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Seeram Chapter 5: Data Acquisition in CT

CT. Seeram Chapter 5: Data Acquisition in CT. Data Collection Basics. Patient. X-ray source & detector must be in & stay in alignment Beam moves (scans) around patient many transmission measurements. X-Ray beams. Data Collection Basics. Pre-patient beam

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Seeram Chapter 5: Data Acquisition in CT

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  1. CT Seeram Chapter 5: Data Acquisition in CT

  2. Data Collection Basics Patient • X-ray source & detector must be in & stay in alignment • Beam moves (scans) around patient • many transmission measurements X-Ray beams

  3. Data Collection Basics • Pre-patient beam • collimated to pass only through slice of interest • shaped by special bow tie filter for uniformity Patient Filter

  4. Data Collection Basics (cont) • Beam attenuated by patient • Transmitted photons detected by scanner • Detected photon intensity converted to electrical signal (analog) • Electrical signal converted to digital value • A to D converter • Digital value sent to reconstruction computer

  5. CT “Ray” • That part of beam falling onto a single detector Ray

  6. Each CT Ray • attenuated by patient • projected onto one detector • detector produces electrical signal • produces single data sample

  7. CT View • # of simultaneously collected rays

  8. Scan Requires Many Data Samples • # Data Samples = [# data samples per view] X [# views] • # Data Samples = [# detectors] X [# data samples per detector]

  9. Acquisition Geometries • Pencil Beam • Fan Beam • Spiral • Multislice

  10. Pencil Beam Geometry Tube Detector • Tube-detector assembly translates left to right • Entire assembly rotates 1o 1st Generation CT 1o Pencil Beam

  11. Fan Beam Geometry Tube Detectors 3nd Generation Fan Beams 2nd Generation 4th Generation

  12. Comparing Long vs. Short Geometry Long Geometry • Smaller fan angle • Longer source-detector distance • Lower beam intensity • Lower patient dose • More image noise • Less image blur • Requires larger gantry Scan FOV Scan FOV Short Long

  13. Spiral Geometry Interconnect Wiring Tube Slip Rings Detector • X-ray tube rotates continuously around patient • Patient continuously transported through gantry • No physical wiring between gantry & x-ray tube • Requires “Slip Ring” technology

  14. What’s a Slip Ring?

  15. Slip Rings • Electrical connections made by stationary brushes pressing against rotating circular conductor • Similar to electric motor / generator design

  16. X-Ray Generator Configurations with Slip Ring Technology • Problem: • Supply high voltage to a continually rotating x-ray tube? • Options • #1 • Stationary Generator & Transformer • #2 • Stationary Generator • Transformer & x-ray tube rotate in gantry • #3 • Transformer, generator & tube rotate in gantry

  17. Option #1: Stationary High Voltage Transformer Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

  18. Option #1: Stationary High Voltage Transformer Secondary Voltage Line Voltage Primary Voltage Tube Slip Rings Detector Generator • high voltage must pass through slip rings HV Transformer

  19. Option #2: Rotating High Voltage Transformer Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

  20. Option #2: Rotating High Voltage Transformer Line Voltage Primary Voltage HV Transformer Tube Slip Rings Detector Generator • low voltage must pass through slip rings

  21. Rotating Generator Primary Voltage Secondary Voltage Incoming AC Power X-Ray Generator High Voltage Transformer X-Ray Tube Stationary Rotating Slip Rings

  22. Rotating Generator Line Voltage Generator Tube Slip Rings HV Transformer • low line voltage must pass through slip rings

  23. Spiral CT Advantages • Faster scan times • minimal interscan delays • no need to stop / reverse direction of rotation • Slip rings solve problem of cabling to rotating equipment • Continuous acquisition protocols possible

  24. X-Ray System Components • X-Ray Generator • X-Ray Tube • Beam Filter • Collimators

  25. X-Ray Generator • 3 phase originally used • Most vendors now use high frequency generators • relatively small • small enough to rotate with x-ray tube • can fit inside gantry

  26. X-Ray Tube

  27. X-Ray Tube • Must provide sufficient intensity of transmitted radiation to detectors • Radiation incident on detector depends upon • beam intensity from tube • patient attenuation • beam’s energy spectrum • patient • thickness • atomic # • density

