A CT scan , also known as computed tomography scan , utilizes a computer-processed combination of many X-ray measurements taken from different angles to produce cross-sectional (tomographic) images (virtual "slices") of a certain area of ​​the scanned object, allowing the user to look into the object without cutting. Other terms include computed axial tomography (CAT scan) and computer aided tomography.
Digital geometry processing is used to produce a further three-dimensional volume of the inside of an object from a large set of two-dimensional radiographic images taken about one rotation axis. Medical imaging is the most common X-ray CT application. Its cross-section images are used for diagnostic and therapeutic purposes in various medical disciplines. The rest of this article discusses X-ray medical imaging CTs; the X-ray CT industry application is discussed on industrial tomography scanning .
The term "computed tomography" (CT) is often used to refer to CT X-rays, as this is the most recognizable form. However, many other types of CT, such as positron emission tomography (PET) and single photon emission tomography (SPECT). X-ray tomography, the predecessor of CT, is one form of radiography, along with many forms of tomographic and non-tomographic radiography.
CT produces data that can be manipulated to demonstrate the various body structures based on its ability to absorb X-ray light. Although, historically, the resulting image is in axial or transverse plane, perpendicular to the body's long axis, modern scanners allow the volume of this data to be reformatted in various fields or even as a representation of volumetric (3D) structures. Although most common in the medical world, CT is also used in other fields, such as testing nondestructive materials. Other examples are archaeological use such as imaging of sarcophagus content. Individuals responsible for conducting CT examinations are called radiographers or radiologists.
The use of CT has increased dramatically over the past two decades in many countries. An estimated 72 million scans were performed in the United States in 2007 and more than 80 million per year by 2015. One study estimates that as many as 0.4% of current cancers in the United States are caused by CT done in the past and that this can increase to 1 , 5-2% with rate of use of CT 2007; However, this estimate is disputed, as there is no consensus about the existence of damage from low radiation levels. Lower doses of radiation are often used in many areas, such as in renal colic investigation. Side effects of intravenous contrast are used in several types of research including kidney problems.
Video CT scan
Medical use
Since its introduction in the 1970s, CT has become an important tool in medical imaging to complement X-rays and medical ultrasonography. Recently it has been used for preventive or screening treatment for diseases, such as CT colonography for people with high risk of colon cancer, or full-motion heart scan for people with high risk of heart disease. A number of agencies offer a full body scan for the general population although this practice contradicts the advice and official position of many professional organizations in the field mainly because of the applied radiation dose.
Head
CT scan head is usually used to detect infarction, tumor, calcification, bleeding and bone trauma. Above, the structure of hypodens (dark) may indicate edema and infarct, hyperdense structure (light) indicates calcification and bleeding and bone trauma can be seen as a disjunction in the bone window. The tumor can be detected by the swelling and anatomical distortion caused, or by the surrounding edema. Ambulances equipped with small multi-slore CT scans respond to cases involving stroke or head trauma. Head CT scans are also used in CT-guided stereotactic and radiosurgery surgery for the treatment of intracranial tumors, arteriovenous malformations and other surgically manageable conditions using a device known as N-localizer.
Magnetic resonance imaging (MRI) from the head provides superior information compared to CT scans when seeking information about headaches to confirm the diagnosis of neoplasm, vascular disease, posterior cranial fossa lesions, cervicomedullary lesions, or intracranial pressure disorders. It also does not carry the risk of exposing patients to ionizing radiation. CT scans can be used to diagnose headaches when neuroimaging is indicated and MRI is not available, or in emergency settings when bleeding, stroke, or traumatic brain injury is suspected. Even in emergency situations, when minor head injuries as determined by the evaluation of the physician and based on established guidelines, head CT should be avoided for adults and delayed delayed clinical observation in the emergency department for children.
Lung
CT scans can be used to detect acute and chronic changes in the pulmonary parenchyma, the internal part of the lung. This is particularly relevant here because normal two-dimensional X-rays do not exhibit such defects. Various techniques are used, depending on suspected suspicions. For the evaluation of chronic interstitial processes (emphysema, fibrosis, etc.), thin sections with high spatial frequency reconstruction are used; often the scan is done both in inspiration and expiration. This special technique is called high resolution CT. Therefore, it produces lung sampling rather than continuous images.
