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 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
 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
 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

 CT imaging of the head and brain can detect tumors, show blood clots and blood vessel defects, show enlarged ventricles (caused by a buildup 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
 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 highspeed rotation
 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 lowenergy low-penetration part of the x-ray beam
• 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.
• 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
» 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 detector the 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
 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