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COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION

COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION. Dr. Halima Hawesa Lecture 3-RAD 466. OUTLINE. CT SYSTEM COMPONENTS – DEFINITION OF A SCANNER SCANNER COORDINATE SYSTEM – XYZ, ISOCENTER IMAGING SYSTEM COMPUTER SYSTEM DISPLAY, RECORDING, AND STORAGE SYSTEMS. CT MAIN SYSTEMS.

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COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION

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  1. COMPUTED TOMOGRAPHY INSTRUMENTATION AND OPERATION Dr. Halima Hawesa Lecture 3-RAD 466

  2. OUTLINE • CT SYSTEM COMPONENTS – DEFINITION OF A SCANNER • SCANNER COORDINATE SYSTEM – XYZ, ISOCENTER • IMAGING SYSTEM • COMPUTER SYSTEM • DISPLAY, RECORDING, AND STORAGE SYSTEMS

  3. CT MAIN SYSTEMS • IMAGING SYSTEM • COMPUTER SYSTEM • DISPLAY, RECORDING, STORAGE SYSTEM • DATA ACQUISITION SYSTEM

  4. CT SYSTEM GANTRY X-RAY TUBE GANTRY CONTROL DETECTORS HIGH VOLTAGE GENERATOR DAC S/H ADC SCAN CONTROLLER ARRAY PROCESSOR HOST COMPUTER CONSOLE STORAGE

  5. SCANNER

  6. SCANNER • GANTRY • PATIENT COUCH

  7. GANTRY HOUSES: • X-RAY TUBE • GENERATOR (LOW VOLTAGE DESIGN) • COLLIMATORS • DETECTORS

  8. GANTRY CHARACTERISTICS • APERTURE • TILTING RANGE

  9. MOST OF THE SCANNERS HAVE 70CM APERTURE

  10. 70 CM

  11. COORDINATE SYSTEM X

  12. COORDINATE SYSTEM Y

  13. COORDINATE SYSTEM Z

  14. ISOCENTER

  15. TILTING RANGE OF MOST SCANNERS- +30 TO -30 DEGREES

  16. PATIENT COUCH :450 LBS (204 KG) DISTRIBUTED WEIGHT LIMIT

  17. SCANNABLE RANGE:COVERAGE FROM HEAD TO THIGH (162CM)

  18. MAX. SCANNABLE RANGE

  19. IMAGING SYSTEM • PRODUCTION OF X-RAYS • SHAPING OF X-RAY BEAM ENERGY • FILTERING X-RAY BEAM

  20. IMAGING SYSTEM COMPONENTS • X-RAY TUBE • GENERATOR –HIGH VOLTAGE • COLLIMATORS • FILTER • DETECTORS • DETECTOR ELECTRONICS

  21. X-RAY TUBE AND X-RAY PRODUCTION

  22. CATHODE --------MADE OF TUNGSTEN IN CT – STILL SMALL AND LARGE

  23. THERMIONIC EMISSION CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS TO LIBERATE ELECTRONS FOR TRANSIT TO ANODE

  24. FOCAL SPOT- CT UTILIZES DIFFERENT FOCAL SPOTS • THE FILAMENT SIZE – LENGTH – FOCAL SPOT SMALLER FOCAL SPOT - Low mA SMALLER FOCAL SPOT – sharper image

  25. ANODE +++++ MADE OF TUNGSTEN AND MOLYBDENUM TUNGSTEN TARGET TARGET MADE OF TUNGSTEN AND RHENIUM

  26. mA – tube current • The number of electrons flowing from cathode to anode

  27. kVp • Potential difference between cathode and anode (Volts) kilo means 1,000 x.

  28. S –time of exposuremAs tube current for certain length of time

  29. X-RAY PRODUCTION RESULTS IN A LOT OF HEAT AND VERY LITTLE X-RAYS BEING GENERATED HEAT UNITS CALCULATION HU= kVp X mA x time MOST CT TUBES HEAT CAPACITY 3-5 MILLION HU

  30. REDUCTION OF HEAT UNITS – TECHNIQUE COMPENSATION • kVp • mA • Time INCREASED NOISE

  31. TOO LOW OF kVp: • NOISE !!!!

  32. Why changing mA or time • Avoiding motion – mA time • Pediatric technique modification • Reducing noise - mAs MOTION NOISE

  33. Tube voltage (kVp) CHANGE INTENSITY - ENERGY– kVp 15% INCREASE OF KVP = 2 * mAs

  34. kVp IN CT • 80-140 • TOO LOW – NOISE (NOT ENOUGH PENETRATION OF THE PATIENT ) PHOTON STARVATION - NOISE!!!!!

  35. HIGH VOLTAGE GENERATOR –(HVG) • GENERATES HIGH VOLTAGE POTENTIAL BETWEEN CATHODE AND ANODE OF AN X-RAY TUBE

  36. CT GENERATOR • 5-50 kHz • 30-60 kW KVP SELECTION: 80, 100, 120, 130,140 mA selection: 30, 50, 65, 100, 125, 150, 175, 200, 400

  37. CT Imaging Steps Filter DEFINES SLICE THICKNESS Patient REDUCES SCATTER RECHING THE PATIENT Detector

  38. COLLIMATION IN CTBASIC DATA AQUSITION SCHEME IN CT FILTRATION PRE-PATIENT COLLIMATION POST-PATIENT COLLIMATION ADC

  39. X-RAY EMISSIONTUBE CURRENT CHANGE INTENSITY ENERGY – NO CHANGE CURRENT 2 * mA = 2 * number of photons 4 * mA = 4 * number of photons

  40. FILTRATION CHANGE INTENSITY ENERGY FILTRATION

  41. FILTRATION MATERIAL • ALUMINIUM ( SPECIAL FILTER IN CT) TO MAKE THE BEAM HARDER AND MORE MONOENERGETIC

  42. CT DETECTORS

  43. DETECTOR TYPES: SCINTILLATION S. CRYSTAL S. CRYSTAL PHOTODIODE PM TUBE

  44. SCINTILLATION CRYSTALS USED WITH PM TUBES: • SODIUM IODIDE • CALCIUM FLUORIDE • BISMUTH GERMANATE

  45. S. CRYSTAL USED WITH PHOTODIODE • CALCIUM TUNGSTATE • RARE EARTH OXIDES - CERAMIC

  46. DETECTOR TYPE: GAS IONIZATION XENON GAS 30 ATM

  47. EFFICIENCY OF DETECTORS- QDE • SCINTILLATION – 95% - 100%- COMMONLY USED IN III & IV GENERATION SCANNERS • GAS – 50% - 60%

  48. COMPUTER SYSTEM • RECONSTRUCTION AND POSTPROCESSING • CONTROL OF ALL SCANNER COMPONENTS • CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY. • DATA FLOW DIRECTION

  49. COMPUTER SYSTEM IN CT • MINICOMPUTERS

  50. COMPUTER SYSTEM COMPOSED OF: • HARDWARE • SOFTWARE

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