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. effects of radiation further information: radiobiology 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; 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. 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. because of increased susceptibility of fetuses to radiation exposure, the radiation dosage of a ct scan is an important consideration in the choice of medical imaging in pregnancy. excess doses 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 were exposed to radiations for over 18-month period. over 40% of them lost patches of hair, and prompted the editorial to 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." similar problems have been reported at other centers. these incidents are believed to be due to human error. procedure ct scan procedure varies according to the type of the study and the organ being imaged. the patient is made to lie 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 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. preparation patient preparation may vary according to the type of scan. the general patient preparation includes. signing the informed consent. removal of metallic objects and jewelry from the region of interest. changing to the hospital gown according to hospital protocol. checking of kft rate (in case of cect) mechanism ct scanner with cover removed to show internal components. legend: t: x-ray tube d: x-ray detectors x: x-ray beam r: gantry rotation left image is a sinogram which is a graphic representation of the raw data obtained from a ct scan. at right is an image sample derived from the raw data. main article: operation of computed tomography computed tomography operates by using an x-ray generator that rotates around the object; x-ray detectors are positioned on the opposite side of the circle from the x-ray source. as the x-rays pass through the patient, they are attenuated differently by various tissues according to the tissue density. a visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation. once the scan data has been acquired, the data must be processed using a form of tomographic reconstruction, which produces a series of cross-sectional images. these cross-sectional images are made up of small units of pixels or voxels. pixels in an image obtained by ct scanning are displayed in terms of relative radiodensity. the pixel itself is displayed according to the mean attenuation of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on the hounsfield scale. a pixel is a two dimensional unit based on the matrix size and the field of view. when the ct slice thickness is also factored in, the unit is known as a voxel, which is a three-dimensional unit. water has an attenuation of 0 hounsfield 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 cause artifacts. 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. hence, the left side of the image is to the patient's right and vice versa, while anterior in the image also is the patient's anterior and vice versa. this left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients. initially, the images generated in ct scans were in the transverse (axial) anatomical plane, perpendicular to the long axis of the body. modern scanners allow the scan data to be reformatted as images in other planes. digital geometry processing can generate a three-dimensional image of an object inside the body from a series of two-dimensional radiographic images taken by rotation around a fixed axis. these cross-sectional images are widely used for medical diagnosis and therapy. contrast main article: contrast ct contrast media used for x-ray ct, as well as for plain film x-ray, are called radiocontrasts. radiocontrasts for ct are, in general, iodine-based. 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. history main article: history of computed tomography the history of x-ray computed tomography goes back to at least 1917 with the mathematical theory of the radon transform. 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". the first commercially viable ct scanner was invented by godfrey hounsfield in 1972. etymology the word "tomography" is derived from the greek tome (slice) and graphein (to write). 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. it was later known as computed axial tomography (cat or ct scan) and body section röntgenography. the term "cat scan" is no longer used because current ct scans enable for multiplanar reconstructions. this makes "ct scan" the most appropriate term, which is used by radiologists in common vernacular as well as in textbooks and scientific papers. in medical subject headings (mesh), "computed axial tomography" was used from 1977 to 1979, but the current indexing explicitly includes "x-ray" in the title. the term sinogram was introduced by paul edholm and bertil jacobson in 1975. society and culture campaigns 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 the society for pediatric radiology. 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. 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 the image 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 called image wisely. the world health organization and international atomic energy agency (iaea) of the united nations have also been working in this area and have ongoing projects designed to broaden best practices and lower patient radiation dose. prevalence number of ct scanners by country (oecd)as of 2017(per million population) countryvalue japan111.49 australia64.35 iceland43.68 united states42.64 denmark39.72 switzerland39.28 latvia39.13 south korea38.18 germany35.13 italy34.71 greece34.22 austria28.64 finland24.51 chile24.27 lithuania23.33 ireland19.14 spain18.59 estonia18.22 france17.36 slovakia17.28 poland16.88 luxembourg16.77 new zealand16.69 czech republic15.76 canada15.28 slovenia15.00 turkey14.77 netherlands13.48 russia13.00 israel9.53 hungary9.19 mexico5.83 colombia1.24 use of ct has increased dramatically over the last two decades. an estimated 72 million scans were performed in the united states in 2007, accounting for close to half of the total per-capita dose rate from radiologic and nuclear medicine procedures. of the ct scans, six to eleven percent are done in children, an increase of seven to eightfold from 1980. similar increases have been seen in europe and asia. 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 the emergency physician who saw them from 1.8% to 25%. in the emergency department in the united states, ct or mri imaging is done in 15% of people who present with injuries as of 2007 (up from 6% in 1998). 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). as of 2007, in the united states a proportion of ct scans are performed unnecessarily. some estimates place this number at 30%. there are a number of reasons for this including: legal concerns, financial incentives, and desire by the public. for example, some healthy people avidly pay to receive full-body ct scans as screening. in that case, it is not at all clear that the benefits outweigh the risks and costs. deciding whether and how to treat incidentalomas is complex, radiation exposure is not negligible, and the money for the scans involves opportunity cost. manufacturers major manufacturers of ct scanners devices and equipment are: ge healthcare siemens healthineers canon medical systems corporation (formerly toshiba medical systems) koninklijke philips n.v. fujifilm healthcare (formerly hitachi medical systems) neusoft medical systems united imaging healthcare research photon-counting computed tomography is a ct technique currently under development. 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. 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. 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. some early research has found the dose reduction potential of photon counting ct for breast imaging to be very promising. 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.