Bronchial wall thickening can be seen in lung CT, and generally (but not always) shows bronchial inflammation. Typically, the ratio of bronchial wall thickness and bronchial diameter is between 0.17 and 0.23.
A nodule found by chance in the absence of symptoms (sometimes referred to as incidentaloma) can cause concern that it may be a tumor, either benign or malignant. Perhaps persuaded by fear, patients and doctors occasionally approve an intensive CT scan schedule, sometimes up to every three months and beyond the recommended guidelines, in an effort to supervise the nodules. However, the established guidelines suggest that patients without previous cancer history and whose solid nodules have not grown over a two-year period are unlikely to have malignant cancers. For this reason, and because no studies provide supporting evidence that intensive surveillance results in better outcomes, and because of the risks associated with CT scans, patients should not receive CT screening more than recommended by the established guidelines.
Angiography
Computed tomography angiography (CTA) is a CT contrast to visualize arteries and veins throughout the body. These range from arteries that serve the brain to those who carry blood to the lungs, kidneys, arms and legs. An example of this type of examination is a CT pulmonary angiogram (CTPA) used to diagnose pulmonary embolism (PE). It uses computed tomography and an iodine-based contrast agent to get a pulmonary artery image.
Cardiac
A CT scan of the heart is performed to gain knowledge of heart or coronary anatomy. Traditionally, a CT scan of the heart is used to detect, diagnose or follow up coronary artery disease. Recently CT has played a key role in the rapidly developing field of structural cardiac transeleter intervention, more specifically in transkateter repair and valve replacement heart.
The main forms of heart CT scan are:
- Coronary CT angiography (CTA): the use of CT to assess cardiac coronary arteries. The subjects received intravenous radiocontrasion injection and then cardiac scanning using a high-speed CT scanner, allowing radiologists to assess the occlusion rate in the coronary arteries, usually to diagnose coronary artery disease.
- CT scan of coronary calcium: also used to assess the severity of coronary artery disease. In particular, it looks for calcium deposits in the coronary arteries that can narrow the arteries and increase the risk of heart attack. Coronary CT scan coronary CT is performed without the use of radiocontras, but can also be done from a contrast-enhanced image.
To better visualize anatomy, post-processing images are common. The most common are multi-plan reconstruction (MPR) and rendering volumes. For more complex anatomy and procedures, such as heart valve intervention, correct 3D reconstruction or 3D printing are made based on this CT image to gain a deeper understanding.
Abdomen and pelvis
CT is an accurate technique for the diagnosis of stomach diseases. Its uses include cancer diagnosis and staging, as well as follow-up after cancer treatment to assess the response. Usually used to investigate acute abdominal pain.
extremities
CT is often used to image complex fractures, especially those around the joint, due to its ability to reconstruct areas of interest on multiple planes. Fractures, ligament injuries and dislocations can be easily recognized with a resolution of 0.2 mm. With modern dual energy CT scans, new usage areas have been established, such as helping with gout diagnosis.
Maps CT scan
Benefits
There are several advantages that CT has over traditional 2D medical radiography. First, CT completely eliminates the superimposition of structural images outside of the desired area. Secondly, because of the inherent high contrast CT resolution, the difference between different tissues in the physical density of less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of multiple multiple or one helical scan can be viewed as an image in the axial, coronal, or sagittal field, depending on the diagnostic task. These are referred to as multiplanar reformed imagery.
CT is considered a moderate to high radiation diagnostic technique. Increased CT resolution has enabled the development of new investigations, which may have advantages; compared with conventional radiography, for example, CT angiography avoids invasive insertion of a catheter. CT colonography (also known as virtual colonoscopy or VC for short) is much more accurate than barium enema to detect tumors, and uses lower radiation doses. 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 colonoscopy in some cases.
The radiation dose for a particular study depends on several factors: the volume being scanned, the number of patients, the number and type of scan sequences, as well as the resolution and quality of the desired image. In addition, two easily adjustable helical CT scan parameters that have a profound effect on the radiation dose are tube and pitch currents. Computed tomography (CT) scans have proven to be more accurate than radiographs in evaluating anterior interbody fusion but may still read too much of the fusion level.
