Multi-Slice CT

1. CT Multi-Slice CT

2. Third Generation CTSingle or Multislice Z-axis orientation perpendicular to page Patient

3. Single Slice Thickness Determined by Collimation Z Detector

4. Single-Slice Detectors • Many detectors rotate around patient • Single row in z-direction • Slice thickness determined by collimation Z-Axis

5. Single Slice CT: Slice Thickness Collimated Beam Thickness Collimated Beam Thickness Thin slice Thick slice Z-Axis Z-Axis

6. Tube Multi-slice CT Post-collimator Detectors

7. What’s Different for Multislice CT?

8. Multislice CT • Multiple rows of detectors • Open collimators in “Z” direction 4 3 2 1 http://www.veterinary-imaging.com/images/MSS_CT.gif

9. 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/multidetector/multislice_ct.jpg

10. Single Slice vs. Multislice Detector Collimated Beam Thickness Multislice detector Single slice detector Z-Axis

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 Multi Single

12. Multi-slice CT • Size & distribution of detectors in non-axial direction similar to previous CT’s • Similar spatial & contrast resolution

13. Distribution of detectors in axial direction varies with manufacturer All detectors same width Variable widths “Z”Direction

14. Multi-slice CTUniform Detector Thickness “z” direction • 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 4 1 2 3 Four thicker slices

15. 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 fromdetector arrays

16. Detectors vs. Channels Example • 16 detectors • 4 channels

17. Multi-Slice Detector Example16 Detector Rows, 4 Channels

18. Detectors vs. Channels4 X 1.25 mm • Beam collimated to 4 detector rows • 1 detector row per DAS channel Effective Detector

19. Detectors vs. Channels4 X 2.5 mm • Beam collimated to 8 detector rows • 2 detector rows per DAS channel Effective Detector

20. Detectors vs. Channels4 X 3.75 mm • Beam collimated to 12 detector rows • 3 detector rows per DAS channel Effective Detector

21. Detectors vs. Channels4 X 5 mm • Beam collimated to 16 detector rows • 4 detector rows per DAS channel Effective Detector

22. Capture Efficiency • Fraction of detector area that is active detector

23. Equal-width Detectors Disadvantage • Many gaps • Gaps are dead space • Reduce capture efficiency

24. Multi-slice CT“Adaptive Array Detectors” “z” direction • Some scanners use detectors of various widths • Post-collimators used to partially block wider elements for thinner slices 3 1 2 Three thicker slices Post-collimators 2 1 3 4 Four thinner slices

25. 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. Single Slice Pitch Definition table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

27. Beam thickness Beam PitchDefined only for Multi-slice scanners table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness

28. Beam PitchDefined only for Multi-slice scanners Beam Pitch = 1 Beam Pitch > 1

29. CT Beam Pitch

30. Example • 5 mm slices • 4 simultaneous slices • Beam pitch = 1 • 1 revolution / sec. • Beam thickness? • Table speed? table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness

31. Beam Thickness • 5 mm slices • 4 simultaneous slices • Beam pitch = 1 • 1 revolution / sec. table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Beam Thickness = 5 X 4 = 20 mm

32. Table Speed • 5 mm slices • 4 simultaneous slices • Beam pitch = 1 • 1 revolution / sec. • 20 mm beam thickness table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Table speed = 20 mm rotation (1 sec) = 20 mm / sec

33. Tube Slice Thickness Defined at Rotational Center Rotational Center

34. Detector Field must be Larger than Slice Thickness at Rotational Center Diverging Beam Rotational Center

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. 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. 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. Multislice CT Advantage? Speed!

39. Single slice / Multislice Images about the same!

40. Speed = Power • Speed enables new applications

41. How do we spend our new speed?

42. Multi-slice CT ImagingClinical Advantages • Thinner slices for improved z-direction resolution • Same acquisition in shorter time • Scan larger volumes in same time

43. Multi-slice CT ImagingClinical 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. Multi-slice CT ImagingClinical 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

45. Multi-slice CT ImagingClinical Advantage: Angiography • Simplifies contrast bolus timing • Continuous observation of target vessel • Can reduce amount of contrast required • Coverage from aorta to lower extremities • Runoff CTA ~ 13% of all CT procedures

46. Continuous CT ImagingInterventional 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. Multi-Slice Compared to Single-slice helical • Much Faster • No significant image quality differences • Equivalent Patient Dose • Ref: • Willi Kalender, Ph.DInstitute of Medical PhysicsUniversity of Erlanger, Germany

48. Organ Coverage Time Depends On • Beam Pitch • Rotation Time • Detector Acquisition Length

49. 64 Slice CT Typical Coverage Times • Heart & coronary arteries / brain / lungs • 5 seconds • Whole body coverage for blood clot search • 30 seconds Philips

50. 64-Slice Commercial Cardiac CT www.impactscan.org