CT scan

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This article is about X-ray computed tomography as used in medicine. CT scan_sentence_0

For cross-sectional images used in industry, see Industrial computed tomography. CT scan_sentence_1

For means of tomography other than X-ray, see Tomography. CT scan_sentence_2

CT scan_table_infobox_0

CT scanCT scan_header_cell_0_0_0
Other namesCT scan_header_cell_0_1_0 X-ray computed tomography (X-ray CT), computerized axial tomography scan (CAT scan), computer aided tomography, computed tomography scanCT scan_cell_0_1_1
ICD-10-PCSCT scan_header_cell_0_2_0 B?2CT scan_cell_0_2_1
ICD-9-CMCT scan_header_cell_0_3_0 CT scan_cell_0_3_1
MeSHCT scan_header_cell_0_4_0 CT scan_cell_0_4_1
OPS-301 codeCT scan_header_cell_0_5_0 CT scan_cell_0_5_1
MedlinePlusCT scan_header_cell_0_6_0 CT scan_cell_0_6_1

A CT scan or computed tomography scan (formerly known as a computed axial tomography or CAT scan) is a medical imaging technique that uses computer-processed combinations of multiple X-ray measurements taken from different angles to produce tomographic (cross-sectional) images (virtual "slices") of a body, allowing the user to see inside the body without cutting. CT scan_sentence_3

The personnel that perform CT scans are called radiographers or radiologic technologists. CT scan_sentence_4

The 1979 Nobel Prize in Physiology or Medicine was awarded jointly to South African American physicist Allan M. Cormack and British electrical engineer Godfrey N. Hounsfield "for the development of computer assisted tomography." CT scan_sentence_5

Initially, the images generated in CT scans were in the transverse (axial) anatomical plane, perpendicular to the long axis of the body. CT scan_sentence_6

Modern scanners allow the scan data to be reformatted as images in other planes. CT scan_sentence_7

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. CT scan_sentence_8

These cross-sectional images are widely used for medical diagnosis and therapy. CT scan_sentence_9

Use of CT scans has increased dramatically over the last two decades in many countries. CT scan_sentence_10

An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015. CT scan_sentence_11

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 rates of CT use in 2007. CT scan_sentence_12

Others dispute this estimate, as there is no consensus that the low levels of radiation used in CT scans cause damage. CT scan_sentence_13

Lower radiation doses are often used in many areas, such as in the investigation of renal colic. CT scan_sentence_14

Side effects from contrast agents, administered intravenously in some CT scans, might impair kidney performance in patients that have kidney disease. CT scan_sentence_15

Medical use CT scan_section_0

Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. CT scan_sentence_16

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. CT scan_sentence_17

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. CT scan_sentence_18

Head CT scan_section_1

Main article: Computed tomography of the head CT scan_sentence_19

CT scanning of the head is typically used to detect infarction, tumors, calcifications, haemorrhage, and bone trauma. CT scan_sentence_20

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 scan_sentence_21

Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. CT scan_sentence_22

Ambulances equipped with small bore multi-slice CT scanners respond to cases involving stroke or head trauma. CT scan_sentence_23

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. CT scan_sentence_24

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. CT scan_sentence_25

It also does not carry the risks of exposing the patient to ionizing radiation. CT scan_sentence_26

CT scans may be used to diagnose headache when neuroimaging is indicated and MRI is not available, or in emergency settings when hemorrhage, stroke, or traumatic brain injury are suspected. CT scan_sentence_27

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. CT scan_sentence_28

Neck CT scan_section_2

Contrast CT is generally the initial study of choice for neck masses in adults. CT scan_sentence_29

CT of the thyroid plays an important role in the evaluation of thyroid cancer. CT scan_sentence_30

Also, CT scans often incidentally find thyroid abnormalities, and thereby practically becomes the first investigation modality. CT scan_sentence_31

Lungs CT scan_section_3

A CT scan can be used for detecting both acute and chronic changes in the lung parenchyma, the tissue of the lungs. CT scan_sentence_32

It is particularly relevant here because normal two-dimensional X-rays do not show such defects. CT scan_sentence_33

