A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body.[2] The personnel that perform CT scans are called radiographers or radiology technologists.[3][4] CT scanners use a rotating X-ray tube and a row of detectors placed in a gantry to measure X-ray attenuations by different tissues inside the body. The multiple X-ray measurements taken from different angles are then processed on a computer using tomographic reconstruction algorithms to produce tomographic (cross-sectional) images (virtual "slices") of a body. CT scans can be used in patients with metallic implants or pacemakers, for whom magnetic resonance imaging (MRI) is contraindicated.
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Since its development in the s, CT scanning has proven to be a versatile imaging technique. While CT is most prominently used in medical diagnosis, it can also be used to form images of non-living objects. The Nobel Prize in Physiology or Medicine was awarded jointly to South African-American physicist Allan MacLeod Cormack and British electrical engineer Godfrey Hounsfield "for the development of computer-assisted tomography".[5][6]
On the basis of image acquisition and procedures, various type of scanners are available in the market.
Sequential CT, also known as step-and-shoot CT, is a type of scanning method in which the CT table moves stepwise. The table increments to a particular location and then stops which is followed by the X-ray tube rotation and acquisition of a slice. The table then increments again, and another slice is taken. The table movement stops while taking slices. This results in an increased time of scanning.[7]
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. 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. The main limitation of this type of CT 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. Some designs use two X-ray sources and detector arrays offset by an angle, as a technique to improve temporal resolution.[8][9]
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.[10] 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.[11] 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.[12]
Dual energy CT, also known as spectral CT, is an advancement of computed Ttmography in which two energies are used to create two sets of data.[13] A dual energy CT may employ dual source, single source with dual detector layer, single source with energy switching methods to get two different sets of data.[14]
CT perfusion imaging is a specific form of CT to assess flow through blood vessels whilst injecting a contrast agent.[21] Blood flow, blood transit time, and organ blood volume, can all be calculated with reasonable sensitivity and specificity.[21] This type of CT may be used on the heart, although sensitivity and specificity for detecting abnormalities are still lower than for other forms of CT.[22] This may also be used on the brain, where CT perfusion imaging can often detect poor brain perfusion well before it is detected using a conventional spiral CT scan.[21][23] This is better for stroke diagnosis than other CT types.[23]
Positron emission tomography–computed tomography is a hybrid CT modality which combines, in a single gantry, a positron emission tomography (PET) scanner and an X-ray computed tomography (CT) scanner, to acquire sequential images from both devices in the same session, which are combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning.[24]
PET-CT gives both anatomical and functional details of an organ under examination and is helpful in detecting different type of cancers.[25][26]
Since its introduction in the s,[27] CT has become an important tool in medical imaging to supplement conventional X-ray imaging 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 a high risk of heart disease. Several 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.[28]
The use of CT scans has increased dramatically over the last two decades in many countries.[29] An estimated 72 million scans were performed in the United States in and more than 80 million in .[30][31]
CT scanning of the head is typically used to detect infarction (stroke), tumors, calcifications, haemorrhage, and bone trauma.[32] 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. 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.[33][34][35][36][37][38]
Contrast CT is generally the initial study of choice for neck masses in adults. CT of the thyroid plays an important role in the evaluation of thyroid cancer.[40] CT scan often incidentally finds thyroid abnormalities, and so is often the preferred investigation modality for thyroid abnormalities.[40]
A CT scan can be used for detecting both acute and chronic changes in the lung parenchyma, the tissue of the lungs.[41] It is particularly relevant here because normal two-dimensional X-rays do not show such defects. A variety of techniques are used, depending on the suspected abnormality. For evaluation of chronic interstitial processes such as emphysema, and fibrosis,[42] thin sections with high spatial frequency reconstructions are used; often scans are performed both on inspiration and expiration. This special technique is called high resolution CT that produces a sampling of the lung, and not continuous images.[43]
Bronchial wall thickening can be seen on lung CTs and generally (but not always) implies inflammation of the bronchi.[44]
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.[45] 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.[46] 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.[46] 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.[46]
Computed tomography angiography (CTA) is a type of contrast CT to visualize the arteries and veins throughout the body.[47] 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.[48][49][50] CT scans can reduce the risk of angiography by providing clinicians with more information about the positioning and number of clots prior to the procedure.[51][52]
A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy.[53] Traditionally, cardiac CT scans are used to detect, diagnose, or follow up coronary artery disease.[54] 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.[55][56][57]
The main forms of cardiac CT scanning are:
To better visualize the anatomy, post-processing of the images is common.[54] 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.[62][63][64][65]
