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[] A CT scan, also known as computed tomography scan, makes use of computer-processed combinations of many measurements taken from different angles to produce cross-sectional () images (virtual 'slices') of specific areas of a scanned object, allowing the user to see inside the object without cutting. Other terms include computed axial tomography (CAT scan) and computer aided tomography. Is used to further generate a volume of the inside of the object from a large series of two-dimensional images taken around a single. Is the most common application of X-ray CT.
Its cross-sectional images are used for and therapeutic purposes in various medical disciplines. The rest of this article discusses medical-imaging X-ray CT; industrial applications of X-ray CT are discussed. The term 'computed tomography' (CT) is often used to refer to X-ray CT, because it is the most commonly known form. But, many other types of CT exist, such as (PET) and (SPECT). X-ray tomography, a predecessor of CT, is one form of, along with many other forms of tomographic and non-tomographic radiography.
CT produces data that can be manipulated in order to demonstrate various bodily structures based on their ability to absorb the X-ray beam. Although, historically, the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.
Although most common in medicine, CT is also used in other fields, such as. Another example is archaeological uses such as imaging the contents of sarcophagi. Individuals responsible for performing CT exams are called or radiologic technologists. Use of CT has increased dramatically over the last two decades in many countries. An estimated 72 million scans were performed in the United States in 2007. One study estimated that as many as 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5 to 2% with 2007 rates of CT use; however, this estimate is disputed, as there is not a consensus about the existence of damage from low levels of radiation.
Side effects from used in some types of studies include. Picture of a CT scout ( scanogram or topogram) as used for planning every scan slice.
Since its introduction in the 1970s, CT has become an important tool in to supplement and. It has more recently been used for or for disease, for example CT colonography for people with a high risk of colon cancer, or full-motion heart scans for people with high risk of heart disease. A number of institutions offer 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.
Computed tomography of, from to top. Taken with intravenous contrast medium.
CT scanning of the head is typically used to detect, tumors,, and bone trauma. Of the above, hypodense (dark) structures can indicate and infarction, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. Ambulances equipped with small bore multi-sliced CT scanners respond to cases involving stroke or head trauma. CT scanning of the head is also used in CT- and for treatment of intracranial tumors, arteriovenous malformations and other surgically treatable conditions using a device known as the. (MRI) of the head provides superior information as compared to CT scans when seeking information about headache to confirm a diagnosis of,, lesions, cervicomedullary lesions, or disorders. It also does not carry the risks of exposing the patient to.
CT scans may be used to diagnose headache when is indicated and MRI is not available, or in emergency settings when hemorrhage,, or are suspected. Even in emergency situations, when a head injury is minor as determined by a physician's evaluation and based on established guidelines, CT of the head should be avoided for adults and delayed pending clinical observation in the emergency department for children. Of a normal thorax, taken in the, and, respectively. CT scan can be used for detecting both acute and chronic changes in the parenchyma, that is, the internals of the lungs.
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 (,, and so forth), thin sections with high spatial frequency reconstructions are used; often scans are performed both in inspiration and expiration. This special technique is called high resolution CT. Therefore, it produces a sampling of the lung and not continuous images. An incidentally found nodule in the absence of symptoms (sometimes referred to as an ) may raise concerns that it might represent a tumor, either benign. 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.
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. 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. Angiography [ ]. Main article: Computed tomography angiography (CTA) is to visualize and vessels throughout the body. This ranges from arteries serving the to those bringing blood to the,, and.
An example of this type of exam is (CTPA) used to diagnose (PE). It employs computed tomography and an iodine based contrast agent to obtain an image of the. Cardiac [ ] A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy. Traditionally, cardiac CT scans are used to detect, diagnose or follow up coronary artery disease. 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. The main forms of cardiac CT scanning are: • (CTA): the use of CT to assess the of the. The subject receives an of and then the heart is scanned using a high speed, allowing radiologists to assess the extent of occlusion in the, usually in order to diagnose.
•: also used for the assessment of severity of. Specifically, it looks for calcium deposits in the coronary arteries that can narrow arteries and increase the risk of heart attack. A typical coronary CT calcium scan is done without the use of, but it can possibly be done from contrast-enhanced images as well. To better visualize the anatomy, post-processing of the images is common. 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. Abdominal and pelvic [ ]. Of 11 cm of right kidney in 13-month-old.