Adverse effects
Cancer
Radiation used in CT scans can damage the body's cells, including DNA molecules, which can cause cancer. According to the National Council on Radiation Protection and Measurements, between the 1980s and 2006, the use of CT scans has increased sixfold (500%). The dose of radiation received from the CT scan is variable. Compared with the lowest dose x-ray technique, CT scans can be 100 to 1,000 times higher than conventional X-rays. However, lumbar spine x-rays have the same dose as head CT. Articles in the media often overestimate the relative dose of CT by comparing the lowest dose x-ray (chest x-rays) technique with the highest dose CT technique. In general, the radiation dose associated with abdominal CT routine has the same radiation dose as 3 years of average background radiation (from cosmic radiation).
Some experts note that CT scans are known to be "overused," and "there is little sad evidence of better health outcomes associated with high scans today."
The initial estimate of damage from CT is based partly on similar radiation exposure experienced by those present during the atomic bomb explosion in Japan after the Second World War and those of nuclear industry workers. A more recent study by the National Cancer Institute in 2009, based on a scan made in 2007, estimates that 29,000 cases of excessive cancer and 14,500 excessive deaths will occur during the lifetime of the patient. Some experts project that in the future, between three and five percent of all cancers will result from medical imaging.
An Australian study of 10.9 million people reported that the increased incidence of cancer after CT scan exposure in this cohort was largely due to irradiation. In this group one in each CT scan 1800 followed by excessive cancer. If the lifetime risk of developing cancer is 40% then absolute risk increases to 40.05% after CT.
A person's age plays an important role in subsequent cancer risk. The estimated lifetime risk of cancer mortality from a 1 year CT abdomen is 0.1% or 1: 1000 scan. The risk for a 40-year-old is half that of a 20-year-old man with much less risk in the elderly. The International Commission on Radiological Protection estimates that the risk to a fetus exposed to 10 mGy (radiation exposure unit, see Gray (unit)) increases cancer rates before the age of 20 from 0.03% to 0.04% (for CT references the lung angiogram exposes the fetus up to 4 mGy). A review of 2012 found no association between medical radiation and cancer risk in children who noted however limited the evidence on which the review was based.
CT scans can be performed with different settings for lower exposure in children with most CT scan manufacturers in 2007 having this function built. Furthermore, certain conditions may require children to have CT scans. Study support tells parents about the risk of pediatric CT scans.
Contrast reactions
In the United States half of CT scans are contrasted CT using an intravenously injected radiocontrast agent. The most common reactions of these agents are mild, including nausea, vomiting, and an itchy rash; however, more severe reactions may occur. The overall reaction occurs in 1 to 3% with nonionic contrast and 4 to 12% of people with ionic contrast. Skin rashes can appear within a week to 3% of people.
The old radiocontrast agent causes anaphylaxis in 1% of cases whereas the newer agent, lower osmolar causes a 0.01-0.04% reaction of the case. Death occurs in about two to 30 people per 1,000,000 administrations, with newer agents becoming more secure. There is a higher risk of death in those who are women, elderly or in poor health, usually secondary to either anaphylaxis or acute renal failure.
Contrast agents may induce contrast-induced nephropathy. This occurs in 2 to 7% of people receiving this agent, at greater risk for those with pre-existing renal insufficiency, pre-existing diabetes, or intravascular volume depletion. People with mild renal impairment are usually advised to ensure full hydration for several hours before and after the injection. For moderate renal failure, use of iodinated contrast should be avoided; this may mean using an alternative technique than CT. Those with severe kidney failure who require dialysis require more stringent precautions, because their kidneys have fewer functions left so that further damage will not be seen and dialysis will eliminate contrast agents; it is usually recommended, however, to regulate dialysis as soon as possible after contrasting to minimize the adverse effects of contrast.
