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COMPUTED TOMOGRAPHY

COMPUTED TOMOGRAPHY. Merrill’s V. 3: Ch. 31 & Bushong Ch. 23 . No other development in x-ray imaging over the past 50 years has been as significant !. Snook transformer Coolidge hot-cathode x-ray tube Potter-Bucky diaphragm Image-intensifier tube. Wow… look at that image

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COMPUTED TOMOGRAPHY

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  1. COMPUTED TOMOGRAPHY Merrill’s V. 3: Ch. 31 & Bushong Ch. 23

  2. No other development in x-ray imaging over the past 50 years has been as significant ! • Snook transformer • Coolidge hot-cathode x-ray tube • Potter-Bucky diaphragm • Image-intensifier tube Wow… look at that image quality & detail

  3. FUNDAMENTALS • CREATING A CROSS-SECTIONAL TOMOGRAPHIC PLANE OF ANY BODY PART • A PATIENT IS SCANNED BY AN X-RAY TUBE ROTATING AROUND THE BODY • A DETECTOR ASSEMBLY MEASURES THE RADITION EXITING THE PATIENT.

  4. Equipment arrangement

  5. Tomo = image // to long axis of the bodyCT = image is transverse to the body

  6. In its simplest form, a CT imaging system consists of a finely collimated x-ray beam and a single detector. • both moving synchronously in a translate rotate mode. • Translation = one rotation of source and detector

  7. FUNDAMENTALS • EXITING RADIATION: PRIMARY DATA • PRIMARY DATA IS COLLECT BY DETECTORS • THE COMPUTER COMPILES AND CALCULATES THE DATA BASED ON PRESELECTED ALGORITHM AND AN IMAGE IS FORMED

  8. PRESELECTED ALGORITHM

  9. IMAGE • EACH IMAGE IS DISPLAYED IN AN AXIAL FORM INITALLY • THE IMAGES ARE DISPLAYED ON A CATHODE RAY TUBE (CRT) OR LCD

  10. Cathode Ray Tube (CRT): is an air evacuated glass envelope containing an e- gun and a fluorescent screen. When e- hit the fluorescent screen light is emitted

  11. LCD

  12. CT • Conventional Radiographs: Frequently body structures are superimposed • In CT: A tightly collimated x-ray beam is directed thought the patient from different angles – “cross sectional image” • Essentially eliminating superimposition of body structures

  13. CT • Claim to fame: Exceptional Contrast Resolution

  14. Contrast resolution = differentiation of densities, capable of differentiating among tissues with similar densities

  15. The contrast of an object is expressed relative to its surrounding background.  That is what determines its visibility.

  16. CT • Due to the reduction in amount of scattered radiation • Reducing over lapping structures and 2 collimators • Digitized image: because of this numerous image manipulation techniques can be used to enhance and optimize the diagnostic information. • Window/Level, Axial, Sagittal, Coronal

  17. CT Collimation

  18. CT vs CAT

  19. Historical Development • CT was first demonstrated successfully in 1970. Dedicated to head CT only • Dr. Godfrey Hounsfield (Engineer) • Allan Macleod Cormack (Nuclear Physicist) • Nobel Prize in medicine and physiology in 1979

  20. First full-scale commercial unit • Installed in 1971 • Physicians recognized its value for providing diagnostic neurologic information • U.S.– June 1973 at the Mayo Clinic (MN) & General Hospital (MA) • These early units were also dedicated head CT scanners.

  21. Early CAT scanners • Hounsfield’s discovery parallels Rontgen’s discover of x-rays • Early CAT scans required nine days to produce a single section image

  22. Whole-body scanners • 1974 – Dr. Robert S. Ledley of Georgetown University Medical Center, developed the first whole-body scanner • Many different companies began manufacturing scanners.

  23. Generations • First Generation Scanners • Translation/Rotation • Tube produced a finely collimated beam or pencil beam • 1 to 3 detectors were placed opposite the tube for radiation detection • 4.5 minutes to gather enough information for one slice • Tube was only able to rotate 180 degrees

  24. Second Generation • Fan-shaped x-ray beam • 30 or more detectors • 20 seconds per slice or 10 minutes for a 40 slice exam • 180 degree rotation • Long data reconstruction time

  25. Third Generation • Fan-shaped x-ray beam • 960 detectors opposite the x-ray tube • Complete 360 degree rotation Rotate/Rotate movement • One rotation = one slice • Second data acquisition could be made as the tube and detectors move in the opposite direction. • Time reduced to 1 sec per slice

  26. 3rd generation configuration

  27. Fourth Generation • Developed in 1980’s • Fixed ring of as many as 4800 detectors, completely surrounding the patient, Rotate only movement • Rotating x-ray tube provides short bursts of radiation • Detectors collect the remnant radiation to reconstruct into an image • 1 minute for multiple slices

  28. 4th generation configuration

  29. Fifth GenerationEBCT

  30. Modern Scanners • No longer categorize into Generations • Contemporary CT scanners are either third or fourth generation designs • Scanners are categorized by tube and detector movement • Slip Ring Technology: connects generator with tube (no cables)

  31. Slip Rings

  32. Technical Aspects • Optimum imaging: patient/area of interest and gantry are perpendicular to each other • Tube rotates around the patient, irradiating the area of interest. • Detectors measure the transmitted x-ray values, covert them in to an electric signal, and relay the signal to the computer.

  33. Raw Data • The remnant radiation that is converted into an electrical signal values are called projections, scan profiles or raw data. • Raw data is collected and digitized. • This process assigns a whole number to each signal. • The value assigned is directly proportional to the strength of the signal.

  34. Tube Interactions

  35. Digital Image • Array of numbers arranged in a grid of rows and columns called a matrix. • Single square, or picture element, with in the matrix is called a pixel. • Slice thickness gives the pixel and added dimension called the volume element, or voxel

  36. Voxel • Each pixel in the image corresponds to the volume of tissue in the body section being imaged. • The voxel volume is a product of the pixel area and slice thickness

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