CT is an accurate technique for diagnosis of diseases. Its uses include diagnosis and staging of cancer, as well as follow up after cancer treatment to assess response.
It is commonly used to investigate acute abdominal pain. Extremities [ ] CT is often used to image complex, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries and dislocations can easily be recognised with a 0.2 mm resolution. Advantages [ ] There are several advantages that CT has over traditional 2D.
First, CT completely eliminates the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the,, or planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging. CT is regarded as a moderate- to high- diagnostic technique. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional radiography, for example, CT angiography avoids the invasive insertion of a catheter.
(also known as virtual colonoscopy or VC for short) is far more accurate than a for detection of tumors, and uses a lower radiation dose. CT VC is increasingly being used in the and US as a screening test for colon polyps and colon cancer and can negate the need for a in some cases. The radiation dose for a particular study depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. In addition, two helical CT scanning parameters that can be adjusted easily and that have a profound effect on radiation dose are tube current and pitch. Computed tomography (CT) scan has been shown to be more accurate than radiographs in evaluating anterior interbody fusion but may still over-read the extent of fusion. Adverse effects [ ] Cancer [ ] The used in CT scans can damage body cells, including DNA molecules, which can lead to cancer. According to the, between the 1980s and 2006, the use of CT scans has increased sixfold (+500%).
The radiation doses received from CT scans is variable. Compared to the lowest dose x-ray techniques, CT scans can have 100 to 1,000 times higher dose than conventional X-rays. However, a lumbar spine x-ray has a similar dose as a head CT. Articles in the media often exaggerate the relative dose of CT by comparing the lowest dose x-ray techniques (chest x-ray) with the highest dose CT techniques. In general, the radiation dose associated with a routine abdominal CT has a radiation dose similar to 3 years average background radiation (from cosmic radiation).
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.' Early estimates of harm from CT are partly based on similar radiation exposures experienced by those present during the explosions in Japan after the and those of workers.
A more recent study by the in 2009, based on scans made in 2007, estimated that 29,000 excess cancer cases and 14,500 excess deaths would be caused over the lifetime of the patients. Some experts project that in the future, between three and five percent of all cancers would result from medical imaging.
An Australian study of 10.9 million people reported that the increased incidence of cancer after CT scan exposure in this cohort was mostly due to irradiation. In this group one in every 1800 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. A person's age plays a significant role in the subsequent risk of cancer.
Estimated lifetime cancer mortality risks from an abdominal CT of a 1-year-old is 0.1% or 1:1000 scans. The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly. The estimates that the risk to a fetus being exposed to 10 mGy (a unit of radiation exposure, see ) increases the rate of cancer before 20 years of age from 0.03% to 0.04% (for reference a CT pulmonary angiogram exposes a fetus to 4 mGy). A 2012 review did not find an association between medical radiation and cancer risk in children noting however the existence of limitations in the evidences over which the review is based. CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of 2007 having this function built in. Furthermore, certain conditions can require children to be exposed to multiple CT scans. Studies support informing parents of the risks of pediatric CT scanning.
Contrast reactions [ ] In the United States half of CT scans are using intravenously injected. The most common reactions from these agents are mild, including nausea, vomiting and an itching rash; however, more severe reactions may occur.
Overall reactions occur in 1 to 3% with and 4 to 12% of people with. Skin rashes may appear within a week to 3% of people.
The old caused in 1% of cases while the newer, lower-osmolar agents cause reactions in 0.01–0.04% of cases. Death occurs in about two to 30 people per 1,000,000 administrations, with newer agents being safer. There is a higher risk of mortality in those who are female, elderly or in poor health, usually secondary to either anaphylaxis or acute renal failure. The contrast agent may induce. This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting, preexisting, 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 should be avoided; this may mean using an alternative technique instead of CT.
Those with severe renal failure requiring dialysis require less strict precautions, as their kidneys have so little function remaining that any further damage would not be noticeable and the dialysis will remove the contrast agent; it is normally recommended, however, to arrange dialysis as soon as possible following contrast administration to minimize any adverse effects of the contrast. In addition to the use of intravenous contrast, orally administered contrast agents are frequently used when examining the abdomen. These are frequently the same as the intravenous contrast agents, merely diluted to approximately 10% of the concentration. However, oral alternatives to iodinated contrast exist, such as very dilute (0.5–1% w/v) 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. Main article: Computed tomography operates by using an that rotates around the object; are positioned on the opposite side of the circle from the X-ray source. A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation.