In addition to the use of intravenous contrast, oral contrast agents are often used when examining the stomach. This is often the same as an intravenous contrast agent, only diluted to about 10% of the concentration. However, oral alternatives to iodinated contrast exist, such as a very dilute barium sulfate suspension (0.5-1% w/v). Dilute barium sulfate has the advantage that it does not cause any type-allergic or renal failure, but can not be used in patients with bowel perforation suspected or suspected of intestinal injury, because barium sulphate leak from damaged intestine can cause fatal peritonitis.
Process
Computed tomography operates using an X-ray generator that rotates around an object; The X-ray detector is positioned on the opposite side of the circle from the X-ray source. The visual representation of the raw data obtained is called a sinogram, but not sufficient for interpretation. Once the scanning data is obtained, the data must be processed using a form of tomographic reconstruction, resulting in a series of cross-sectional images. The pixels in the images obtained by CT scans are shown in terms of relative radiodensity. The pixel itself is shown according to the average attenuation of the network (s) corresponding to the scale of 3071 (most debilitating) to -1024 (at least lessening) on ​​the Hounsfield scale. Pixels are two-dimensional units based on the size of the matrix and field of view. When CT slice thickness is also taken into account, this unit is known as Voxel, which is a three-dimensional unit. The phenomenon that one part of the detector can not distinguish between different networks is called "Effect Volume Section" . That means that large amounts of cartilage and a thin layer of bone can produce the same attenuation in voxel as hyperfense cartilage alone. Water has attenuation of 0 units Hounsfield (HU), while air is -1000Ã, HU, 400c cancellous bone, HU, skull bones can reach 2000Ã, HU or more (os temporale) and can cause artifacts. The attenuation of a metal implant depends on the atomic number of the element used: Titanium usually has 1000Ã, HU, steel can fully extinguish the X-ray and, therefore, is responsible for the well-known artifact line in the calculated tomogram. Artifacts are caused by sudden transitions between low and high density materials, resulting in data values ​​that exceed the dynamic range of electronic processing. Conventional CT images are conventionally given so that the view is as if looked up from the patient's legs. Therefore, the left side of the image is to the patient's right and vice versa, while the anterior in the image is also anterior to the patient and vice versa. This left-right exchange is consistent with the view that physicians generally have reality when positioned in front of the patient. A collection of CT data has a very high dynamic range that should be reduced for display or printing. This is usually done through the "windowing" process, which maps the range ("window") of the pixel value to the grayscale gradient. For example, brain CT images are usually seen with windows that extend from 0 HU to 80 HU. The pixel value 0 and lower, shown as black; 80 and higher values ​​are shown as white; the values ​​in the window are displayed as a gray intensity that is proportional to the position inside the window. The window used for the display should be matched to the X-ray density of the desired object, to optimize the visible detail.
Contrast
Contrast media used for X-ray CT, as well as for regular X-ray films, are called radiocontrasts. Radiocontrasts for X-ray CT, in general, iodine-based. This is useful for highlighting structures such as blood vessels that would otherwise be difficult to describe from the surroundings. Using contrasting materials can also help gain functional information about the network. Often, pictures are taken with and without radiocontras.
Dose scan
This table reports average radiation exposure, however, there can be a wide variation in radiation doses between the same types of scans, where the highest dose can be as much as 22 times higher than the lowest dose. A typical typical X-ray film involves a radiation dose of 0.01-0.15 mGy, while a typical CT can involve 10-20 mGy for a particular organ, and can rise up to 80 mGy for a particular particular CT.
For comparison purposes, the world average dose rate from a natural source of background radiation is 2.4 mSv per year, the same for practical purposes in this application up to 2.4 mGy per year. Although there are some variations, most people (99%) receive less than 7 mSv per year as background radiation. Medical imaging in 2007 accounted for half of their radiation exposure in the United States with a CT scan that makes up two-thirds of this amount. In the United Kingdom it accounts for 15% of radiation exposure. The average radiation dose from medical sources was 0.6 mSv per person globally in 2007. Those in the nuclear industry in the United States are limited to doses of 50 mSv per year and 100 mSv every 5 years.
Lead is the main ingredient used by radiographic personnel to protect against scattered X-rays.
Unit radiation dose
The reported dose of radiation in a gray or mGy unit is proportional to the amount of energy expected by the irradiated body part, and the physical effects (such as multiple DNA strands) on the cell's chemical bond by X-ray radiation are proportional to that energy.
