PMID28187798
^van der Bijl N, Joemai RM, Geleijns J, Bax JJ, Schuijf JD, de Roos A, Kroft LJ (2010). "Assessment of Agatston Coronary Artery Calcium Score Using Contrast-Enhanced CT Coronary Angiography".American Journal of Roentgenology.195 (6):1299–1305.doi:10.2214/AJR.09.3734.ISSN0361-803X.PMID21098187.
^Miller OF, Kane CJ (September 1999). "Time to stone passage for observed ureteral calculi: a guide for patient education".Journal of Urology.162 (3 Part 1):688–691.doi:10.1097/00005392-199909010-00014.PMID10458343.
^"Ankle Fractures".orthoinfo.aaos.org. American Association of Orthopedic Surgeons. Archived fromthe original on 30 May 2010. Retrieved30 May 2010.
^Buckwalter, Kenneth A., et al. (11 September 2000). "Musculoskeletal Imaging with Multislice CT".American Journal of Roentgenology.176 (4):979–986.doi:10.2214/ajr.176.4.1760979.PMID11264094.
^Ramon A, Bohm-Sigrand A, Pottecher P, Richette P, Maillefert JF, Devilliers H, Ornetti P (2018-03-01). "Role of dual-energy CT in the diagnosis and follow-up of gout: systematic analysis of the literature".Clinical Rheumatology.37 (3):587–595.doi:10.1007/s10067-017-3976-z.ISSN0770-3198.PMID29350330.S2CID3686099.
^P. Babaheidarian, D. Castanon (2018). "Joint reconstruction and material classification in spectral CT". In Greenberg JA, Gehm ME, Neifeld MA, Ashok A (eds.).Anomaly Detection and Imaging with X-Rays (ADIX) III. p. 12.doi:10.1117/12.2309663.ISBN978-1-5106-1775-9.S2CID65469251.
^P. Jin, E. Haneda, C. A. Bouman (November 2012)."Implicit Gibbs prior models for tomographic reconstruction"(PDF).Signals, Systems and Computers (ASILOMAR), 2012 Conference Record of the Forty Sixth Asilomar Conference on. IEEE. pp. 613–636. Archived fromthe original(PDF) on 2015-04-11. Retrieved2015-04-05.
^abReis EP, Nascimento F, Aranha M, Mainetti Secol F, Machado B, Felix M, Stein A, Amaro E (29 July 2020). "Brain Hemorrhage Extended (BHX): Bounding box extrapolation from thick to thin slice CT images v1.1".PhysioNet.101 (23):215–220.doi:10.13026/9cft-hg92.
^Arthur W. Toga, John C. Mazziotta, eds. (2002).Brain mapping: the methods (2nd ed.). Amsterdam: Academic Press.ISBN0-12-693019-8.OCLC52594824.
^Jerrold T. Bushberg, J. Anthony Seibert, Edwin M. Leidholdt, John M. Boone (2002).The essential physics of medical imaging (2nd ed.). Philadelphia, PA: Lippincott Williams & Wilkins. p. 358.ISBN0-683-30118-7.OCLC47177732.
^Dalrymple NC, Prasad SR, Freckleton MW, Chintapalli KN (September 2005). "Informatics in radiology (infoRAD): introduction to the language of three-dimensional imaging with multidetector CT".Radiographics.25 (5):1409–1428.doi:10.1148/rg.255055044.ISSN1527-1323.PMID16160120.
^R. A. Crowther, D. J. DeRosier, A. Klug (1970). "The Reconstruction of a Three-Dimensional Structure from Projections and its Application to Electron Microscopy".Proc. R. Soc. Lond. A.317 (1530):319–340.Bibcode:1970RSPSA.317..319C.doi:10.1098/rspa.1970.0119.S2CID122980366.
^Boas FE, Fleischmann D (2011). "Evaluation of Two Iterative Techniques for Reducing Metal Artifacts in Computed Tomography".Radiology.259 (3):894–902.doi:10.1148/radiol.11101782.PMID21357521.