Once the scan data has been acquired, the data must be processed using a form of, which produces a series of cross-sectional images. In an image obtained by CT scanning are displayed in terms of relative. The pixel itself is displayed according to the mean of the tissue(s) that it corresponds to on a scale from +3071 (most attenuating) to −1024 (least attenuating) on the. 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, which is a three-dimensional unit. The phenomenon that one part of the detector cannot differentiate between different tissues is called the 'Partial Volume Effect'. That means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone.
Water has an attenuation of 0 (HU), while air is −1000 HU, cancellous bone is typically +400 HU, cranial bone can reach 2000 HU or more (os temporale) and can cause artifacts. The attenuation of metallic implants depends on atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is, therefore, responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics.
Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. Hence, the left side of the image is to the patient's right and vice versa, while anterior in the image also is the patient's anterior and vice versa. This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients.
CT data sets have a very high which must be reduced for display or printing. This is typically done via a process of 'windowing', which maps a range (the 'window') of pixel values to a grayscale ramp. For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU. Pixel values of 0 and lower, are displayed as black; values of 80 and higher are displayed as white; values within the window are displayed as a grey intensity proportional to position within the window. The window used for display must be matched to the X-ray density of the object of interest, in order to optimize the visible detail. Contrast [ ].
Main article: used for X-ray CT, as well as for, are called. Radiocontrasts for X-ray CT are, in general, iodine-based. This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast. Scan dose [ ] Examination Typical () to the whole body Typical () to the organ in question Annual background radiation 2.4 2.4 Chest X-ray 0.02 0.01–0.15 Head CT 1–2 56 Screening 0.4 3 Abdomen CT 8 14 Chest CT 5–7 13 6–11 Chest, abdomen and pelvis CT 9.9 12 Cardiac CT angiogram 9–12 40–100 15 15 Neonatal abdominal CT 20 20 The table reports average radiation exposures, however, there can be a wide variation in radiation doses between similar scan types, where the highest dose could be as much as 22 times higher than the lowest dose.
A typical plain film X-ray involves radiation dose of 0.01 to 0.15 mGy, while a typical CT can involve 10–20 mGy for specific organs, and can go up to 80 mGy for certain specialized CT scans. For purposes of comparison, the world average dose rate from naturally occurring sources of is 2.4 mSv per year, equal for practical purposes in this application to 2.4 mGy per year. While there is some variation, most people (99%) received less than 7 mSv per year as background radiation. Medical imaging as of 2007 accounted for half of the radiation exposure of those in the United States with CT scans making up two thirds of this amount. In the United Kingdom it accounts for 15% of radiation exposure. The average radiation dose from medical sources is ≈0.6 mSv per person globally as of 2007.
Those in the nuclear industry in the United States are limited to doses of 50 mSv a year and 100 mSv every 5 years. Is the main material used by radiography personnel for against scattered X-rays. Radiation dose units [ ] The radiation dose reported in the unit is proportional to the amount of energy that the irradiated body part is expected to absorb, and the physical effect (such as DNA ) on the cells' chemical bonds by X-ray radiation is proportional to that energy. The unit is used in the report of the. The sievert unit, in the context of CT scans, does not correspond to the actual radiation dose that the scanned body part absorbs but to another radiation dose of another scenario, the whole body absorbing the other radiation dose and the other radiation dose being of a magnitude, estimated to have the same probability to induce cancer as the CT scan. Thus, as is shown in the table above, the actual radiation that is absorbed by a scanned body part is often much larger than the effective dose suggests.
A specific measure, termed the (CTDI), is commonly used as an estimate of the radiation absorbed dose for tissue within the scan region, and is automatically computed by medical CT scanners. The 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. Effects of radiation [ ]. Further information: Most adverse health effects of radiation exposure may be grouped in two general categories: • deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and • stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells. The added lifetime risk of developing cancer by a single abdominal CT of 8 is estimated to be 0.05%, or 1 one in 2,000. Because of increased susceptibility of fetuses to radiation exposure, the radiation dosage of a CT scan is an important consideration in the choice of.