The sievert unit is used in the effective dose report. The sievert unit, in the context of CT scan, does not match the actual dose of radiation absorbed by the scanned body part but for other radiation doses of other scenarios, the entire body absorbs other radiation doses and other radiation doses of magnitude, is estimated to have the same probability of inducing cancer as a CT scan. Thus, as shown in the table above, the actual radiation absorbed by the scanned body part is often much greater than the effective dose. The specific size, called computed tomographic dose index (CTDI), is generally used as an approximate dose of absorbed radiation for tissues within the scanning region, and is automatically calculated by medical CT scans.
An equivalent dose is an effective dose of a case, in which the whole body will actually absorb the same radiation dose, and the sievert unit is used in the report. In the case of non-uniform radiation, or radiation given only on body parts, which is common for CT examination, using locally equivalent doses alone would overstate the biological risks in all organisms.
Radiation effects
Most of the adverse health effects of radiation exposure can be grouped into two general categories:
- deterministic effects (dangerous tissue reactions) due in large part to cell killing/malfunctions after high doses; and
- Stochastic effects, ie, cancer and inherited effects that involve the development of cancer in individuals exposed to somatic cell mutations or hereditary diseases in their offspring due to mutation of reproductive cells (germs).
The additional lifetime risk of developing cancer by a single abdominal CT of 8 mSv is estimated to be 0.05%, or 1 one in 2,000.
Because of the increased susceptibility of the fetus to radiation exposure, the CT scan radiation dose is an important consideration in the selection of medical imaging in pregnancy.
Overdose
In October 2009, the US Food and Drug Administration (FDA) initiated a CT scan of the perfusion brain (PCT), based on radiation overdose caused by faulty settings at one facility specific to this type of CT scan. More than 256 patients over an 18 month period were exposed, more than 40% lost hair patches, and asked the editorial to request an improvement of the CT quality assurance program, while also noting that "while unnecessary exposure to radiation should be avoided, CT is medically required. acquired with proper acquisition parameters has a greater benefit than radiation risk. "Similar problems have been reported in other centers. This incident is believed to be due to human error.
Campaign
In response to increased public awareness and ongoing advancement of best practices, The Alliance for Radiation Safety in Pediatric Imaging is formed within the Society for Pediatric Radiology. Together 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 develops and launches an Image Gently Campaign designed to sustain high quality imaging studies using the lowest dose. and best radiation safety practices available in pediatric patients. This initiative has been supported and implemented by a growing list of professional medical organizations worldwide and has received support and assistance from companies that produce equipment used in Radiology.
Following 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 problem in an adult population called Images Wisely .
The World Health Organization and the International Atomic Energy Agency (IAEA) of the United Nations have also worked in this field and have ongoing projects designed to extend best practice and reduce patient radiation doses.
Prevalence
The use of CT has increased dramatically over the past two decades. An estimated 72 million scans were performed in the United States in 2007. Of these, six to eleven percent were performed in children, up seven to eight times that of 1980. Similar increases have been seen in Europe and Asia. In Calgary, Canada, 12.1% of people who come to the emergency room with an urgent complaint receive a CT scan, most often in the head or stomach. The percentage receiving CT, however, varies greatly by emergency physicians who see them from 1.8% to 25%. In the emergency department in the United States, CT or MRI imaging was performed in 15% of people injured in 2007 (up from 6% in 1998).
Increased use of CT scans has become the largest in two areas: adult screening (lung CT scan in smokers, virtual colonoscopy, heart CT screening, and whole-body CT in asymptomatic patients) and children's CT scanning. Shortening the scan time by about 1 second, eliminating the strict need for the subject to remain silent or anesthetized, is one of the main reasons for the large increase in pediatric populations (especially for the diagnosis of appendicitis). In 2007 in the United States, the proportion of CT scans was not necessary. Some estimates put this figure at 30%. There are a number of reasons for this including: legal issues, financial incentives, and wishes by the public. For example, some healthy people pay diligently to receive a whole-body CT scan 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 full of complexity, cumulative radiation exposure and can not be ignored, and money for scans involves opportunity costs (may be more effectively spent on more targeted screening or other health care strategies).