  28. Maximizing X-Ray Tube Heat Capacity • rotating anode • high rotational speed • small target angle • large anode diameter • focal spot size appropriate to geometry • distances • detector size

  29. Special Considerations for Slip Ring Scanners • continuous scanning means • Heat added to tube faster • No cooling between slices • Need • more heat capacity • faster cooling

  30. Why not use a Radioactive Source instead of an X-Ray Tube? • High intensity required • X-ray tubes produce higher intensities than sources • Single energy spectrum desired • Produced by radioactive source • X-ray tubes produce spectrum of energies • Coping with x-ray tube energy spectrum • heavy beam filtering (see next slide) • reconstruction algorithm corrects for beam hardening

  31. CT Beam Filtration • Hardens beam • preferentially removes low-energy radiation • Removes greater fraction of low-energy photons than high energy photons • reduces patient exposure • Attempts to produce uniform intensity & beam hardening across beam cross section Patient Filter

  32. CT Beam Collimation • Pre-collimators • between tube & patient Tube • Post-collimators • between patient & detector Detector

  33. Pre-Collimation Tube Detector • Constrains size of beam • Reduces production of scatter • May have several stages or sets of jaws Pre-collimator

  34. Post-Collimation Tube Detector • Reduces scatter radiation reaching detector • Helped define slice (beam) thickness for some scanners Post-collimator

  35. CT Detector Technology:Desirable Characteristics • High efficiency • Quick response time • High dynamic range • Stability

  36. CT Detector Efficiency • Ability to absorb & convert x-ray photons to electrical signals

  37. Efficiency Components • Capture efficiency • fraction of beam incident on active detector • Absorption efficiency • fraction of photons incident on the detector which are absorbed • Conversion efficiency • fraction of absorbed energy which produce signal

  38. Overall Detector Efficiency Overall detector efficiency = capture efficiencyXabsorption efficiencyXconversion efficiency

  39. Capture Efficiency • Fraction of beam incident on active detector

  40. Absorption Efficiency • Fraction of photons incident on the detector which are absorbed • Depends upon detector’s • atomic # • density • size • thickness • Depends on beam spectrum capture efficiencyXabsorption efficiencyXconversion efficiency

  41. Conversion Efficiency • Ability to convert x-ray energy to light GE “Gemstone Detector” made of garnet

  42. Conversion Efficiency • Ability to convert x-ray energy to light • Siemens UltraFastCeramic (UFC) CT Detector • Proprietary • Fast afterglow decay UFC Plate UFC Material

  43. Response Time • Minimum time after detection of 1st event until detector can detect 2nd event • If time between events < response time, 2nd event may not be detected • Shorter response time better

  44. Stability • Consistency of detector signal over time • Short term • Long term • The less stable, the more frequently calibration required

  45. Dynamic Range • Ratio of largest to smallest signal which can be faithfully detected • Ability to faithfully detect large range of intensities • Typical dynamic range: 1,000,000:1 • much better than film

  46. Detector Types: Gas Ionization Electrical Signal X-Rays • X-rays converted directly to electrical signal Filled with Air Ionization Chamber + - + -

  47. CT Ionization Detectors • Many detectors (chambers) used • adjacent walls shared between chambers • Techniques to increase efficiency • Increase chamber thickness • x-rays encounter longer path length • Pressurize air (xenon) • more gas molecules encountered per unit path length thickness X-Rays

  48. Older Style Scintillation Detectors • X-rays fall on crystal material • Crystal glows • Light flash directed toward photomultiplier (PM) tube • Light directed through light pipe or conduit • PM tube converts light to electrical signal • signal proportional to light intensity PM Electrical Signal

  49. Detector Types: Scintillation Light X-Rays • X-ray energy converted to light • Light converted to electrical signal Photomultiplier Tube Electrical Signal Scintillation Crystal

  50. Photomultiplier Tubes • Light incident on Photocathode of PM tube • Photocathode releases electrons + - Light X-Rays Scintillation Crystal Photocathode PM Tube Dynodes

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