A variety of techniques are used, depending on the suspected abnormality. CT scan_sentence_34

For evaluation of chronic interstitial processes such as emphysema, and fibrosis, thin sections with high spatial frequency reconstructions are used; often scans are performed both on inspiration and expiration. CT scan_sentence_35

This special technique is called high resolution CT that produces a sampling of the lung, and not continuous images. CT scan_sentence_36

Bronchial wall thickening can be seen on lung CTs and generally (but not always) implies inflammation of the bronchi. CT scan_sentence_37

Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23. CT scan_sentence_38

An incidentally found nodule in the absence of symptoms (sometimes referred to as an incidentaloma) may raise concerns that it might represent a tumor, either benign or malignant. CT scan_sentence_39

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. CT scan_sentence_40

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. CT scan_sentence_41

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. CT scan_sentence_42

Angiography CT scan_section_4

Main article: Computed tomography angiography CT scan_sentence_43

Computed tomography angiography (CTA) is contrast CT to visualize the arteries and veins throughout the body. CT scan_sentence_44

This ranges from arteries serving the brain to those bringing blood to the lungs, kidneys, arms and legs. CT scan_sentence_45

An example of this type of exam is CT pulmonary angiogram (CTPA) used to diagnose pulmonary embolism (PE). CT scan_sentence_46

It employs computed tomography and an iodine-based contrast agent to obtain an image of the pulmonary arteries. CT scan_sentence_47

Cardiac CT scan_section_5

A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy. CT scan_sentence_48

Traditionally, cardiac CT scans are used to detect, diagnose, or follow up coronary artery disease. CT scan_sentence_49

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. CT scan_sentence_50

The main forms of cardiac CT scanning are: CT scan_sentence_51

CT scan_unordered_list_0

  • Coronary CT angiography (CTA): the use of CT to assess the coronary arteries of the heart. The subject receives an intravenous injection of radiocontrast, and then the heart is scanned using a high-speed CT scanner, allowing radiologists to assess the extent of occlusion in the coronary arteries, usually in order to diagnose coronary artery disease.CT scan_item_0_0
  • 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 heart attack. 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.CT scan_item_0_1

To better visualize the anatomy, post-processing of the images is common. CT scan_sentence_52

Most common are multiplanar reconstructions (MPR) and volume rendering. CT scan_sentence_53

For more complex anatomies and procedures, such as heart valve interventions, a true 3D reconstruction or a 3D print is created based on these CT images to gain a deeper understanding. CT scan_sentence_54

Abdominal and pelvic CT scan_section_6

Main article: Abdominal and pelvic CT CT scan_sentence_55

CT is an accurate technique for diagnosis of abdominal diseases. CT scan_sentence_56

Its uses include diagnosis and staging of cancer, as well as follow up after cancer treatment to assess response. CT scan_sentence_57

It is commonly used to investigate acute abdominal pain. CT scan_sentence_58

Axial skeleton and extremities CT scan_section_7

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. CT scan_sentence_59

Fractures, ligamentous injuries, and dislocations can easily be recognised with a 0.2 mm resolution. CT scan_sentence_60

With modern dual-energy CT scanners, new areas of use have been established, such as aiding in the diagnosis of gout. CT scan_sentence_61

Geological use CT scan_section_8

X-ray CT is used in geological studies to quickly reveal materials inside a drill core. CT scan_sentence_62

Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images. CT scan_sentence_63

Cultural heritage use CT scan_section_9

X-ray CT and micro-CT can also be used for the conservation and preservation of objects of cultural heritage. CT scan_sentence_64

For many fragile objects, direct research and observation can be damaging and can degrade the object over time. CT scan_sentence_65

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. CT scan_sentence_66

These scans have been optimal for research focused on the workings of the Antikythera mechanism or the text hidden inside the charred outer layers of the En-Gedi Scroll. CT scan_sentence_67

However, they are not optimal for every object subject to these kinds of research questions, as there are certain artifacts like the Herculaneum papyri in which the material composition has very little variation along the inside of the object. CT scan_sentence_68

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 the En-Gedi scroll and the Herculaneum papyri. CT scan_sentence_69