Presentations
The result of CT scan is volume voxels, which can be presented to human observers by various methods, which extensively fall into the following categories:
- Slice thinly. These are generally considered to be fields representing a thickness of less than 3 mm.
- Projection, including maximum intensity projection and average intensity projection
- Rendering volume (VR)
Technically, all rendering volumes are projected when viewed on a 2-dimensional display, making the distinction between projection and rendering volumes rather fuzzy. However, the epitome model of volume rendering displays mixes eg staining and shadows to create realistic and observable representations.
Conventional CT images are conventionally given so that the view is as if looked up from the patient's legs. Therefore, the left side of the image is to the patient's right and vice versa, while the anterior in the image is also anterior to the patient and vice versa. This left-right exchange is consistent with the view that physicians generally have reality when positioned in front of the patient.
Grayscale
The pixels in the images obtained by CT scans are shown in terms of relative radiodensity. The pixel itself is shown according to the average attenuation of the network (s) corresponding to the scale of 3071 (most debilitating) to -1024 (at least lessening) on ​​the Hounsfield scale. Pixels are two-dimensional units based on the size of the matrix and field of view. When CT slice thickness is also taken into account, this unit is known as Voxel, which is a three-dimensional unit. The phenomenon that one part of the detector can not distinguish between different networks is called "Effect Volume Section" . That means that large amounts of cartilage and a thin layer of bone can produce the same attenuation in voxel as hyperfense cartilage alone. Water has attenuation of 0 units Hounsfield (HU), while air is -1000Ã, HU, 400c cancellous bone, HU, skull bones can reach 2000Ã, HU or more (os temporale) and can cause artifacts. The attenuation of a metal implant depends on the atomic number of the element used: Titanium usually has 1000Ã, HU, steel can fully extinguish the X-ray and, therefore, is responsible for the well-known artifact line in the calculated tomogram. Artifacts are caused by sudden transitions between low and high density materials, resulting in data values ​​that exceed the dynamic range of electronic processing.
A collection of CT data has a very high dynamic range that should be reduced for display or printing. This is usually done through the "windowing" process, which maps the range ("window") of the pixel value to the grayscale gradient. For example, brain CT images are usually seen with windows that extend from 0 HU to 80 HU. The pixel value 0 and lower, shown as black; 80 and higher values ​​are shown as white; the values ​​in the window are displayed as a gray intensity that is proportional to the position inside the window. The window used for the display should be matched to the X-ray density of the desired object, to optimize the visible detail.
Projection
The projection includes the maximum intensity projection (MIP), which increases the area with high radiodensity, and is very useful for angiography studies. MIP reconstruction tends to increase the air space so it is useful for assessing the structure of the lungs.
The mean intensity projection essentially mimics conventional projection radiography, but can be used for a certain volume in the human body.
Multiplanar reconstruction
Multiplanary reconstruction (MPR) is the creation of incisions in the anatomical plane more than the usual (usually transverse) used for the acquisition of early tomography. It can be used for thin slices as well as projections. Multiplanar reconstruction is feasible because contemporary CT scans offer isotropic or near isotropic resolution.
MPR is often used to check the spine. Axial images through the spine will show only one vertebral body at a time and can not show intervertebral discs. By reformatting the volume, it becomes easier to visualize the position of one vertebral body in relation to the other.
Modern software allows reconstruction in a non-orthogonal (oblique) field so that the optimal field can be selected to display an anatomical structure. It may be very useful to visualize the bronchial structure because it does not lie orthogonally against the direction of scanning.
For vascular imaging, aircraft-curved reconstruction can be performed. This allows bends in the vessel to be "straightened" so that the entire length can be visualized on a single image, or a series of short drawings. Once the vessel has been "straightened out" in this way, quantitative measurements of the length and extent of the cross-section can be made, so that the operation or maintenance of the intervention can be planned.
Rendering of volume
The radiodensity threshold value is determined by the operator (eg, bone-related level). From this, a three-dimensional model can be constructed using an edge detection image processing algorithm and displayed on the screen. Some models can be constructed of various thresholds, allowing different colors to represent any anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.
