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CT scan

A CT scan or computed tomography scan (formerly computerized axial tomography scan or CAT scan)[2]

makes use of computer-processed combinations of many X-ray measurements taken from different angles to produce cross-sectional (tomographic) images (virtual 'slices') of specific areas of a scanned object, allowing the user to see inside the object without cutting.

Digital geometry processing is used to further generate a three-dimensional volume of the inside of the object from a small series of two-dimensional radiographic images taken around a single axis of rotation.[4]

Although, historically, the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

One study estimated that as many as 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5 to 2% with 2007 rates of CT use;[12]

Side effects from intravenous contrast used in some types of studies include the possibility of exacerbating kidney problems in the setting of pre-existing kidney disease.[15]

It has more recently been used for preventive medicine or screening for disease, for example CT colonography for people with a high risk of colon cancer, or full-motion heart scans for people with high risk of heart disease.

A number of institutions offer full-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 the radiation dose applied.[16]

Of the above, hypodense (dark) structures can indicate edema and infarction, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows.

CT scanning of the head is also used in CT-guided stereotactic surgery and radiosurgery for treatment of intracranial tumors, arteriovenous malformations and other surgically treatable conditions using a device known as the N-localizer.[17][18][19][20][21][22]

Magnetic resonance imaging (MRI) of the head provides superior information as compared to CT scans when seeking information about headache to confirm a diagnosis of neoplasm, vascular disease, posterior cranial fossa lesions, cervicomedullary lesions, or intracranial pressure disorders.[23]

Even in emergency situations, when a head injury is minor as determined by a physician's evaluation and based on established guidelines, CT of the head should be avoided for adults and delayed pending clinical observation in the emergency department for children.[24]

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.[30]

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.[30]

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.[30]

More recently CT has played a key role in the fast evolving field of transcatheter structural heart interventions, more specifically in the transcatheter repair and replacement of heart valves.[33][34][35]

For the axial skeleton and extremities, CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes.

Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task.

The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality.[45]

Early estimates of harm from CT are partly based on similar radiation exposures experienced by those present during the atomic bomb explosions in Japan after the Second World War and those of nuclear industry workers.[12]

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,[52]

International Commission on Radiological Protection estimates that the risk to a fetus being exposed to 10 mGy (a unit of radiation exposure, see Gray (unit)) 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).[56]

Those with severe renal failure requiring dialysis 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;

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 fatal peritonitis.

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.

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.[55]

For purposes of comparison, the world average dose rate from naturally occurring sources of background radiation is 2.4 mSv per year, equal for practical purposes in this application to 2.4 mGy per year.[68]

The radiation dose reported in the gray or mGy unit is proportional to the amount of energy that the irradiated body part is expected to absorb, and the physical effect (such as DNA double strand breaks) on the cells' chemical bonds by X-ray radiation is proportional to that energy.[73]

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.[74]

A specific measure, termed the computed tomography dose index (CTDI), is commonly used as an estimate of the radiation absorbed dose for tissue within the scan region, and is automatically computed by medical CT scanners.[75]

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.

In October, 2009, the US Food and Drug Administration (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based on radiation burns caused by incorrect settings at one particular facility for this particular type of CT scan.

Over 256 patients over an 18-month period were exposed, over 40% lost patches of hair, and prompted the editorial to call for increased CT quality assurance programs, while also noting 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.'[55][78]

In concert with The American Society of Radiologic Technologists, The American College of Radiology and The American 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.[79]

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.

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 of appendicitis).[12]

For example, some healthy people avidly pay to receive full-body CT scans as screening, but it is not at all clear that the benefits outweigh the risks and costs, because deciding whether and how to treat incidentalomas is fraught with complexity, radiation exposure is cumulative and not negligible, and the money for the scans involves opportunity cost (it may have been more effectively spent on more targeted screening or other health care strategies).[58]

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.

However, increased dosage raises the adverse side effects, including the risk of radiation-induced cancer – a four-phase abdominal CT gives the same radiation dose as 300 chest X-rays (See the Scan dose section).

CT scanning has also found an application in transport security (predominantly airport security where it is currently used in a materials analysis context for explosives detection CTX (explosive-detection device)[107][108][109][110]

and is also under consideration for automated baggage/parcel security scanning using computer vision based object recognition algorithms that target the detection of specific threat items based on 3D appearance (e.g.

Computed tomography was originally known as the 'EMI scan' as it was developed in the early 1970s at a research branch of EMI, a company best known today for its music and recording business.

Although the term 'computed tomography' could be used to describe positron emission tomography or single photon emission computed tomography (SPECT), in practice it usually refers to the computation of tomography from X-ray images, especially in older medical literature and smaller medical facilities.

The main limitation of this type 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.

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 the electrons, travelling between the cathode and anode of the X-ray tube, are spun using deflection coils.

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.

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.[122][123]