CT is an accurate technique for diagnosis of abdominal diseases like Crohn's disease,[66] GIT bleeding, and diagnosis and staging of cancer, as well as follow-up after cancer treatment to assess response.[67] It is commonly used to investigate acute abdominal pain.[68]
Non-contrast-enhanced CT scans are the gold standard for diagnosing kidney stone disease.[69] They allow clinicians to estimate the size, volume, and density of stones, helping to guide further treatment; with size being especially important in predicting the time to spontaneous passage of a stone.[70]
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 recognized with a 0.2 mm resolution.[71][72] With modern dual-energy CT scanners, new areas of use have been established, such as aiding in the diagnosis of gout.[73]
CT is used in biomechanics to quickly reveal the geometry, anatomy, density and elastic moduli of biological tissues.[74][75]
Industrial CT scanning (industrial computed tomography) is a process which uses X-ray equipment to produce 3D representations of components both externally and internally. Industrial CT scanning has been used in many areas of industry for internal inspection of components. Some of the key uses for CT scanning have been flaw detection, failure analysis, metrology, assembly analysis, image-based finite element methods[76] and reverse engineering applications. CT scanning is also employed in the imaging and conservation of museum artifacts.[77]
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)[78][79][80][81] 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).[82][83][84] Its usage in airport security pioneered at Shannon Airport in March has ended the ban on liquids over 100 ml there, a move that Heathrow Airport plans for a full roll-out on 1 December and the TSA spent $781.2 million on an order for over 1,000 scanners, ready to go live in the summer.
X-ray CT is used in geological studies to quickly reveal materials inside a drill core.[85] Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images.[86]
Traditional methods of studying fossils are often destructive, such as the use of thin sections and physical preparation. X-ray CT is used in paleontology to non-destructively visualize fossils in 3D.[87] This has many advantages. For example, we can look at fragile structures that might never otherwise be able to be studied. In addition, one can freely move around models of fossils in virtual 3D space to inspect it without damaging the fossil.
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 certain artifacts like the Herculaneum papyri in which the material composition has very little variation along the inside of the object. 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.[88] Micro-CT has also proved useful for analyzing more recent artifacts such as still-sealed historic correspondence that employed the technique of letterlocking (complex folding and cuts) that provided a "tamper-evident locking mechanism".[89][90] Further examples of use cases in archaeology is imaging the contents of sarcophagi or ceramics.[91]
Recently, CWI in Amsterdam has collaborated with Rijksmuseum to investigate art object inside details in the framework called IntACT.[92]
Varied types of fungus can degrade wood to different degrees, one Belgium research group has been used X-ray CT 3 dimension with sub-micron resolution unveiled fungi can penetrate micropores of 0.6 μm[93] under certain conditions.
Sawmills use industrial CT scanners to detect round defects, for instance knots, to improve total value of timber productions. Most sawmills are planning to incorporate this robust detection tool to improve productivity in the long run, however initial investment cost is high.[94]
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:
Technically, all volume renderings become projections when viewed on a 2-dimensional display, making the distinction between projections and volume renderings a bit vague. The epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations.[99][100]
Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet.[101] 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.[102]
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.[103] 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.[104] The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of + HU, iron steel can completely block 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.[105]
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 gray intensity proportional to position within the window.[106] 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.[107] Window width and window level parameters are used to control the windowing of a scan.[108]
Multiplanar reconstruction (MPR) is the process of converting data from one anatomical plane (usually transverse) to other planes. It can be used for thin slices as well as projections. Multiplanar reconstruction is possible as present CT scanners provide almost isotropic resolution.[109]
MPR is used almost in every scan. The spine is frequently examined with it.[110] An image of the spine in axial plane can only show one vertebral bone at a time and cannot show its relation with other vertebral bones. By reformatting the data in other planes, visualization of the relative position can be achieved in sagittal and coronal plane.[111]
New software allows the reconstruction of data in non-orthogonal (oblique) planes, which help in the visualization of organs which are not in orthogonal planes.[112][113] It is better suited for visualization of the anatomical structure of the bronchi as they do not lie orthogonal to the direction of the scan.[114]
Curved-plane reconstruction (or curved planar reformation = CPR) is performed mainly for the evaluation of vessels. This type of reconstruction helps to straighten the bends in a vessel, thereby helping to visualize a whole vessel in a single image or in multiple images. After a vessel has been "straightened", measurements such as cross-sectional area and length can be made. This is helpful in preoperative assessment of a surgical procedure.[115]
For 2D projections used in radiation therapy for quality assurance and planning of external beam radiotherapy, including digitally reconstructed radiographs, see Beam's eye view.