The surface rendering is limited because it will display only the surface that meets the threshold density, and will only show the surface closest to the imaginary viewer. In volume rendering, transparency, color and shadow are used to allow better volume representation to be displayed in a single image. For example, the pelvic bone can be shown as semi-transparent, so that, even at an angle, one part of the image does not hide the other.
Image quality
Artifact
Although the image generated by CT is generally a faithful representation of the scanned volume, this technique is susceptible to a number of artifacts, as follows: Chapter 3 and 5
- Streak Artefact
- The lines are often seen around the material that blocks most of the X-rays, such as metal or bone. Many factors contribute to these lines: undersampling, starving photons, motion, hardening of light, and Compton landing. This type of artifact generally occurs in the posterior fossa of the brain, or if there is a metal implant. The lines can be reduced using newer reconstruction techniques or approaches such as reduction of metal artifacts (MAR). MAR techniques include spectral imaging, in which CT images are taken with different energy level photons, and then synthesized into monochromatic images with special software such as GSI (Gemstone Spectral Imaging).
- Partial volume effect
- This looks as "blurred" from the edge. This is because the scanner can not distinguish between small amounts of high density materials (eg, bone) and larger lower density (eg, cartilage). Reconstruction assumes that the X-ray damping in each voxel is homogeneous; this is unlikely to happen at the sharp end. This is most often seen in the z-direction, due to the highly anisotropic use of conventional voxels, which have much lower out-of-plane resolution, than in-field resolution. This can be partially solved by scanning using thinner slices, or isotropic acquisitions on modern scanners.
- Ring artifact
- Perhaps the most common mechanical artifact, the image of one or more "rings" appears in an image. They are usually caused by variations in the response of individual elements in a two-dimensional X-ray detector because of defects or miscalculations. Most ring artifacts can be reduced by the normalization of intensity, also referred to as flat field correction. The remaining rings can be suppressed by the transformation into the polar spaces, where they become linear lines. The evaluation of the ratio of reduction of ring artifacts to X-ray tomographic images shows that the Sijbers and Postnov methods can effectively suppress ring artifacts.
- Noise
- This appears as a grain on the image and is caused by a low signal to noise ratio. This happens more often when a thin slice thickness is used. This can also occur when the power supplied to the X-ray tube is not sufficient to penetrate the anatomy.
- Motion artifact
- This looks blurred and/or streaked, caused by the movement of the object being imaged. Motion blurring can be reduced by using a new technique called IFT (incompressible flow tomography).
- Windmill
- Streaky appearance can occur when the detector bypasses the reconstruction field. This can be reduced by filter or pitch reduction.
- Beam harden
- This can give "dotted look" when grayscale is visualized as high. This is because conventional sources, such as X-ray tubes emit polychromatic spectra. Photons of higher-energy photon levels are usually less attenuated. Therefore, the average energy of the spectrum increases as it passes through an object, often described as increasingly "difficult". This causes the effect of material thickness to be underestimated, if not corrected. Many algorithms exist to improve these artifacts. They can be divided into mono and multi-material methods.
Dose compared image quality
An important issue in radiology today is how to reduce the radiation dose during CT testing without compromising image quality. In general, higher radiation doses produce higher resolution images, while lower doses lead to an increase in image and image noise that is not sharp. However, increased doses increase the side effects, including the risk of radiation-induced cancer - a four-phase stomach CT provides the same radiation dose as 300 chest x-rays (See Scan dose section). Some methods that can reduce exposure to ionizing radiation during CT scan exist.
- New software technologies can significantly reduce the required radiation dose.
- Individualize the examination and adjust the radiation dose to the body type and the organs of the examined body. Different types and organs require different amounts of radiation.
- Before each CT examination, the evaluation of whether the exam is motivated or if another type of examination is more suitable. Higher resolutions are not always suitable for any particular scenario, such as small pulmonary mass detection.
