Computed tomography (CT) scans are completed with the use of a 360-degree x-ray beam and computer production of images. These scans allow for cross-sectional views of body organs and tissues. Computed tomography is also known as computerized axial tomography or CAT scan.
CT scans are used to image a wide variety of body structures and internal organs. Since the 1990s, CT equipment has become more affordable and available. In some diagnoses, CT scans have become the first imaging exam of choice. Because the computerized image is so sharp, focused, and three-dimensional, many tissues can be better differentiated than on standard x rays. Common CT indications include:
- Sinus studies. The CT scan can show details of sinusitis and bone fractures. Physicians may order a CT scan of the sinuses to provide an accurate map for surgery.
- Brain studies. Brain scans can detect tumors, strokes, and hematomas (collections of blood that have escaped from the vessels). The introduction of CT scanning, especially spiral CT, has helped reduce the need for more invasive procedures such as cerebral angiography.
- Body scans. CT scans of the body will often be used to observe abdominal organs, such as the liver, kidneys, adrenal glands, spleen, pancreas, biliary tree and lymph nodes, and extremities.
- Aorta scans. CT scans can focus on the thoracic or abdominal sections of the aorta to locate aneurysms and other possible aortic diseases.
- Chest scans. CT scans of the chest are useful in distinguishing tumors and in detailing accumulation of fluid in chest infections.
Computed tomography is a combination of focused x-ray beams, a detector array, and computerized production of an image. Introduced in the early 1970s, this radiologic procedure has advanced rapidly and is now widely used, sometimes in the place of standard x rays.
A CT scan may be performed in a hospital or outpatient imaging center. Although the equipment looks large and intimidating, it is very sophisticated and fairly comfortable. The patient is asked to lie on a narrow table that slides into the center of the scanner, called the gantry. The scanner looks like a square doughnut with a round opening in the middle, which allows the x-ray beam to rotate around the patient. The scanner's gantry section may also be tilted slightly to allow for certain cross-sectional angles.
The patient will feel the table move very slightly as the precise adjustments for each sectional image are made. A technologist watches the procedure from a window and views the images on a monitor.
It is essential that the patient lie very still during the procedure to prevent motion blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and referring physician to review. A radiologist can also interpret CT exams on a special viewing console. The procedure time will vary in length depending on the area being imaged. Average study times are from 30 to 60 minutes. Some patients may be concerned about claustrophobia, but the width of the gantry portion of the scanner is wide enough to preclude problems with claustrophobia, in most instances.
The CT image
Traditional x rays image organs in two dimensions, with the possibility that organs in the front of the body are superimposed over those in the back. CT scans allow for a more three-dimensional effect. Some have compared CT images to slices in a loaf of bread. Precise sections of the body can be located and imaged as cross-sectional views. The technologist's console displays a computerized image of each section captured by the xray beam and detector array. Thus, various densities of tissue can be easily distinguished.
Contrast agents are often used in CT exams and in other radiology procedures to demonstrate certain anatomic details that, otherwise, may not be seen easily. Some contrast agents are natural, such as air or water. Other times, a water-based contrast agent is administered for specific diagnostic purposes. Barium sulfate is commonly used in gastrointestinal procedures. The patient may drink this contrast medium, or receive it in an enema. Oral and rectal contrasts are usually given when examining the abdomen or gastrointestinal tract, and not used when scanning the brain or chest. Iodine-based contrast media are the most widely used intravenous contrast agents and are usually administered through an antecubital (in front of the elbow) vein.
If contrast agents are used in the CT exam, these will be administered several minutes before the study begins. Abdominal CT patients may be asked to drink a contrast medium. Some patients may experience a salty taste, flushing of the face, warmth, slight nausea, or hives from an intravenous contrast injection. Technologists and radiologists have equipment and training to help patients through these minor reactions and to handle more severe reactions. Severe reactions to contrast are rare, but do occur.