Examples of different algorithms of thickening multiplanar reconstructions[116] Type of projection Schematic illustration Examples (10 mm slabs) Description Uses Average intensity projection (AIP) 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. Useful for identifying the internal structures of a solid organ or the walls of hollow structures, such as intestines. Maximum intensity projection (MIP) The voxel with the highest attenuation is displayed. Therefore, high-attenuating structures such as blood vessels filled with contrast media are enhanced. Useful for angiographic studies and identification of pulmonary nodules. Minimum intensity projection (MinIP) The voxel with the lowest attenuation is displayed. Therefore, low-attenuating structures such as air spaces are enhanced. Useful for assessing the lung parenchyma.A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). With the help of edge detection image processing algorithms a 3D model can be constructed from the initial data and displayed on screen. Various thresholds can be used to get multiple models, each anatomical component such as muscle, bone and cartilage can be differentiated on the basis of different colours given to them. However, this mode of operation cannot show interior structures.[117]
Surface rendering is limited technique as it displays only the surfaces that meet a particular threshold density, and which are towards the viewer. However, In volume rendering, transparency, colours and shading are used which makes it easy to present a volume in a single image. For example, Pelvic bones could be displayed as semi-transparent, so that, even viewing at an oblique angle one part of the image does not hide another.[118]
An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. In general, higher radiation doses result in higher-resolution images,[119] while lower doses lead to increased image noise and unsharp images. 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.[120] Several methods that can reduce the exposure to ionizing radiation during a CT scan exist.[121]
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:[125][126]Chapters 3 and 5
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.[141] 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.[142] 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.[143]
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.[144][145] Virtual colonography is far more accurate than a barium enema for detection of tumors and uses a lower radiation dose.[146]
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.[147] 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.[148]
The radiation used in CT scans can damage body cells, including DNA molecules, which can lead to radiation-induced cancer.[149] 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.[150] However, a lumbar spine X-ray has a similar dose as a head CT.[151] 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.[152]
Large scale population-based studies have consistently demonstrated that low dose radiation from CT scans has impacts on cancer incidence in a variety of cancers.[153][154][155][156] For example, in a large population-based Australian cohort it was found that up to 3.7% of brain cancers were caused by CT scan radiation.[157] Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.[150] 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. The risks of CT scan radiation are especially important in patients undergoing recurrent CT scans within a short time span of one to five years.[158][159][160]
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."[150] On the other hand, a recent paper analyzing the data of patients who received high cumulative doses showed a high degree of appropriate use.[161] 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,[159] which counteracts existing criticisms some investigators may have on the effects of protracted versus acute exposure.