Advantages CT scan_section_10

CT scanning has several advantages over traditional two-dimensional medical radiography. CT scan_sentence_70

First, CT eliminates the superimposition of images of structures outside the area of interest. CT scan_sentence_71

Second, CT scans have greater image resolution, enabling examination of finer details. CT scan_sentence_72

CT can distinguish between tissues that differ in radiographic density by 1% or less. CT scan_sentence_73

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. CT scan_sentence_74

The improved resolution of CT has permitted the development of new investigations. CT scan_sentence_75

For example, CT angiography avoids the invasive insertion of a catheter. CT scan_sentence_76

CT scanning can perform a virtual colonoscopy with greater accuracy and less discomfort for the patient than a traditional colonoscopy. CT scan_sentence_77

Virtual colonography is far more accurate than a barium enema for detection of tumors and uses a lower radiation dose. CT scan_sentence_78

CT VC is increasingly being used in the UK and US as a screening test for colon polyps and colon cancer and can negate the need for a colonoscopy in some cases. CT scan_sentence_79

CT is a moderate- to high-radiation diagnostic technique. CT scan_sentence_80

The radiation dose for a particular examination depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. CT scan_sentence_81

Two helical CT scanning parameters, tube current and pitch, can be adjusted easily and have a profound effect on radiation. CT scan_sentence_82

CT scanning is more accurate than two-dimensional radiographs in evaluating anterior interbody fusion, although they may still over-read the extent of fusion. CT scan_sentence_83

Adverse effects CT scan_section_11

Cancer CT scan_section_12

Further information: Radiobiology CT scan_sentence_84

The radiation used in CT scans can damage body cells, including DNA molecules, which can lead to radiation-induced cancer. CT scan_sentence_85

The radiation doses received from CT scans is variable. CT scan_sentence_86

Compared to the lowest dose x-ray techniques, CT scans can have 100 to 1,000 times higher dose than conventional X-rays. CT scan_sentence_87

However, a lumbar spine x-ray has a similar dose as a head CT. CT scan_sentence_88

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. CT scan_sentence_89

In general, the radiation dose associated with a routine abdominal CT has a radiation dose similar to three years average background radiation. CT scan_sentence_90

Recent studies 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. CT scan_sentence_91

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." CT scan_sentence_92

On the other hand, a recent paper analyzing the data of patients who received high cumulative doses showed a high degree of appropriate use. CT scan_sentence_93

This creates an important issue of cancer risk to these patients. CT scan_sentence_94

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. CT scan_sentence_95

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. CT scan_sentence_96

Some experts project that in the future, between three and five percent of all cancers would result from medical imaging. CT scan_sentence_97

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. CT scan_sentence_98

In this group, one in every 1,800 CT scans was followed by an excess cancer. CT scan_sentence_99

If the lifetime risk of developing cancer is 40% then the absolute risk rises to 40.05% after a CT. CT scan_sentence_100

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. CT scan_sentence_101

A person's age plays a significant role in the subsequent risk of cancer. CT scan_sentence_102

Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1% or 1:1000 scans. CT scan_sentence_103

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. CT scan_sentence_104

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). CT scan_sentence_105

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 scan_sentence_106

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. CT scan_sentence_107

Furthermore, certain conditions can require children to be exposed to multiple CT scans. CT scan_sentence_108

Studies support informing parents of the risks of pediatric CT scanning. CT scan_sentence_109

Contrast reactions CT scan_section_13

Further information: Iodinated contrast § Adverse effects CT scan_sentence_110

In the United States half of CT scans are contrast CTs using intravenously injected radiocontrast agents. CT scan_sentence_111

The most common reactions from these agents are mild, including nausea, vomiting and an itching rash; however, more severe reactions may occur. CT scan_sentence_112

Overall reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast. CT scan_sentence_113

Skin rashes may appear within a week to 3% of people. CT scan_sentence_114

The old radiocontrast agents caused anaphylaxis in 1% of cases while the newer, lower-osmolar agents cause reactions in 0.01–0.04% of cases. CT scan_sentence_115

Death occurs in about two to 30 people per 1,000,000 administrations, with newer agents being safer. CT scan_sentence_116