Spiral CT, also called helical CT, is a newer version of CT scanning that is continuous in motion and allows for three-dimensional re-creation of images. For example, traditional CT allows the technologist to take slices at very small and precise intervals one after the other. Spiral CT allows for a continuous flow of images, without stopping the scanner to move to the next image slice. A major advantage of spiral CT is higher resolution and the ability to reconstruct images anywhere along the length of the study area. The procedure also speeds up the imaging process, meaning less time for the patient to lie still. The ability to image the contrast medium more rapidly after it is injected and when it is at its highest level, is another advantage of spiral CT's high speed.
Some facilities have both spiral and conventional CT available. Although spiral is more advantageous for many applications, conventional CT is still a superior and precise method for imaging many tissues and structures. The physician will evaluate which type of CT works best for the specific exam purpose.
If a contrast medium must be administered, the patient may be asked to fast from about four to six hours prior to the procedure. This is so if a patient experiences nausea, vomiting will not occur. Patients will usually be given a gown (like a typical hospital gown) to be worn during the procedure. All metal and jewelry should be removed to avoid artifacts on the film. Pregnant women or those who could possibly be pregnant should not have a CT scan unless the diagnostic benefits outweigh the risks. Contrast agents are often used in CT exams and the use of these agents should be discussed with the medical professional prior to the procedure. Patients should be asked to sign a consent form concerning the administration of contrast media. One common ingredient in contrast agents, iodine, can cause allergic reactions. Patients who are known to be allergic to iodine (or shellfish) should inform the physician prior to the CT scan.
No aftercare is generally required following a CT scan. Immediately following the exam, the technologist will continue to watch the patient for possible adverse contrast reactions. Patients are instructed to advise the technologist of any symptoms, particularly respiratory difficulty. The site of contrast injection will be bandaged and may feel tender following the exam. Hives may develop later and usually do not require treatment.
Radiation exposure from a CT scan is similar to, though higher than, that of a conventional x ray. Although this is a risk to pregnant women, the exposure to other adults is minimal and should produce no effects. Although severe contrast reactions are rare, they are a risk of many CT procedures. There is also a small risk of renal failure in high-risk patients.
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues and fluid will show as various shades of gray, and fat will be dark gray or black. Air will also look black and darker than fat tissue. Intravenous, oral, and rectal contrast appear as white areas. The radiologist can determine if tissues and organs appear normal by the different gradations of the gray scale. In CT, the images that can cut through a section of tissue or organ provide three-dimensional viewing for the radiologist and referring physician.
Abnormal results may show different characteristics of tissues within organs. Accumulations of blood or other fluids where they do not belong may be detected. Radiologists can differentiate among types of tumors throughout the body by viewing details of their makeup.
The increasing availability and lowered cost of CT scanning has led to its increased use in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an abnormal sinus radiograph. The sensitivity of CT allows for location of areas of sinus infection, particularly chronic infection, and is useful for planning prior to functional endoscopic sinus surgery . CT scans can show the extent and location of tiny fractures of the sinus and nasal bones. Foreign bodies in the sinus and nasal area are also easily detected by CT. CT imaging of the sinuses is important in evaluating trauma or disease of the sphenoid bone (the wedge-shaped bone at the base of the skull). Sinus tumors will show as shades of gray indicating the difference in their density from that of normal tissues in the area.
The precise differences in density allowed by CT scanning can clearly show tumors, strokes, or other lesions in the brain area as altered densities. These lighter or darker areas on the image may indicate a tumor or hemorrhage within the brain. Different types of tumors can be identified by the presence of edema, by the tissue's density, or by abnormal contrast enhancement. Congenital abnormalities in children, such as hydrocephalus, may also be confirmed with CT. Hydrocephalus is suggested by enlargement of the fluid structures, called ventricles, of the brain.
The body scan can identify abnormal body structures and organs. Throughout the body, a CT scan may indicate tumors or cysts; enlarged lymph nodes; abnormal collections of fluid, blood, or fat; and metastasis of cancer. Fractures or damage to soft tissues can be more easily seen on the sensitive images produced by CT scanning. Liver conditions, such as cirrhosis, abscess, and fatty liver, may be observed with a CT body scan.