There are contrarian views and the debate is ongoing. 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,[162] concluding that no evidence indicates such low doses cause any long-term harm.[163][164][165] 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 .[149] Others dispute this estimate,[166] 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.[167]
A person's age plays a significant role in the subsequent risk of cancer.[168] Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1%, or 1: scans.[168] 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.[168] 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).[169] A 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.[170] CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of having this function built in.[171] Furthermore, certain conditions can require children to be exposed to multiple CT scans.[149]
Current recommendations are to inform patients of the risks of CT scanning.[172] However, employees of imaging centers tend not to communicate such risks unless patients ask.[173]
In the United States half of CT scans are contrast CTs using intravenously injected radiocontrast agents.[174] The most common reactions from these agents are mild, including nausea, vomiting, and an itching rash. Severe life-threatening reactions may rarely occur.[175] Overall reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast.[176] Skin rashes may appear within a week to 3% of people.[175]
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.[175][177] Death occurs in about 2 to 30 people per 1,000,000 administrations, with newer agents being safer.[176][178] 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.[174]
The contrast agent may induce contrast-induced nephropathy.[179] This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting kidney failure,[179] 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.[180] 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.[181]
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.[182][179]
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.[168] 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.[185]
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.[183] While there is some variation, most people (99%) received less than 7 mSv per year as background radiation.[187] Medical imaging as of accounted for half of the radiation exposure of those in the United States with CT scans making up two thirds of this amount.[168] In the United Kingdom it accounts for 15% of radiation exposure.[169] The average radiation dose from medical sources is ≈0.6 mSv per person globally as of .[168] Those in the nuclear industry in the United States are limited to doses of 50 mSv a year and 100 mSv every 5 years.[168]
Lead is the main material used by radiography personnel for shielding against scattered X-rays.
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.[188]
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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.[189] 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.[190]
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. 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.[191][192][193]
Most adverse health effects of radiation exposure may be grouped in two general categories:
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.[196]
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.[197][198]
In October, , 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 200 patients were exposed to radiation at approximately eight times the expected dose for an 18-month period; over 40% of them lost patches of hair. This event prompted a 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."[168][199] Similar problems have been reported at other centers.[168] These incidents are believed to be due to human error.[168]
CT scan procedure varies according to the type of the study and the organ being imaged. The patient lies 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[clarification needed] 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.[200]
Patient preparation may vary according to the type of scan. The general patient preparation includes.[200]
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.[203] As the X-rays pass through the patient, they are attenuated differently by various tissues according to the tissue density.[204] A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation.[205] 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.[206] These cross-sectional images are made up of small units of pixels or voxels.[207]
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.[207]
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 + 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.[101] 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.[125] These cross-sectional images are widely used for medical diagnosis and therapy.[208]
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.[209] 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.[210]
The history of X-ray computed tomography goes back to at least with the mathematical theory of the Radon transform.[211][212] In October , William H. Oldendorf received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material".[213] The first commercially viable CT scanner was invented by Godfrey Hounsfield in .[214]
It is often claimed that revenues from the sales of The Beatles' records in the s helped fund the development of the first CT scanner at EMI. The first production X-ray CT machines were in fact called EMI scanners.[215]
The word tomography is derived from the Greek tome 'slice' and graphein 'to write'.[216] Computed tomography was originally known as the "EMI scan" as it was developed in the early s at a research branch of EMI, a company best known today for its music and recording business.[217] It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography.[218]
The term CAT scan is no longer in technical use 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.[219][220][221]
In Medical Subject Headings (MeSH), computed axial tomography was used from to , but the current indexing explicitly includes X-ray in the title.[222]
The term sinogram was introduced by Paul Edholm and Bertil Jacobson in .[223]
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.[225] 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.[226]
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.[227][228]
Use of CT has increased dramatically over the last two decades.[29] An estimated 72 million scans were performed in the United States in ,[30] accounting for close to half of the total per-capita dose rate from radiologic and nuclear medicine procedures.[229] Of the CT scans, six to eleven percent are done in children,[169] an increase of seven to eightfold from .[168] Similar increases have been seen in Europe and Asia.[168] 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%.[230] In the emergency department in the United States, CT or MRI imaging is done in 15% of people who present with injuries as of (up from 6% in ).[231]
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).[149] As of , in the United States a proportion of CT scans are performed unnecessarily.[171] Some estimates place this number at 30%.[169] There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public.[171] 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.[171]
Major manufacturers of CT scanning devices and equipment are:[232]
Photon-counting computed tomography is a CT technique currently under development.[as of?] 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.[233] 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.[234][235] 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 , photon counting CT is in use at three sites.[236] Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising.[237] 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.[238][159][160][161]
Spontaneous intraparenchymal hemorrhages (ICH) are nontraumatic hemorrhages affecting the brain parenchyma and account for 10–15% of cases of acute stroke1,2,3. Although most cases of ICH are caused by hypertension, amyloid angiopathy, or impaired coagulation, many occur because of aneurysms, arteriovenous malformations (AVM), Dural vein thrombosis and Dural Arteriovenous Fistulas (AVF)3,4.
