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COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS . Without reference, identify principles relating to Computed Tomography Clinical Applications with at least 70 percent accuracy. COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS . COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS .
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COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS Without reference, identify principles relating to Computed Tomography Clinical Applications with at least 70 percent accuracy
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • The History of Computed Tomography • Computed Tomography (CT) imaging is also known as "CAT scanning" (Computed Axial Tomography) • Tomography is from the Greek word "tomos" meaning "slice" or "section" and "graphia" meaning "describing" • CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories, England, and independently by South African born physicist Allan Cormack of Tufts University, Massachusetts • Hounsfield was later awarded the Nobel Peace Prize and honored with Knighthood in England for his contributions to medicine and science
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Applications of CT • Unlike other medical imaging techniques, such as conventional x-ray imaging, CT enables direct imaging and differentiation of soft tissue structures, such as • Liver • Lung tissue • Fat • Therefore CT is a valuable tool, for instance, in searching for large space occupying lesions, tumors and metastasis • CT scans can not only reveal the presence but also the size, spatial location and extent of a tumor
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • CT imaging of the head and brain can detect tumors, show blood clots and blood vessel defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid) and image other abnormalities such as those of the nerves or muscles of the eye • Due to the short scan times of 500 milliseconds to a few seconds, CT can be used for all anatomic regions, including those susceptible to patient motion and breathing • For example, in the thorax, CT can be used for visualization of nodular structures, infiltration of fluids, and fibrosis • CT exams are fast and simple and enable a quick overview of possibly life-threatening pathology and rapidly enable a dedicated surgical treatment. Therefore, CT is becoming the method of choice for imaging trauma patients
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • The first clinical CT scanners were installed between 1974 and 1976 • The original systems were dedicated to head imaging only, but "whole body" systems with larger patient openings became available in 1976 • CT became widely available by about 1980 • There are now about 6,000 CT scanners installed in the U.S. and about 30,000 installed worldwide • The latest multi-slice CT systems can collect up to 4 slices of data in about 350 ms and reconstruct a 512 x 512 matrix image from millions of data points in less than a second
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • The X-ray System • Tube and gantry • The x-ray tube is mounted on a circular gantry assembly, which rotates around the patient's body • There are two ways to supply power to the tube while it rotates • Cables • Designed to only make a couple of rotations • The gantry must be stopped and rotated in the other direction to uncoil the cables • Sliding electrical contacts (or slip rings) - they permit continuous high-speed rotation
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Collimation - two sets of collimators • One set of collimators determines the angular span of the beam • The other set of collimators determines the thickness of the beam • Filtration - CT x-ray beams are filter for two purposes • Beam hardening • In CT imaging the x-ray beam creates an image artifact because of the peripheral tissue is exposed to a lower average photon energy than the inner portion of the slice • This filtration reduces patient exposure by selectivity removing the low-energy low-penetration part of the x-ray beam
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Compensation • Compensates for the non-uniform thickness of the human body • It is thicker near the edges and is sometimes referred to as the bow-tie filter • Power supply - typically a constant potential type that can produce relatively high KV and MA values for a sustained period of time • Detectors - the radiation receptor is an array of many small detectors that are mounted within the gantry assembly • Function • Absorbs the radiation it intercepts • Produces an electrical signal proportional to the radiation intensity.
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Configurations • The way in which the detectors are arranged and moved during the scanning process has changed during the evolution of the CT scanner and is different among scanners used today • There are four generations of detector configuration • First Type A. Used a single detector element that was moved, along with the tube, in a straight line across the patient's body to form one view B. Then the tube and detector assembly was rotated 1°and scanned across the body to form the second view • Second type - used multiple detectors and reduced the number of rotations required to achieve a full scan
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Third type (Rotate-rotate scanner) A. An array of individual detector elements that is just large enough to form one view is mounted on the gantry B. It rotates along with the x-ray tube • Fourth Type (Stationary-rotate scanner) A. A ring of detectors that completely encircles the patient B. The detectors remain stationary as the tube rotates around the patient • Computer - performs several functions • Control • After the operator selects the appropriate scanning factors and initiates the scan, the procedure progresses under the control of the computer • It coordinates and times the sequence of events during the scan • Turning the beam and detectors on and off at the appropriate times • Transferring data • Monitoring the system operation
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • Processing - directly involved in the formation of the CT image through processing data into the image • Storage and retrieval - it transfers, stores, and retrieves images and data • Display unit and camera • Display unit displays an image on a CRT or video monitor • Camera converts image to film • CT image formation • The formation of a CT image is a distinct three phase process • The CT image is, for all practical purposes, an image of three densities of the tissue • The scanning phase • The x-ray beam is scanned around the body • The amount of radiation that penetrates the body is measured by the detectors and converted into data
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • The reconstruction phase • Back projection • The data produced is not a complete image, but a profile of the objects that have been x-rayed • Only enough data in the profile allows the computer to draw in streaks • As the x-ray beam rotates around the body, obtaining different views, we see the beginnings of an image • CT number • A digital image of CT is in the form of a matrix of pixels • A part of the reconstruction phase is to calculate a CT number for each image pixel • Water is the reference material for CT numbers and has an assigned value of zero • Materials with density greater than water will have a positive CT number • Materials that is less dense than water will have a negative Ct number • CT numbers are measured in Hounsfield Units
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • The digital to analog conversion phase • The digital image, consisting of a matrix of pixels with each pixel having a CT number, is converted into a visible image represented by different shades of gray or brightness levels by windowing • Windowing controls contrast in CT imaging • The window is the range of CT numbers that will be displayed with the different shades of gray, ranging from black to white • Tissues within the window will have different shades of gray (brightness) and will have visible contrast • All tissues and materials that have CT numbers above the window will be all white and no contrast within this range
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • All that have CT numbers below the window will be all black and without contrast • The level control adjusts the center of the window • The width control adjusts the range of CT numbers that will be displayed with contrast • The width controls the contrast in the displayed image • Reducing window width increases the displayed image contrast among the tissues • The ability to window is what gives CT a very high contrast sensitivity • This is because a window can be set to display and make visible very small differences in tissue densities
COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS • All tissues and materials that have CT numbers above the window will be all white and no contrast within this range • All that have CT numbers below the window will be all black and without contrast • The level control adjusts the center of the window • The width control adjusts the range of CT numbers that will be displayed with contrast • The width controls the contrast in the displayed image • Reducing window width increases the displayed image contrast among the tissues • The ability to window is what gives CT a very high contrast sensitivity • This is because a window can be set to display and make visible very small differences in tissue densities