Industrial use
Industrial CT scanning (industrial computed tomography) is a process that utilizes X-ray equipment to produce a 3D representation of components both externally and internally. CT scan industry has been used in many areas of industry for internal component inspection. Some of the keys used for CT scanning are defect detection, failure analysis, metrology, assembly analysis, image-based finite element method, and reversed engineering applications. CT scans are also used in the imaging and conservation of museum artefacts.
CT scans have also found applications in transport security (especially airport security where currently used in the context of material analysis for the detection of explosive CTX (blast detection equipment) and are also being considered for automated baggage/packet security scanning using computer vision based on object recognition algorithms which targets the detection of certain threat items based on 3D appearance (eg weapons, knives, liquid containers).
History
The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of the Radon transformation. In October 1963, Oldendorf received a US patent for "radiation energy equipment to investigate certain areas of interior objects obscured by solid materials". The first commercial CT scanner invented by Sir Godfrey Hounsfield in 1967.
Etymology
The word "tomography" comes from the Greek tome (slice) and graphein (for writing). Computed tomography was originally known as "EMI scan" as it was developed in the early 1970s at an EMI research branch, a company well known today for its music and recording business. It then became known as computed axial tomography ( CAT or CT scan ) and rntnography of body parts .
Although the term "computed tomography" can be used to describe positron emission tomography or single photon emission tomography (SPECT), in practice it usually refers to the tomographic calculations of X-ray images, especially in older medical literature and smaller medical facilities..
In MESH, "computed axial tomography" was used from 1977 to 1979, but current indexing explicitly included "X-rays" in the title.
The term sinogram was introduced by Paul Edholm and Bertil Jacobson in 1975.
Machine types
The rotating tube, usually called the CT spiral, or helical CT is an imaging technique in which all X-ray tubes revolve around the central axis of the area being scanned. This is the dominant type of scanner on the market because they have been manufactured longer and offer lower production and purchase costs. The main limitations of this type are the bulk and inertia of the equipment (X-ray tube assembly and array detector on the opposite side of the circle) which limits the speed at which the equipment can rotate. Some designs use two X-ray sources and an array of detectors offset by angles, as a technique for increasing temporal resolution.
Electron beam tomography (EBT) is a special form of CT in which a large X-ray tube is constructed so that only the electron path, traveling between the cathode and the anode of the X-ray tube, is rotated using a deflection coil.. This type has a great advantage because sweeping speeds can be faster, allowing the imaging of less opaque movement structures, such as heart and arteries. Fewer of these design scanners have been manufactured when compared to the type of rotating tube, largely because of the higher costs associated with the construction of far-flung tubes and a much larger array detector and limited anatomical coverage. Only one producer (Imatron, later acquired by General Electric) has ever produced this design scanner. Production stopped in early 2006.
In multislice computed tomography (MSCT), the higher number of tomographic pieces allows for high resolution imaging.
Direction of research
Photon count computed tomography is a CT technique under development. A typical CT scanner uses an energy-integrating detector; the photon is measured as a voltage on the capacitor proportional to the detected x-rays. However, this technique is susceptible to noise and other factors that can affect the voltage linearity to the x-ray intensity relationship. The photon counting sensor (PCD) is still affected by noise but does not change the number of measured photons. PCD has several potential advantages including improving signal (and contrast) to noise ratio, reducing dosage, increasing spatial resolution and, through the use of multiple energies, differentiating multiple contrast agents. PCD has recently become feasible in CT scans due to improved detector technology that can cope with the volume and data rate required. As of February 2016, CT photon counting is being used on three sites. Several preliminary studies have found the potential dose reduction of CT photon counting for breast imaging becomes very promising.
See also
References
External links
- Dose Radiation in X-Ray and CT Exam - American College of Radiology and Radiologic Society of North America
- Radiation Risk Calculator - American Society of Radiologic Technologists
- CTisus - CT scan protocols, drawings, and learning materials
- CTCases - Computerized Case Studies, drawings and protocols
- CT imaging development
- CT Artefacts - PPT by David Platten
- CT historic violin scanning
- Filler, Aaron (July 12, 2009). "History, Development and Impact of Computed Imaging in Neurological and Neurosurgery Diagnosis: CT, MRI, and DTI". Precedings Nature . doi: 10.1038/npre.2009.3267.5.
Source of the article : Wikipedia
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