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. CT scan_sentence_117

The contrast agent may induce contrast-induced nephropathy. CT scan_sentence_118

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. CT scan_sentence_119

People with mild kidney impairment are usually advised to ensure full hydration for several hours before and after the injection. CT scan_sentence_120

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. CT scan_sentence_121

In addition to the use of intravenous contrast, orally administered contrast agents are frequently used when examining the abdomen. CT scan_sentence_122

These are frequently the same as the intravenous contrast agents, merely diluted to approximately 10% of the concentration. CT scan_sentence_123

However, oral alternatives to iodinated contrast exist, such as very dilute (0.5–1% w/v) barium sulfate suspensions. CT scan_sentence_124

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. CT scan_sentence_125

Process CT scan_section_14

Main article: Operation of computed tomography CT scan_sentence_126

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. CT scan_sentence_127

A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation. CT scan_sentence_128

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. CT scan_sentence_129

Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. CT scan_sentence_130

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. CT scan_sentence_131

Pixel is a two dimensional unit based on the matrix size and the field of view. CT scan_sentence_132

When the CT slice thickness is also factored in, the unit is known as a voxel, which is a three-dimensional unit. CT scan_sentence_133

The phenomenon that one part of the detector cannot differentiate between different tissues is called the partial volume effect. CT scan_sentence_134

This means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone. CT scan_sentence_135

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. CT scan_sentence_136

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. CT scan_sentence_137

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. CT scan_sentence_138

Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. CT scan_sentence_139

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. CT scan_sentence_140

This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients. CT scan_sentence_141

CT data sets have a very high dynamic range which must be reduced for display or printing. CT scan_sentence_142

This is typically done via a process of "windowing", which maps a range (the "window") of pixel values to a grayscale ramp. CT scan_sentence_143

For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU. CT scan_sentence_144

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 grey intensity proportional to position within the window. CT scan_sentence_145

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. CT scan_sentence_146

Contrast CT scan_section_15

Main article: Contrast CT CT scan_sentence_147

Contrast media used for X-ray CT, as well as for plain film X-ray, are called radiocontrasts. CT scan_sentence_148

Radiocontrasts for X-ray CT are, in general, iodine-based. CT scan_sentence_149

This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. CT scan_sentence_150

Using contrast material can also help to obtain functional information about tissues. CT scan_sentence_151

Often, images are taken both with and without radiocontrast. CT scan_sentence_152

Scan dose CT scan_section_16

CT scan_table_general_1

ExaminationCT scan_header_cell_1_0_0 Typical effective dose (mSv)
to the whole bodyCT scan_header_cell_1_0_1
Typical absorbed dose (mGy)
to the organ in questionCT scan_header_cell_1_0_2
Annual background radiationCT scan_cell_1_1_0 2.4CT scan_cell_1_1_1 2.4CT scan_cell_1_1_2
Chest X-rayCT scan_cell_1_2_0 0.02CT scan_cell_1_2_1 0.01–0.15CT scan_cell_1_2_2
Head CTCT scan_cell_1_3_0 1–2CT scan_cell_1_3_1 56CT scan_cell_1_3_2
Screening mammographyCT scan_cell_1_4_0 0.4CT scan_cell_1_4_1 3CT scan_cell_1_4_2
Abdomen CTCT scan_cell_1_5_0 8CT scan_cell_1_5_1 14CT scan_cell_1_5_2
Chest CTCT scan_cell_1_6_0 5–7CT scan_cell_1_6_1 13CT scan_cell_1_6_2
CT colonographyCT scan_cell_1_7_0 6–11CT scan_cell_1_7_1 CT scan_cell_1_7_2
Chest, abdomen and pelvis CTCT scan_cell_1_8_0 9.9CT scan_cell_1_8_1 12CT scan_cell_1_8_2
Cardiac CT angiogramCT scan_cell_1_9_0 9–12CT scan_cell_1_9_1 40–100CT scan_cell_1_9_2
Barium enemaCT scan_cell_1_10_0 15CT scan_cell_1_10_1 15CT scan_cell_1_10_2
Neonatal abdominal CTCT scan_cell_1_11_0 20CT scan_cell_1_11_1 20CT scan_cell_1_11_2
Further information: Template:Effective dose by medical imaging typeCT scan_cell_1_12_0