Using NCCT signs in patients with ICH, alone or accompanied by clinical variables on the triage and monitoring of patients with ICH is desirable, particularly when there are low-resource settings or interpreting the CTA is challenging. Proper management of ICH depends mainly on finding the etiology of the bleeding, especially in cases where surgical or endovascular intervention reduces the risk of bleeding5.
Digital catheter angiography (DSA) as the modality of choice for the diagnosis of vascular abnormalities is being largely replaced by computed tomography angiography (CTA) as the standard of diagnosis6. DSA is an invasive procedure that may have some potential risks of causing neurologic deficits7.
Also, CTA offers several advantages over conventional angiography. It is an available, rapid (only a few minutes using a multidetector row CT), and non-invasive modality that is more applicable in critically ill patients. It is more cost-effective than conventional angiography because of its lower cost, favorable risk profile, and high sensitivity in detecting vascular lesions8,9,10. However, there are some troubles with CTA. One is an added radiation dose of approximately 2.5 mSV after the NCCT which is also 2.5 mSV (total of 5 mSV). Another is that intravenous contrast agents may probably cause contrast nephropathy, especially in patients with a history of nephropathy11.
It is of good practice to have clinical and imaging criteria for selecting the patients who would benefit most from CTA, especially if there are limited resources for CTA and/or DSA.
Previous studies have identified clinical and non-contrast computed tomography (NCCT) features such as younger age, neither known hypertension nor impaired coagulation, presence of subarachnoid or intraventricular hemorrhage12,13, and temporal or frontal lobe location14, which are associated with a higher detection rate of vascular abnormalities on CTA or DSA.
To date, there are limited studies about the stratification of patients with ICH according to the risk of harboring an underlying vascular etiology. The purpose of this study is to evaluate the diagnostic accuracy of NCCT findings in detecting patients with an underlying vascular etiology of ICH.
The ethics committee of Mashhad university of medical sciences approved the study (approval code: IR.MUMS.MEDICAL.REC..097) and waived the need for informed consent.
We searched our Picture Archiving and Communicating System (PACS) of our tertiary-level academic hospital between and for four years and collected all consecutive patients hospitalized for intra-parenchymal brain hemorrhage. Inclusion criteria were patients with acute neurologic symptoms who were diagnosed with ICH in the non-contrast brain CT scan (NCCT) at presentation, age above eighteen years, and available brain CTA obtained within 48 h from primary NCCT. The exclusion criteria were history of head trauma within the previous two weeks, evidence of ischemic stroke on the site of hemorrhage, evidence of aneurysmal hemorrhage in the CTA, history of known vascular malformation or vascular mass within the brain, known amyloid angiopathy according to Boston’s criteria, and the presence of severe artifacts in NCCT or CTA making the interpretation challenging and incomplete imaging protocol. A total of 334 patients were enrolled in this study.
All MDCTA and NCCT examinations were performed with a commercially available 16-MDCT scanner (Neusoft, Neuviz 16). NCCT was performed in a head holder by an axial technique with 120 kilovolts (peak), 150 mA, and 5-mm thickness reconstruction. MDCTA was performed by scanning from the base of the C1 body to the vertex using the following parameters: pitch (1.2); collimation, 1.25 mm; maximal mA, 250; kilovolt (peak), 120; FOV, 22 cm; and 100 mL of iodinated contrast material (Iodixanol 320 mg/100 mL) with the flow rate of 4 mL/s, followed by 50 mL of saline chaser injected with the same flow rate into the antecubital vein with a 25-s delay between starting the contrast injection and the start of scanning.
Two radiologists, including an interventional radiologist with 10 years of experience in vascular imaging, and a general radiologist with 4 years of experience in general radiology, assessed the non-contrast CT scans and MDCTA images. In cases of discrepancy, a third opinion was sought from another radiologist with 5 years of experience. Image interpretation was performed on a standard PACS workstation. They were asked to record the location of ICH (lobar, deep grey matter or pons, and infratentorial), the presence of intraventricular hemorrhage (IVH), or subarachnoid hemorrhage (SAH) and the presence of considerable perilesional edema in NCCTs.
