1. Without reference, identify principles relating to Computed Tomography Clinical Applications with at least 70 percent accuracy

3.  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

4.  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

5.  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

6.  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

7.  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

8.  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

9. 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.

10. 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

11. » 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

12. 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

13.  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

14.  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

15. 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

16. 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