CT of the aorta
CT provides the ability to visualize and measure the thickness of the aorta, which is very helpful in diagnosing aortic aneurysms. The use of contrast will help define details within the aorta. In addition, increased areas of density can identify calcification, which helps differentiate between acute and chronic problems. An abnormal CT scan may indicate signs of aortic clots. Aortic rupture is suggested by signs, such as a hematoma around the aorta or the escape of blood or contrast from its cavity.
In addition to those findings which may indicate aortic aneurysms, chest CT studies can show other problems in the heart and lungs. The computer will not only show differences between air, water, tissues, and bone, but will also assign numerical values to the various densities. Mass lesions in the lungs may be indicative of tuberculosis or tumors. CT will help distinguish between the two. Enlarged lymph nodes in the chest area may indicate lymphoma. Spiral CT is particularly effective at identifying pulmonary emboli (clots in the lung's blood vessels).
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A computed axial tomograph is an axial (cross sectional) view computed from a large set of values, each corresponding to the attenuation of an x-ray beam passed transversely (i.e. in the plane of the desired image) through the subject. In medical applications the subject is a patient; however, applications are not limited to medicine. Computed tomography (fr Greek tomos and graphikos for section or slice and drawing, respectively) has applications in areas as diverse as material sciences, archeology, manufacturing, and facility security.
A CAT Scanner is a machine designed to collect and process the x ray transmission data required to construct and display a computed axial tomograph. "CT scanner" is nearly synonymous with CAT scanner, but recognizes that computed views are not limited to axial views.
X rays (comprised of photons) diminish exponentially in intensity as they pass through a material of thickness Δx according to the relationship:
(1) Iout = Iin * e−aΔx. Where "I" is the intensity, "e" is the Naperian constant, "a" is the attenuation coefficient of the material, and "Δx" is the distance traveled. When "n" adjacent materials are transversed, the formula can be applied repeatedly to yield
(2) Iout = Iin * e−a0Δx0 * e−a1Δx1 * e−a2Δx2 * … * e−anΔxn. Which is equivalent to
(3) Iout = Iin * e−a0Δx0 + −a1Δx1 + −a2Δx2 + … + −anΔxn, which yields
(4) −ln(Iout/Iin) = a0Δx0 + a1Δx1 + a2Δx2 + … + anΔxn
Thus the logarithm of the attenuation along any ray is a linear function of the distances traveled through the materials encountered and their respective attenuation coefficients.
Scope of computation
The fundamental computational problem of axial tomography is to solve a group of equations for the attenuation coefficients of each voxel. A voxel–is a volume element, in particular the smallest volume element the system can manage—similar concept to pixel. The solution grid is chosen from the points where rays from different rotation angles intersect. After the attenuation coefficients have been determined for the solution grid, averaging and interpolation are used to superimpose a display grid of cubical voxels. For useful resolution, voxels must be small, consequently the system of equations
becomes very large, i.e., for a resolution of 2mm x 2mm across a body 500mm (19.7 inches) in diameter the minimum number of equations for a solution will be on the order of 64,000 for each transverse slice or "cut." If the detector arc contains 256 detectorsper slice, recordings from about 250 positions around the axis are required. Scanning a region 10 cm long with a corresponding resolution (2mm/cut) will require 50 cuts and the total number of equations becomes 3,200,000. Fortunately mathematicians and computer scientists have been pursuing efficient computational methods for similar problems for decades. A profound improvement was made in 1965 when J. W. Cooley and J. W. Tukey introduced the fast Fourier transform. Present methods require working knowledge of linear algebra, calculus, differential equations, matrices, vector calculus, Fourier transforms, and some programming experience to be appreciated.
By 2002, scans of the resolution and extent mentioned above could be acquired and solved within a few seconds. Consequently the radiation exposure of the patient was limited to safe levels despite the amount of information obtained.