They also evaluated the probability of underlying vascular lesions into three categories of high probable, indeterminate, and low probable according to the following criteria used in the previous literature 10:
High probable: The presence of enlarged vessels, with associated calcifications around the lesion or increased dural vein attenuation. (Fig. 1).
Low probable: ICH in the pons or deep grey matter, without associated high-probable criteria. (Fig. 2).
Indeterminate: the lesions which do not fall in either of the above criteria.
CTA images were then interpreted by the same radiologists. The CTA images were evaluated at least two weeks after the NCCT image to prevent recall bias. The final CTA outcome was recorded as positive/negative according to the presence of vascular lesions (AVM, dural AVF, dural vein thrombosis, etc.) in the CTA images.
Other diagnostic data including MRA/MRV results, surgical and pathological reports, and digital subtraction angiography (DSA), etc. were also recorded. If other diagnostic methods revealed a vascular lesion not detected by CTA, the final analysis included the results.
Furthermore, we used the independent predictors of a positive CTA (NCCT probability, age, hypertension, impaired coagulation, IVH or SAH, location of ICH, associated edema), to construct a practical scoring system to predict the risk of vascular etiology in ICH patients, so called the Vascular ICH score (VICH score).
Medical records were reviewed for patient age, sex, presence of known hypertension, and presence of coagulopathy. We divided our patients according to their age into one of the following two categories: group 1, 18–45 years of age; and group 2, patients 46 years of age and older.
Patients were also classified as hypertensive if they had a history of hypertension on medical records or were taking antihypertensive medications at presentation. Patients were classified as having coagulopathy if, at presentation, they were receiving daily anti-platelet therapy with aspirin (at least 81 mg) and/or clopidogrel had a platelet count of < 50,000 cells per cubic millimeter of blood, were on anticoagulation with warfarin and had an international normalization ratio (INR) > 1.5, or were on anticoagulation with heparin and had an active partial thromboplastin time (aPTT) of > 80 s.
All the obtained data were collected on a database. Demographic, historical and clinical characteristics are summarized using descriptive statistics. A comparison between the groups.was made by conducting T tests. Categorical variables were compared using the χ2 test. Multivariable logistic regression models were conducted to investigate the association between the VICH scores and positive CTAs. Data were analyzed using IBM SPSS version 26. A p-value of less than 0.05 was considered statistically significant.
The ethics committee of Mashhad university of medical sciences approved the study (approval code: IR.MUMS.MEDICAL.REC..097) and waived the need for informed consent.
Patients' personal information, including names, was removed from the images and was replaced with a code unique to every individual. Patients’ medical and personal information were not shared outside the research group.
From March to March a total of 385 patients presented to our emergency department with ICH on NCCT and were further evaluated with MDCTA of the brain within 48 h of presentation. Fifty-one patients were excluded because of the presence of an intracranial aneurysm in the CTA, and a total of 334 patients were analyzed. The mean age of patients was 54.25 (range, 18–87) years, 204 (61.1%) of whom were men, and 130 (38.9%) were women. The demographic characteristics of patients are summarized in Table 1.
The patients were categorized according to their age into the two groups of 45 years or younger, and older than 45 years. The vascular etiology was found in 19.8% of patients 45 years or younger and in 5.3% of patients older than 46 years old (p-value < 0.000).
Thirty-one patients (9.3%) out of a total of 334, had a vascular underlying factor. Of 204 male patients, 23 (11.3%) and out of 130 female patients, 8 (6.2%) had positive CTAs. (Table 4).
The most common vascular etiology was intracranial AVM seen in 20 (6%) patients (Figs. 3 and 4). Table 2 summarizes the frequency of the different vascular ICH etiologies.
The interobserver agreement between the two radiologists for the NCCT categorization was 0.73, which is acceptable.