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. CT scan_sentence_153

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. CT scan_sentence_154

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. CT scan_sentence_155

While there is some variation, most people (99%) received less than 7 mSv per year as background radiation. CT scan_sentence_156

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. CT scan_sentence_157

In the United Kingdom it accounts for 15% of radiation exposure. CT scan_sentence_158

The average radiation dose from medical sources is ≈0.6 mSv per person globally as of 2007. CT scan_sentence_159

Those in the nuclear industry in the United States are limited to doses of 50 mSv a year and 100 mSv every 5 years. CT scan_sentence_160

Lead is the main material used by radiography personnel for shielding against scattered X-rays. CT scan_sentence_161

Radiation dose units CT scan_section_17

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. CT scan_sentence_162

The sievert unit is used in the report of the effective dose. CT scan_sentence_163

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. CT scan_sentence_164

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. CT scan_sentence_165

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. CT scan_sentence_166

The equivalent 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. CT scan_sentence_167

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. CT scan_sentence_168

Effects of radiation CT scan_section_18

Further information: Radiobiology CT scan_sentence_169

Most adverse health effects of radiation exposure may be grouped in two general categories: CT scan_sentence_170

CT scan_unordered_list_1

  • deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; andCT scan_item_1_2
  • 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.CT scan_item_1_3

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. CT scan_sentence_171

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. CT scan_sentence_172

Excess doses CT scan_section_19

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. CT scan_sentence_173

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." CT scan_sentence_174

Similar problems have been reported at other centers. CT scan_sentence_175

These incidents are believed to be due to human error. CT scan_sentence_176

Campaigns CT scan_section_20

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. CT scan_sentence_177

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. CT scan_sentence_178

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. CT scan_sentence_179

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. CT scan_sentence_180

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. CT scan_sentence_181

Prevalence CT scan_section_21

Use of CT has increased dramatically over the last two decades. CT scan_sentence_182

An estimated 72 million scans were performed in the United States in 2007. CT scan_sentence_183

Of these, six to eleven percent are done in children, an increase of seven to eightfold from 1980. CT scan_sentence_184

Similar increases have been seen in Europe and Asia. CT scan_sentence_185

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. CT scan_sentence_186

The percentage who received CT, however, varied markedly by the emergency physician who saw them from 1.8% to 25%. CT scan_sentence_187

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). CT scan_sentence_188

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. CT scan_sentence_189

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). CT scan_sentence_190

As of 2007 in the United States a proportion of CT scans are performed unnecessarily. CT scan_sentence_191

Some estimates place this number at 30%. CT scan_sentence_192

There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public. CT scan_sentence_193

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). CT scan_sentence_194

Presentation CT scan_section_22

The result of a CT scan is a volume of voxels, which may be presented to a human observer by various methods, which broadly fit into the following categories: CT scan_sentence_195

CT scan_unordered_list_2

  • Thin slice. This is generally regarded as planes representing a thickness of less than 3 mm.CT scan_item_2_4
  • Projection, including maximum intensity projection and average intensity projectionCT scan_item_2_5
  • Volume rendering (VR)CT scan_item_2_6

Technically, all volume renderings become projections when viewed on a 2-dimensional display, making the distinction between projections and volume renderings a bit vague. CT scan_sentence_196

Still, the epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations. CT scan_sentence_197

Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. CT scan_sentence_198

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. CT scan_sentence_199

This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients. CT scan_sentence_200

Grayscale CT scan_section_23

Multiplanar reconstruction and projections CT scan_section_24

Multiplanar reconstruction (MPR) is the creation of slices in more anatomical planes than the one (usually transverse) used for initial tomography acquisition. CT scan_sentence_201

It can be used for thin slices as well as projections. CT scan_sentence_202

Multiplanar reconstruction is feasible because contemporary CT scanners offer isotropic or near isotropic resolution. CT scan_sentence_203

MPR is frequently used for examining the spine. CT scan_sentence_204

Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. CT scan_sentence_205

By reformatting the volume, it becomes much easier to visualise the position of one vertebral body in relation to the others. CT scan_sentence_206

Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. CT scan_sentence_207

This may be particularly useful for visualization of the structure of the bronchi as these do not lie orthogonal to the direction of the scan. CT scan_sentence_208

For vascular imaging, curved-plane reconstruction can be performed. CT scan_sentence_209

This allows bends in a vessel to be "straightened" so that the entire length can be visualised on one image, or a short series of images. CT scan_sentence_210

Once a vessel has been "straightened" in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned. CT scan_sentence_211

CT scan_table_general_2

Examples of different algorithms of thickening multiplanar reconstructionsCT scan_table_caption_2
Type of projectionCT scan_header_cell_2_0_0 Schematic illustrationCT scan_header_cell_2_0_1 Examples (10 mm slabs)CT scan_header_cell_2_0_2 DescriptionCT scan_header_cell_2_0_3
Average intensity projection (AIP)CT scan_cell_2_1_0 CT scan_cell_2_1_1 CT scan_cell_2_1_2 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 conventional projectional radiography as slice thickness increases.CT scan_cell_2_1_3
Maximum intensity projection (MIP)CT scan_cell_2_2_0 CT scan_cell_2_2_1 CT scan_cell_2_2_2 The voxel with the highest attenuation is displayed. Therefore, high-attenuating structures such as blood vessels filled with contrast media are enhanced. May be used for angiographic studies and identification of pulmonary nodules.CT scan_cell_2_2_3
Minimum intensity projection (MinIP)CT scan_cell_2_3_0 CT scan_cell_2_3_1 CT scan_cell_2_3_2 The voxel with the lowest attenuation is displayed. Therefore, low-attenuating structures such as air spaces are enhanced. May be used for assessing the lung parenchyma.CT scan_cell_2_3_3

Volume rendering CT scan_section_25

Main article: Volume rendering CT scan_sentence_212

A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). CT scan_sentence_213

From this, a three-dimensional model can be constructed using edge detection image processing algorithms and displayed on screen. CT scan_sentence_214

Multiple models can be constructed from various thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. CT scan_sentence_215

However, the interior structure of each element is not visible in this mode of operation. CT scan_sentence_216

Surface rendering is limited in that it will display only surfaces that meet a threshold density, and will display only the surface that is closest to the imaginary viewer. CT scan_sentence_217

In volume rendering, transparency, colors and shading are used to allow a better representation of the volume to be shown in a single image. CT scan_sentence_218

For example, the bones of the pelvis could be displayed as semi-transparent, so that, even at an oblique angle, one part of the image does not conceal another. CT scan_sentence_219

Image quality CT scan_section_26

Artifacts CT scan_section_27

Although images produced by CT are generally faithful representations of the scanned volume, the technique is susceptible to a number of artifacts, such as the following: CT scan_sentence_220

CT scan_description_list_3

  • Streak artifact: Streaks are often seen around materials that block most X-rays, such as metal or bone. Numerous factors contribute to these streaks: undersampling, photon starvation, motion, beam hardening, and Compton 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 or approaches such as metal artifact reduction (MAR). MAR techniques include spectral imaging, where CT images are taken with photons of different energy levels, and then synthesized into monochromatic images with special software such as GSI (Gemstone Spectral Imaging).CT scan_item_3_7

CT scan_description_list_4

  • 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). 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, due to the conventional use of highly anisotropic 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.CT scan_item_4_8

CT scan_description_list_5

  • 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. Ring artefacts 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.CT scan_item_5_9

CT scan_description_list_6

  • 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.CT scan_item_6_10

CT scan_description_list_7

  • Windmill: Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.CT scan_item_7_11

CT scan_description_list_8

  • 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 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.CT scan_item_8_12

Dose versus image quality CT scan_section_28

An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. CT scan_sentence_221

In general, higher radiation doses result in higher-resolution images, while lower doses lead to increased image noise and unsharp images. CT scan_sentence_222

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 scan_sentence_223

Several methods that can reduce the exposure to ionizing radiation during a CT scan exist. CT scan_sentence_224