A realistic scanner (Figure 1) has a point as the source of x rays. This point produces a wedge-shaped beam aimed at an arc of detectors (possibly more than one layer thick, to acquire data for adjacent cuts simultaneously). The source and detectors have a constant geometric relationship because they are fixed to a rigid ring (gantry). The ring is rotated about the axis of the subject lying on an x ray–transparent table that moves along the ring axis. Exposures are made at many angular positions to acquire sufficient data to compute the voxel values for the cut(s). For the resulting attenuation data-set to be useful it must be recorded with precise values for the corresponding angular position of the beam and the linear position of the table. If slip rings (or telemetry) connect the power, control, and data cables between the
ring and the frame, the ring can rotate continuously without reversing between cuts. Continuous rotation with continuous linear table motion of one detector-array-width per ring-revolution produces an aptly named helical scan (loosely called a "spiral" scan). If the detectors and processing electronics are fast enough, rotation, exposure, and detection can also be continuous. The fastest systems can complete a scan of a beating heart so rapidly that there is negligible motion artifact in the computed image—similar to stop action photography with fast shutter speeds and fast film.
Austrian mathematician Johann Radon (1887–1956) suspected that images might be created from rays traveling in the plane of a subject and in 1917 presented a proof that this was at least theoretically possible. Decades later, his work proved fundamental in developing methods for computing axial tomographs. CAT scanners could not be built until the late 1960s, as even a rudimentary CAT scanner requires the confluence of imagination and several technologies (just as da Vinci's helicopter had to wait for the invention of the internal combustion engine and light metal alloys).
CAT scanners require affordable computational power, reliable micro-sized solid state detectors, materials dimensionally stable with respect to time and temperature, high power x ray–source tubes with precisely shaped anodes, and considerable sophistication in computational methods. Similarly, the burgeoning field of computational chemistry would be nonexistent without computational power unattainable until very the end of the twentieth century.
British engineer Sir Godfrey Newbold Hounsfield invented the CAT scanner as soon as it was practical to do so. Beginning in the late 1960s he conceived demonstration conceptual laboratory models that took hours to acquire and analyze data for a single low-resolution slice. Hounsfield was assisted in implementing his ideas by radiologists James Ambrose and
Louis Kreel. Hounsfield obtained a British patent in 1972 and a US patent in 1976 and shared the 1979 Nobel Prize for medicine and physiology, with Allan MacLeod Cormack, a physicist at Yale who had independently developed mathematical methods required to solve the inverse problem of axial tomography.
Such a vast quantity of information is collected by high-resolution scanners that additional refinements are needed to help physicians quickly absorb the information. The principal aids are computed color contrasts and images computed from selected viewpoints.
Contrast and Color. The attenuation differences of similar tissues can be enhanced by the administration (intravenously or by mouth) of contrast agents (sometimes called "dyes") that will collect preferentially in one tissue type before eventual elimination by the body. These agents contain salts of heavy atoms, such as iodine, that attenuate x rays more rapidly than living tissue. Even if the attenuation coefficient change might be inapparent to a human viewing a simple gray-level presentation of the data, differences detectable to the imaging system (with or without contrast) permit attenuation coefficients within different ranges to be displayed as different colors: such as red for blood, pink for muscle, silver for tendon, ivory for bone, and yellow for nervous tissue.
Computed Views. By convention, medical CAT views are oriented with the viewer looking from the feet toward the head with the patient supine. However once the grids of multiple slices have been solved for the attenuation coefficient of each voxel, there are no restrictions except imagination and computation costs on how that information can be presented. Such constructed views actually contain no more information than a series-of-slices presentation, but they can present the information in a way that emphasizes clinically important details. Imagine a rash on the inside of the thorax, this would be very difficult to perceive by mentally integrating the subtle chest wall changes from dozens of slice views. In contrast a computed view looking toward the chest wall from inside the thorax might make the rash obvious. However, only a careful medical history and examination would prompt the physician to request that view from the infinite number of possible computed views.
Very high resolution systems (voxel diameters of 1 mm or less) may replace exploratory procedures such as endoscopy. The scan data can be processed interactively to give the physician views corresponding to a "virtual" endoscopic examination under his control. However, parallel studies in which hundreds of patients receive literal or virtual visualization procedures with comparison of diagnostic accuracy, and long term outcome will be necessary before virtual examinations can be accepted as the standard of medical care.
see also Nuclear Magnetic Resonance; Spectroscopy.
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"CT Scans." Chemistry: Foundations and Applications. . Retrieved April 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/ct-scans