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. The personnel that perform CT scans are called radiographers or radiologic technologists.[2][3]
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."[4]
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.[5] These cross-sectional images are widely used for medical diagnosis and therapy.[6]
Use of CT scans has increased dramatically over the last two decades in many countries.[7] An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015.[8][9]
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.[10] Others dispute this estimate,[11] as there is no consensus that the low levels of radiation used in CT scans cause damage. Lower radiation doses are often used in many areas, such as in the investigation of renal colic.[12]
Side effects from contrast agents, administered intravenously in some CT scans, might impair kidney performance in patients that have kidney disease.[13]
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.[5] These cross-sectional images are widely used for medical diagnosis and therapy.[6]
Use of CT scans has increased dramatically over the last two decades in many countries.[7] An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015.[8][9]
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.[10] Others dispute this estimate,[11] as there is no consensus that the low levels of radiation used in CT scans cause damage. Lower radiation doses are often used in many areas, such as in the investigation of renal colic.[12]
Side effects from contrast agents, administered intravenously in some CT scans, might impair kidney performance in patients that have kidney disease.[13]
Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. 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.[14]
CT scanning of the head is typically used to detect infarction, tumors, calcifications, haemorrhage, and bone trauma. 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. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. Ambulances equipped with small bore multi-slice CT scanners respond to cases involving stroke or head trauma. 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.[15][16][17][18][19][20]
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.[21] It also does not carry the risks of exposing the patient to ionizing radiation.[21] 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.[21] 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.[22]
Contrast CT is generally the initial study of choice for neck masses in adults.[23] CT of the thyroid plays an important role in the evaluation of thyroid cancer.[24] Also, CT scans often incidentally find thyroid abnormalities, and thereby practically becomes the first investigation modality.infarction, tumors, calcifications, haemorrhage, and bone trauma. 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. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. Ambulances equipped with small bore multi-slice CT scanners respond to cases involving stroke or head trauma. 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.[15][16][17][18][19][20]
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.[21] It also does not carry the risks of exposing the patient to ionizing radiation.[21] 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.[21] 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.[22]
Bronchial wall thickening can be seen on lung CTs and generally (but not always) implies inflammation of the bronchi.[25] Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23.[26]
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 Bronchial wall thickening can be seen on lung CTs and generally (but not always) implies inflammation of the bronchi.[25] Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23.[26]
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.[27] 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.[28] 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.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.[27] 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.[28] 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.[28] 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.[28]
Computed tomography angiography (CTA) is contrast CT to visualize the arteries and veins throughout the body. This ranges from arteries serving the brain to those bringing blood to the lungs, kidneys, arms and legs. An example of this type of exam is CT pulmonary angiogram (CTPA) used to diagnose pulmonary embolism (PE). It employs computed tomography and an iodine-based contrast agent to obtain an image of the pulmonary arteries.
A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy.[29] Traditionally, cardiac CT scans are used to detect, diagnose, or follow up coronary artery disease.[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.[31][32][33]
The main forms of cardiac CT scanning are:
A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy.[29] Traditionally, cardiac CT scans are used to detect, diagnose, or follow up coronary artery disease.[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.[31][32][33]
The main forms of cardiac CT scanning are:
The main forms of cardiac CT scanning are:
To better visualize the anatomy, post-processing of the images is common.[30] Most common are multiplanar reconstructions (MPR) and volume rendering. 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.[36][37][38][39]
CT is an accurate technique for diagnosis of abdominal diseases. Its uses include diagnosis and staging of cancer, as well as follow up after cancer treatment to assess response. It is commonly used to investigate acute abdominal pain.
For the axial skeleton and extremities, CT is often used to image complex fractures, especially ones around joints,
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. Fractures, ligamentous injuries, and dislocations can easily be recognised with a 0.2 mm resolution.[40][41] With modern dual-energy CT scanners, new areas of use have been established, such as aiding in the diagnosis of gout.[42]
X-ray CT is used in geological studies to quickly reveal materials inside a drill core.[43] Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images.
X-ray CT and micro-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 the Antikythera mechanism or the text hidden inside the charred outer layers of the En-Gedi Scroll. However, they are not optimal for every object subject to these kinds of research questions, as there are ce
X-ray CT is used in geological studies to quickly reveal materials inside a drill core.[43] Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images.
CT scanning has several advant
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.[citation needed] Second, CT scans have greater image resolution, enabling examination of finer details.[citation needed] CT can distinguish between tissues that differ in radiographic density by 1% or less.[citation needed] 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.[citation needed]
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 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.[45][46] Virtual colonography is far more accurate than a barium enema for detection of tumors and uses a lower radiation dose.[citation needed] 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 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 sequences, and desired resolution and image quality.[47] 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.[48]
The radiation used in CT scans can damage body cells, including DNA molecules, which can lead to radiation-induced cancer.[10] 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.[49] However, a lumbar spine x-ray has a similar dose as a head CT.[50] 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, the radiation dose associated with a routine abdominal CT has a radiation dose similar to three years average background radiation.[51]
Recent studies on 2.5 million patients[52] and 3.2 million patients[53] 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."[49] On the other hand, a recent paper analyzing the data of patients who received high Recent studies on 2.5 million patients[52] and 3.2 million patients[53] 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."[49] On the other hand, a recent paper analyzing the data of patients who received high cumulative doses showed a high degree of appropriate use.[54] 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.[55], 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.[10] Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.[49]
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.[56][57]
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,[58] concluding that no evidence indicates such low doses cause any long-term harm.[59][60]
A person's age plays a significant role in the subsequent risk of cancer.[61] Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1% or 1:1000 scans.[61] 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.[61] 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).[62] 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.[63]
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.[64] Furthermore, certain conditions can require children to be exposed to multiple CT scans.[10] Studies support informing parents of the risks of pediatric CT scanning.[65]
In the United States half of CT scans are contrast CTs using intravenously injected radiocontrast agents.[66] The most common reactions from these agents are mild, including nausea, vomiting and an itching rash; however, more severe reactions may occur.[67] Overall reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast.[68] Skin rashes may appear within a week to 3% of people.[67]
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.[67][69] Death occurs in about two to 30 people per 1,000,000 admin
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.[67][69] Death occurs in about two to 30 people per 1,000,000 administrations, with newer agents being safer.[68][70] 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.[66]
The contrast agent may induce contrast-induced nephropathy.[13] This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting kidney failure,[13] 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.
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. 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. 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. 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. The phenomenon that one part of the detector cannot differentiate between different tissues is called the partial volume effect. 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. 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.[72] 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. CT data sets have a very high dynamic 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 grey intensity proportional to position within the window. 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.
Contrast media used for X-ray CT, as well as for plain film X-ray, are called radiocontrasts. Radiocontrasts for X-ray CT are, in general, iodine-based.[73] 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.
Examination | Typical effective dose (mSv) to the whole body |
Typical absorbed dose (mGy) to the organ in question | |||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Annual background radiation | 2.4[74] | 2.4[74] | |||||||||||||||||||
Chest X-ray | 0.02[75] | 0.01–0.15[76] | |||||||||||||||||||
Head CT | 1–2Contrast media used for X-ray CT, as well as for plain film X-ray, are called radiocontrasts. Radiocontrasts for X-ray CT are, in general, iodine-based.[73] 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.
Scan dose
|