Excess doses [ ] In October, 2009, the US Food and Drug Administration (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based on of radiation caused by incorrect settings at one particular facility for this particular type of CT scan. Over 256 patients over an 18-month period were exposed, over 40% lost patches of hair, and prompted the editorial to call for increased CT quality assurance programs, while also noting that 'while unnecessary radiation exposure should be avoided, a medically needed CT scan obtained with appropriate acquisition parameter has benefits that outweigh the radiation risks.'
Similar problems have been reported at other centers. These incidents are believed to be due to. Campaigns [ ] 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. In concert with The, The and The, 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.
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. The and (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. Prevalence [ ] Use of CT has increased dramatically over the last two decades.
An estimated 72 million scans were performed in the United States in 2007. Of these, six to eleven percent are done in children, an increase of seven to eightfold from 1980. Similar increases have been seen in Europe and Asia. Download Buku Discipline Belajar Iqra there. 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 who saw them from 1.8% to 25%. In the emergency department in the United States, CT or MRI imaging is done in 15% of people who present with as of 2007 (up from 6% in 1998). 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 ). As of 2007 in the United States a proportion of CT scans are performed unnecessarily.
Some estimates place this number at 30%. There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public. For example, some healthy people avidly pay to receive full-body CT scans as, but it is not at all clear that the benefits outweigh the risks and costs, because deciding whether and how to treat is fraught with complexity, radiation exposure is cumulative and not negligible, and the money for the scans involves (it may have been more effectively spent on more targeted screening or other health care strategies).
Presentation [ ]. Types of presentations of CT scans: - Average intensity projection - - Thin slice () - by high and low threshold for. The result of a CT scan is a volume of, which may be presented to a human observer by various methods, which broadly fit into the following categories: • Thin slice.
This is generally regarded as planes representing a thickness of less than 3 mm. • Projection, including and average intensity projection • (VR) Technically, all volume renderings become projections when viewed on a, making the distinction between projections and volume renderings a bit vague. Still, the epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations.
Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. 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. Download Nada Dering Pesan Hp Blackberry.
This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients. Grayscale [ ] in an image obtained by CT scanning are displayed in terms of relative. The pixel itself is displayed according to the mean of the tissue(s) that it corresponds to on a scale from +3071 (most attenuating) to −1024 (least attenuating) on the. 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, which is a three-dimensional unit.
The phenomenon that one part of the detector cannot differentiate between different tissues is called the 'Partial Volume Effect'. That means that a big amount of cartilage and a thin layer of compact bone can cause the same attenuation in a voxel as hyperdense cartilage alone. Water has an attenuation of 0 (HU), while air is −1000 HU, cancellous bone is typically +400 HU, cranial bone can reach 2000 HU or more (os temporale) and can cause artifacts. The attenuation of metallic implants depends on atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is, therefore, responsible for well-known line-artifacts in computed tomograms.
Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics. CT data sets have a very high which must be reduced for display or printing. This is typically done via a process of 'windowing', which maps a range (the 'window') of pixel values to a grayscale ramp. For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU.
Pixel values of 0 and lower, are displayed as black; values of 80 and higher are displayed as white; values within the window are displayed as a grey intensity proportional to position within the window. The window used for display must be matched to the X-ray density of the object of interest, in order to optimize the visible detail. Projections [ ] Projections include (MIP), which enhance areas of high radiodensity, and so are useful for angiographic studies. MIP reconstructions tend to enhance air spaces so are useful for assessing lung structure. Average intensity projection essentially imitates conventional, but can be used for specific volumes within the human body.
Multiplanar reconstruction [ ]. Typical screen layout for diagnostic software, showing one volume rendering (VR) and multiplanar view of three thin slices. Multiplanar reconstruction (MPR) is the creation of slices in more than the one (usually ) used for initial tomography acquisition. It can be used for thin slices as well as projections. Multiplanar reconstruction is feasible because contemporary CT scanners offer or near isotropic resolution. MPR is frequently used for examining the spine.
Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral discs. By reformatting the volume, it becomes much easier to visualise the position of one vertebral body in relation to the others. Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display an anatomical structure. This may be particularly useful for visualising the structure of the bronchi as these do not lie orthogonal to the direction of the scan. For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be 'straightened' so that the entire length can be visualised on one image, or a short series of images. Once a vessel has been 'straightened' in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment can be planned.
Volume rendering [ ]. Main article: A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). From this, a three-dimensional model can be constructed using image processing algorithms and displayed on screen. Multiple models can be constructed from various thresholds, allowing different colors to represent each anatomical component such as bone, muscle, and cartilage. However, the interior structure of each element is not visible in this mode of operation.