^abJha D (2014). "Adaptive center determination for effective suppression of ring artifacts in tomography images".Applied Physics Letters.105 (14): 143107.Bibcode:2014ApPhL.105n3107J.doi:10.1063/1.4897441.
^Van de Casteele E, Van Dyck D, Sijbers J, Raman E (2004). "A model-based correction method for beam hardening artefacts in X-ray microtomography".Journal of X-ray Science and Technology.12 (1):43–57.CiteSeerX10.1.1.460.6487.
^abcRehani MM, Yang K, Melick ER, Heil J, Šalát D, Sensakovic WF, Liu B (2020). "Patients undergoing recurrent CT scans: assessing the magnitude".European Radiology.30 (4):1828–1836.doi:10.1007/s00330-019-06523-y.PMID31792585.S2CID208520824.
^abBrambilla M, Vassileva J, Kuchcinska A, Rehani MM (2020). "Multinational data on cumulative radiation exposure of patients from recurrent radiological procedures: call for action".European Radiology.30 (5):2493–2501.doi:10.1007/s00330-019-06528-7.PMID31792583.S2CID208520544.
^abRehani MM, Melick ER, Alvi RM, Doda Khera R, Batool-Anwar S, Neilan TG, Bettmann M (2020). "Patients undergoing recurrent CT exams: assessment of patients with non-malignant diseases, reasons for imaging and imaging appropriateness".European Radiology.30 (4):1839–1846.doi:10.1007/s00330-019-06551-8.PMID31792584.S2CID208520463.
^Rob S, Bryant T, Wilson I, Somani B (2017). "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".Clinical Radiology.72 (1):11–15.doi:10.1016/j.crad.2016.10.005.PMID27810168.
^abcdeDavies HE, Wathen, C. G., Gleeson, F. V. (25 February 2011). "The risks of radiation exposure related to diagnostic imaging and how to minimise them".BMJ.342 (feb25 1): d947.doi:10.1136/bmj.d947.PMID21355025.S2CID206894472.
^Baysson H, Etard C, Brisse HJ, Bernier MO (January 2012). "[Diagnostic radiation exposure in children and cancer risk: current knowledge and perspectives]".Archives de Pédiatrie.19 (1):64–73.doi:10.1016/j.arcped.2011.10.023.PMID22130615.
^abNamasivayam S, Kalra MK, Torres WE, Small WC (Jul 2006). "Adverse reactions to intravenous iodinated contrast media: a primer for radiologists".Emergency Radiology.12 (5):210–5.doi:10.1007/s10140-006-0488-6.PMID16688432.S2CID28223134.
^Hill B, Venning AJ, Baldock C (2005). "A preliminary study of the novel application of normoxic polymer gel dosimeters for the measurement of CTDI on diagnostic X-ray CT scanners".Medical Physics.32 (6):1589–1597.Bibcode:2005MedPh..32.1589H.doi:10.1118/1.1925181.PMID16013718.
^Radon J (1 December 1986). "On the determination of functions from their integral values along certain manifolds".IEEE Transactions on Medical Imaging.5 (4):170–176.doi:10.1109/TMI.1986.4307775.PMID18244009.S2CID26553287.
^Oldendorf WH (1978). "The quest for an image of brain: a brief historical and technical review of brain imaging techniques".Neurology.28 (6):517–33.doi:10.1212/wnl.28.6.517.PMID306588.S2CID42007208.
^Frank Natterer (2001).The Mathematics of Computerized Tomography (Classics in Applied Mathematics). Society for Industrial and Applied Mathematics. p. 8.ISBN978-0-89871-493-7.
^Fred A. Mettler Jr, Mythreyi Bhargavan, Keith Faulkner, Debbie B. Gilley, Joel E. Gray, Geoffrey S. Ibbott, Jill A. Lipoti, Mahadevappa Mahesh, John L. McCrohan, Michael G. Stabin, Bruce R. Thomadsen, Terry T. Yoshizumi (2009). "Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison with Other Radiation Sources — 1950-2007".Radiology.253 (2):520–531.doi:10.1148/radiol.2532082010.PMID19789227.
^Andrew Skelly (Aug 3, 2010). "CT ordering all over the map".The Medical Post.
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