Multi-slice CT

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Multi-slice CT

  1. 1. George David CT Multi-Slice CT
  2. 2. George David Third Generation CT Single or Multislice Patient Z-axis orientation perpendicular to page
  3. 3. Single Slice Thickness Determined by Collimation Detector Z
  4. 4. George David Single-Slice Detectors Many detectors rotate around patient Single row in z-direction Slice thickness determined by collimation Z-Axis
  5. 5. Single Slice CT: Slice Thickness Z-Axis Collimated Beam Thickness Z-Axis Collimated Beam Thickness Thin slice Thick slice
  6. 6. George David Multi-slice CT Detectors Post-collimator Tube
  7. 7. George David What’s Different for Multislice CT?
  8. 8. George David Multislice CT Multiple rows of detectors Open collimators in “Z” direction http://www.veterinary- imaging.com/images/MSS_CT.gif 1 2 3 4
  9. 9. George David Multi-slice CT Developed in late 1990’s Detector array segmented in z- direction Simultaneous acquisition of multiple slices http://www.ctisus.com/gallery/images/mu ltidetector/multislice_ct.jpg
  10. 10. Single Slice vs. Multislice Detector Z-Axis Collimated Beam Thickness Single slice detector Multislice detector
  11. 11. Multi-Slice Detectors Many detectors going around patient Many detector rows in z-direction Slice thickness determined by • Collimation • electronic detector selection “Z” Direction Single Multi
  12. 12. George David Multi-slice CT Size & distribution of detectors in non-axial direction similar to previous CT’s • Similar spatial & contrast resolution
  13. 13. George David Distribution of detectors in axial direction varies with manufacturer All detectors same width “Z” Direction Variable widths
  14. 14. George David Multi-slice CT Uniform Detector Thickness Multiple detectors in axial direction • Size must accommodate thinnest slice Detector signals can be used • Individually • In groups 1 2 3 4 Four thin slices 1 2 3 4 Four thicker slices “z” direction
  15. 15. George David Detectors vs. Channels # Physical Detectors not necessarily equal to # of possible Slices Maximum # slices limited by Digital Acquisition System (DAS) channels • Electronic counters Imaging speed bottleneck • How fast data can be received from detector arrays
  16. 16. George David Detectors vs. Channels Example 16 detectors 4 channels
  17. 17. George David Multi-Slice Detector Example 16 Detector Rows, 4 Channels
  18. 18. Detectors vs. Channels 4 X 1.25 mm Beam collimated to 4 detector rows 1 detector row per DAS channel Effective Detector
  19. 19. Detectors vs. Channels 4 X 2.5 mm Beam collimated to 8 detector rows 2 detector rows per DAS channel Effective Detector
  20. 20. Detectors vs. Channels 4 X 3.75 mm Beam collimated to 12 detector rows 3 detector rows per DAS channel Effective Detector
  21. 21. George David Detectors vs. Channels 4 X 5 mm Beam collimated to 16 detector rows 4 detector rows per DAS channel Effective Detector
  22. 22. George David Capture Efficiency Fraction of detector area that is active detector
  23. 23. George David Equal-width Detectors Disadvantage Many gaps Gaps are dead space • Reduce capture efficiency
  24. 24. George David Multi-slice CT “Adaptive Array Detectors” Some scanners use detectors of various widths Post-collimators used to partially block wider elements for thinner slices “z” direction 1 2 3 Three thicker slices 1 2 4 Four thinner slices 3 Post- collimators
  25. 25. George David Variable Width Detectors Center detectors thinner Thicker detectors can function as thinner ones using collimation Thinner detectors can function a thicker one by combining signals
  26. 26. George David Single Slice Pitch Definition table motion during one rotation Slice Pitch = --------------------------------------- slice thickness
  27. 27. George David Beam Pitch Defined only for Multi-slice scanners table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Beam thickness
  28. 28. George David Beam Pitch Defined only for Multi-slice scanners Beam Pitch = 1Beam Pitch > 1
  29. 29. George David CT Beam Pitch
  30. 30. George David Example table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. Beam thickness? Table speed?
  31. 31. George David Beam Thickness table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. Beam Thickness = 5 X 4 = 20 mm
  32. 32. George David Table Speed table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. 20 mm beam thickness Table speed = 20 mm rotation (1 sec) = 20 mm / sec
  33. 33. George David Slice Thickness Defined at Rotational Center Tube Rotational Center
  34. 34. George David Detector Field must be Larger than Slice Thickness at Rotational Center Rotational Center
  35. 35. Beam Divergence More of a Problem for Multi-Slice Rays diverge  No longer essentially parallel Leads to Cone Angle Artifact Significant for 16, 32, 64 … data channels Requires use of special reconstruction algorithms to compensate
  36. 36. George David Multislice CT Doses Can be 10-30% higher than for single slice units (ICRP #47) Cause • Divergent beam Other considerations • Tendency to cover more volume (anatomy) • Better availability of equipment
  37. 37. George David Other Reasons for High CT Doses Repeat Exams No adjustment of technique factors for different size patients No adjustment for different areas of body
  38. 38. George David Multislice CT Advantage? Speed!
  39. 39. George David Single slice / Multislice Images about the same!
  40. 40. George David Speed = Power Speed enables new applications
  41. 41. George David How do we spend our new speed?
  42. 42. George David Multi-slice CT Imaging Clinical Advantages Same acquisition in shorter time • Trauma • Peds OR Thinner slices for improved z-direction resolution OR Scan larger volumes in same time
  43. 43. George David Multi-slice CT Imaging Clinical Advantages Thinner slices • Improvement in CTA of neck, aorta, renal vessels • Better reconstructions » Sagittal, coronal, oblique » 3-D Fundamental Trade-off • “z” axis resolution vs. image noise
  44. 44. George David Multi-slice CT Imaging Clinical Advantages Better Contrast Utilization • Simplifies timing of contrast bolus • Continuous observation of target vessel • Can reduce amount of contrast required • Possible coverage from aorta to lower extremities » Runoff
  45. 45. George David Multi-slice CT Imaging Clinical Advantages Improved x-ray tube utilization • Reduced x-ray tube loading » 4 slices acquired with same tube loading previously used for 1 » Less need to pause of tube cooling » Reduced wear & tear Other anticipated benefits • CT endoscopy • Diagnosis of pulmonary embolism
  46. 46. George David Continuous CT Imaging Interventional Procedures Biopsy & drainage • Neuro • Chest • Abdominal • Spine • Catheter and tube placement Helps operator avoid critical structures near path of biopsy needle Better visualizing of moving structures • Respiration Functional CT • Brain perfusion
  47. 47. George David Multi-Slice Compared to Single-slice helical Much Faster No significant image quality differences Equivalent Patient Dose Ref: • Willi Kalender, Ph.D Institute of Medical Physics University of Erlanger, Germany
  48. 48. George David Multi-slice challenges Much more raw data • More archival capacity More thin slices possible • May make multi-image printing cumbersome • May require viewing on-line • Requires faster systems • Requires faster communications for remote viewing • Radiologist legally responsible for all images
  49. 49. George David Acknowledgement Many drawings obtained from www.impact.org website
  50. 50. George David The Future More detector channels • More speed Flat panel area detectors ???

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