CT scan_ordered_list_9

  1. New software technology can significantly reduce the required radiation dose. New iterative tomographic reconstruction algorithms (e.g., iterative Sparse Asymptotic Minimum Variance) could offer superresolution without requiring higher radiation dose.CT scan_item_9_13
  2. 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.CT scan_item_9_14
  3. Prior to every CT examination, evaluate the appropriateness of the exam whether it is motivated or if another type of examination is more suitable. Higher resolution is not always suitable for any given scenario, such as detection of small pulmonary masses.CT scan_item_9_15

Industrial use CT scan_section_29

Industrial CT scanning (industrial computed tomography) is a process which utilizes X-ray equipment to produce 3D representations of components both externally and internally. CT scan_sentence_225

Industrial CT scanning has been utilized in many areas of industry for internal inspection of components. CT scan_sentence_226

Some of the key uses for CT scanning have been flaw detection, failure analysis, metrology, assembly analysis, image-based finite element methods and reverse engineering applications. CT scan_sentence_227

CT scanning is also employed in the imaging and conservation of museum artifacts. CT scan_sentence_228

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) 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. guns, knives, liquid containers). CT scan_sentence_229

History CT scan_section_30

Main article: History of computed tomography CT scan_sentence_230

The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of the Radon transform. CT scan_sentence_231

In October 1963, William Henry Oldendorf received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material". CT scan_sentence_232

The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in 1972. CT scan_sentence_233

Etymology CT scan_section_31

The word "tomography" is derived from the Greek tome (slice) and graphein (to write). CT scan_sentence_234

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. CT scan_sentence_235

It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography. CT scan_sentence_236

The term "CAT scan" is not used anymore, since CT scans nowadays allow for multiplanar reconstructions. CT scan_sentence_237

This makes "CT scan" the most appropriate term, which is used by Radiologists in common vernacular as well as in any textbook and any scientific paper. CT scan_sentence_238

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. CT scan_sentence_239

In MeSH, "computed axial tomography" was used from 1977 to 1979, but the current indexing explicitly includes "X-ray" in the title. CT scan_sentence_240

The term was introduced by Paul Edholm and Bertil Jacobson in 1975. CT scan_sentence_241

Types of machines CT scan_section_32

Spinning tube, commonly called spiral CT, or helical CT is an imaging technique in which an entire X-ray tube is spun around the central axis of the area being scanned. CT scan_sentence_242

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. CT scan_sentence_243

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. CT scan_sentence_244

Some designs use two X-ray sources and detector arrays offset by an angle, as a technique to improve temporal resolution. CT scan_sentence_245

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. CT scan_sentence_246

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. CT scan_sentence_247

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. CT scan_sentence_248

Only one manufacturer (Imatron, later acquired by General Electric) ever produced scanners of this design. CT scan_sentence_249

Production ceased in early 2006. CT scan_sentence_250

In multislice computed tomography (MSCT) or multidetector computed tomography (MDCT), a higher number of tomographic slices allow for higher-resolution imaging. CT scan_sentence_251

Modern CT machines typically generate 64-640 slices per scan. CT scan_sentence_252

Manufacturers CT scan_section_33

Major manufacturers of CT Scanners Devices and Equipment are: CT scan_sentence_253

CT scan_unordered_list_10

Research directions CT scan_section_34

Photon counting computed tomography is a CT technique currently under development. CT scan_sentence_254

Typical CT scanners use energy integrating detectors; photons are measured as a voltage on a capacitor which is proportional to the x-rays detected. CT scan_sentence_255

However, this technique is susceptible to noise and other factors which can affect the linearity of the voltage to x-ray intensity relationship. CT scan_sentence_256

Photon counting detectors (PCDs) are still affected by noise but it does not change the measured counts of photons. CT scan_sentence_257

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. CT scan_sentence_258

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. CT scan_sentence_259

As of February 2016 photon counting CT is in use at three sites. CT scan_sentence_260

Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising. CT scan_sentence_261

In view of recent findings of high cumulative doses to patients from recurrent CT scans, there has been a push for sub-mSv CT scans, a goal that has been lingering CT scan_sentence_262

See also CT scan_section_35

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/CT scan.