Surface rendering is limited in that it will display only surfaces that meet a threshold density, and will display only the surface that is closest to the imaginary viewer. In, transparency, colors and are used to allow a better representation of the volume to be shown in a single image. For example, the bones of the pelvis could be displayed as semi-transparent, so that, even at an oblique angle, one part of the image does not conceal another. A series of CT scans converted into an animated image using Photoshop Artifacts [ ] 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: Chapters 3 and 5 Streak artifact Streaks are often seen around materials that block most X-rays, such as metal or bone.
Numerous factors contribute to these streaks: undersampling, photon starvation, motion, beam hardening, and. This type of artifact commonly occurs in the posterior fossa of the brain, or if there are metal implants. The streaks can be reduced using newer reconstruction techniques or approaches such as metal artifact reduction (MAR). MAR techniques include spectral imaging, where CT images are taken with of different energy levels, and then synthesized into images with special software such as GSI (Gemstone Spectral Imaging). Partial volume effect This appears as 'blurring' of edges. It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g., bone) and a larger amount of lower density (e.g., cartilage). The reconstruction assumes that the X-ray attenuation within each voxel is homogenous; this may not be the case at sharp edges.
This is most commonly seen in the z-direction, due to the conventional use of highly voxels, which have a much lower out-of-plane resolution, than in-plane resolution. This can be partially overcome by scanning using thinner slices, or an isotropic acquisition on a modern scanner. Ring artifact Probably the most common mechanical artifact, the image of one or many 'rings' appears within an image. They are usually caused by the variations in the response from individual elements in a two dimensional X-ray detector due to defect or miscalibration.
Ring artefacts can largely be reduced by intensity normalization, also referred to as flat field correction. Remaining rings can be suppressed by a transformation to polar space, where they become linear stripes. A comparative evaluation of ring artefact reduction on X-ray tomography images showed that the method of Sijbers and Postnov can effectively suppress ring artefacts.
Noise This appears as grain on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy. Motion artifact This is seen as blurring and/or streaking, which is caused by movement of the object being imaged.
Motion blurring might be reduced using a new technique called IFT (incompressible flow tomography). Windmill Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.
Beam hardening This can give a 'cupped appearance' when grayscale is visualized as height. It occurs because conventional sources, like X-ray tubes emit a polychromatic spectrum. Photons of higher levels are typically attenuated less. Because of this, the mean energy of the spectrum increases when passing the object, often described as getting 'harder'. This leads to an effect increasingly underestimating material thickness, if not corrected. Many algorithms exist to correct for this artifact.
They can be divided in mono- and multi-material methods. Dose versus image quality [ ] 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, while lower doses lead to increased image noise and unsharp images.
However, increased dosage raises the adverse side effects, including the risk of – a four-phase abdominal CT gives the same radiation dose as 300 chest X-rays (See the section). Several methods that can reduce the exposure to ionizing radiation during a CT scan exist.
• New software technology can significantly reduce the required radiation dose. • Individualize the examination and adjust the radiation dose to the body type and body organ examined. Different body types and organs require different amounts of radiation. • Prior to every CT examination, evaluate the appropriateness of the exam whether it is motivated or if another type of examination is more suitable. Higher resolution is not always suitable for any given scenario, such as detection of small pulmonary masses. Industrial use [ ] (industrial computed tomography) is a process which utilizes X-ray equipment to produce 3D representations of components both externally and internally.
Industrial CT scanning has been utilized 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 and reverse engineering applications.
CT scanning is also employed in the imaging and conservation of museum artifacts. CT scanning has also found an application in transport security (predominantly where it is currently used in a materials analysis context for explosives detection and is also under consideration for automated baggage/parcel security scanning using based object recognition algorithms that target the detection of specific threat items based on 3D appearance (e.g. Guns, knives, liquid containers).
Radiology section, Diagnostic Ultrasound subsection, 9, is usually divided on the basis of the anatomic location of the procedure (chest, pelvis) n Medicine section, Non-Invasive Vascular Diagnostic Studies subsection, 0, divided on the basis of the anatomic location of the procedure (cerebrovascular, extremity) n Medicine section, Echocardiography (ultrasound of the heart and great arteries), 2 An interventional radiologist is a physician who is skilled in both the surgical procedure and the radiology portion of an interventional radiologic service.