Of the 334 patients, 178 were categorized as low probability of the presence of an underlying vascular etiology (53.3%), and 10 were categorized as high probability of the presence of an underlying vascular etiology (3%). The remaining 146 (43.7%) NCCTs were categorized as indeterminate for the presence of an underlying vascular etiology. Of the 10 high probability NCCT examinations, 6 (60%) were true-positive and 4 (40%) were false-positive. Of the 178 low probability NCCT examinations, 170 (95.5%) were true-negative and 14 (4.5%) were false-negative. Hence, for the prediction of a vascular etiology for the ICH, high- and low probability NCCT examinations demonstrated a positive predictive value of 60%, and a negative predictive value of 95.5%, respectively. However, 17 of the 146 indeterminate NCCTs had an underlying vascular etiology for the ICH (11.6%). The results are summarized in Table 3.
The results of univariate and multiple variables logistic regression analysis for the diagnostic yield of NCCT in the entire patient population, patients with lobar ICH, and the 2 different patient age groups are summarized in Table 4. The independent predictors of vascular etiology of ICH were age, known history of hypertension, location of hemorrhage, and presence of considerable edema adjacent to the lesion (Table 4).
We used the independent predictors of a positive CTA, to construct a practical scoring system to predict the risk of vascular etiology in ICH patients (Table 5). The results of the application of this scoring system to the patient population are shown in Table 6. The diagnostic results of NCCT categorization and VICH score are compared in Table 7.
Primary ICH constitutes 10–15% of cases of acute stroke. Although most cases of primary ICH are secondary to hypertension or coagulopathy, many cases have an underlying vascular etiology like AVF or intracranial aneurysm. Correct diagnosis of vascular etiologies is of utmost importance in the timely diagnosis and prevention of episodes of rebleeding.
In our study with a random selection of patients, 61.1% were male and 38.9% were female. The rate of detection of vascular findings in our patient cohort was 9.3% which is roughly similar to other studies. The number of positive CTAs was 11.3% in men and 6.2% in women. These figures were not statistically different (p: 0.13, chi-square test).
Cranial CTA is now accepted as the diagnostic standard for vascular etiologies of ICH. This modality is readily available in most centers and can be performed within seconds. However, the radiation exposure and risks of contrast nephropathy are two drawbacks of this imaging modality11, and it's good to use the NCCT as a guide to predict who may benefit more from cranial CTA10.
In the current study, we retrospectively evaluated the NCCT categorization previously described by Delgado et al.10 as a predictor of vascular etiology in patients with primary ICH. We showed that NCCT categorization had a positive and negative predictive value of 60% and 95.5% to predict the vascular causes of ICH. The results were similar to another study conducted by Almandoz et al. which reported the PPV and NPV of the NCCT to be 84.4% and 98% respectively15. In terms of the independent factors associated with a vascular etiology, we showed that lack of hypertension and coagulopathy, age of 45 years and younger, lobar location of hemorrhage, and presence of significant edema around the lesion were associated with higher risks of harboring a vascular etiology. Our study results agree with those of most previous conventional angiographic studies12,13,16,17,18.
We also developed a scoring system that predicts a given ICH patient’s risk of harboring an underlying vascular etiology based on the dependent factors associated with the presence of a vascular etiology (VICH score). The practicality of the VICH score lies in its ease of calculation because it requires only a review of the NCCT according to the NCCT categorization, location of hemorrhage, perilesional edema, and clinical data routinely obtained on presentation to the emergency department (patient age and history of hypertension).
In our study, one of the patients with a VICH score of 0 had a vascular etiology of ICH (0.9%), and this figure was 1.4% and 5.5% for the VICH scores of 1 and 2 respectively. This finding is very promising, especially in centers where neurovascular evaluation is not readily available, and could serve as a valuable tool to choose the patients who are most likely to harbor an underlying vascular abnormality. The ROC curve analysis showed that the VICH score ≥ 4 has a sensitivity and specificity of 51.6% and 96.4% respectively, which might be the maximum reasonable cut-off. (Fig. 5).
Many medical centers consider a positive rate of more than 10% for a low-risk noninvasive diagnostic tool such as MDCTA to be high enough to merit performing it in all patients10.
The limitations of our study are the retrospective nature of the study, the potential selection bias generated by the inclusion of only patients who presented with ICH and were evaluated with MDCTA, and the lack of independent validation of this scoring system. This scoring system might be helpful when there are limited resources, at the cost of missing some patients with possible vascular etiology.
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