Multi-Slice CT

1. CT Multi-Slice CT

2. Third Generation CTSingle or Multislice

3. Single Slice Thickness Determined by Collimation

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

5. Single Slice CT: Slice Thickness

6. Multi-slice CT

7. What�s Different for Multislice CT?

8. Multislice CT Multiple rows of detectors Open collimators in �Z� direction

9. Multi-slice CT Developed in late 1990�s Detector array segmented in z-direction Simultaneous acquisition of multiple slices

10. Single Slice vs. Multislice Detector

11. Multi-Slice Detectors Many detectors going around patient Many detector rows in z-direction Slice thickness determined by Collimation electronic detector selection

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

14. Multi-slice CTUniform Detector Thickness Multiple detectors in axial direction Size must accommodate thinnest slice Detector signals can be used Individually In groups

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

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

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

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

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� Some scanners use detectors of various widths Post-collimators used to partially block wider elements for 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 PitchDefined only for Multi-slice scanners table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness

28. Beam PitchDefined only for Multi-slice scanners

29. CT Beam Pitch

30. Example

31. Beam Thickness

32. Table Speed

33. Slice Thickness Defined at Rotational Center

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

35. Beam Divergence More of a Problem for Multi-Slice

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 Same acquisition in shorter time Trauma Peds OR Thinner slices for improved z-direction resolutionOR 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 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. 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

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. 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. Acknowledgement Many drawings obtained from www.impact.org website

50. The Future More detector channels More speed Flat panel area detectors ???