partially overcome by scanning using thinner slices, or an isotropic acquisition on a modern scanner. ring artifactprobably 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. ring artifacts can largely be reduced by intensity normalization, also referred to as flat field correction. remaining rings can be suppressed by a transformation to polar space, where they become linear stripes. 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. noisethis 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. windmillstreaking appearances can occur when the detectors intersect the reconstruction plane. this can be reduced with filters or a reduction in pitch. beam hardeningthis can give a "cupped appearance" when grayscale is visualized as height. it occurs because conventional sources, like x-ray tubes emit a polychromatic spectrum. photons of higher photon energy levels are typically attenuated less. because of this, the mean energy of the spectrum increases when passing the object, often described as getting "harder". this leads to an effect increasingly underestimating material thickness, if not corrected. many algorithms exist to correct for this artifact. they can be divided in mono- and multi-material methods. advantages ct scanning has several advantages over traditional two-dimensional medical radiography. first, ct eliminates the superimposition of images of structures outside the area of interest. second, ct scans have greater image resolution, enabling examination of finer details. ct can distinguish between tissues that differ in radiographic density by 1% or less. third, ct scanning enables multiplanar reformatted imaging: scan data can be visualized in the transverse (or axial), coronal, or sagittal plane, depending on the diagnostic task. the improved resolution of ct has permitted the development of new investigations. for example, ct angiography avoids the invasive insertion of a catheter. ct scanning can perform a virtual colonoscopy with greater accuracy and less discomfort for the patient than a traditional colonoscopy. virtual colonography is far more accurate than a barium enema for detection of tumors and uses a lower radiation dose. ct is a moderate-to-high radiation 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. 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. adverse effects cancer main article: radiation-induced cancer the radiation used in ct scans can damage body cells, including dna molecules, which can lead to radiation-induced cancer. 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. however, a lumbar spine x-ray has a similar dose as a head ct. 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 average background radiation. studies published in 2020 on 2.5 million patients and 3.2 million patients have drawn attention to high cumulative doses of more than 100 msv to patients undergoing recurrent ct scans within a short time span of 1 to 5 years. 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." on the other hand, a recent paper analyzing the data of patients who received high cumulative doses showed a high degree of appropriate use. 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, which counteracts existing criticisms some investigators may have on the effects of protracted versus acute exposure. 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. some experts project that in the future, between three and five percent of all cancers would result from medical imaging. 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. 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, concluding that no evidence indicates such low doses cause any long-term harm. 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. others dispute this estimate, 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. a person's age plays a significant role in the subsequent risk of cancer. estimated lifetime cancer mortality risks from an abdominal ct of a one-year-old is 0.1%, or 1:1000 scans. 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. the international commission on radiological protection estimates that the risk to a fetus being exposed to 10 mgy (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). 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. 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. furthermore, certain conditions can require children to be exposed to multiple ct scans. current evidence suggests informing parents of the risks of pediatric ct scanning. contrast reactions further information: iodinated contrast § adverse effects in the united states half of ct scans are contrast cts using intravenously injected radiocontrast agents. the most common reactions from these agents are mild, including nausea, vomiting, and an itching rash. severe life-threatening reactions may rarely occur. overall reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast. skin rashes may appear within a week to 3% of people. the old radiocontrast agents caused anaphylaxis in 1% of cases while the newer, low-osmolar agents cause reactions in 0.01–0.04% of cases. death occurs in about 2 to 30 people per 1,000,000 administrations, with newer agents being safer. there is a higher risk of mortality in those who are female, elderly or in poor health, usually secondary to either anaphylaxis or acute kidney injury. the contrast agent may induce contrast-induced nephropathy. this occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting kidney failure, preexisting diabetes, 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 of iodinated contrast should be avoided; this may mean using an alternative technique instead of ct. those with severe kidney 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; 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. 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 fatal peritonitis. side effects from contrast agents, administered intravenously in some ct scans, might impair kidney performance in patients with kidney disease, although this risk is now believed to be lower than previously thought. scan dose examination typical effective dose (msv) to the whole body typical absorbed dose (mgy) to the organ in question annual background radiation 2.4 2.4 chest x-ray 0.02 0.01–0.15 head ct 1–2 56 screening mammography 0.4 3 abdominal ct 8 14 chest ct 5–7 13 ct colonography 6–11 chest, abdomen and pelvis ct 9.9 12 cardiac ct angiogram 9–12 40–100 barium enema 15 15 neonatal abdominal ct 20 20 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. 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. 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. while there is some variation, most people (99%) received less than 7 msv per year as background radiation. 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. in the united kingdom it accounts for 15% of radiation exposure. the average radiation dose from medical sources is ≈0.6 msv per person globally as of 2007. those in the nuclear industry in the united states are limited to doses of 50 msv a year and 100 msv every 5 years. lead is the main material used by radiography personnel for shielding against scattered x-rays. radiation dose units 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. the sievert unit is used in the report of the effective 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. 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 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. the equivalent dose is the effective dose of a case, in which the whole body would actually