1. CT
Multi-Slice CT

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

3. Single Slice Thickness Determined by CollimationZ
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. Multi-slice CT
Tube
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

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
Collimated Beam Thickness
Single slice detector
Multislice 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
Single
Multi

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 CT Uniform Detector Thickness
Multiple detectors in axial direction
Size must accommodate thinnest slice
Detector signals can be used
Individually
In groups
“z” direction 1 2 3 4
Four thin slices 1 2 3 4
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 from detector arrays

16. Detectors vs. Channels Example

17. Multi-Slice Detector Example 16 Detector Rows, 4 Channels

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

19. Detectors vs. Channels 4 X 2.5 mm
Beam collimated to 8 detector rows
2 detector rows per DAS channel
Effective Detector

20. Detectors vs. Channels 4 X 3.75 mm
Beam collimated to 12 detector rows
3 detector rows per DAS channel
Effective Detector

21. Detectors vs. Channels 4 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 1 2 3
Three thicker slices
Post-collimators 1 2 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 Pitch Defined only for Multi-slice scanners
table motion during one rotation Beam Pitch = Beam thickness
Beam thickness

28. Beam Pitch Defined only for Multi-slice scanners

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. Slice Thickness Defined at Rotational Center
Tube

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 Imaging Clinical Advantages
Thinner slices for improved z-direction resolution
Same acquisition in shorter time
Scan larger volumes in same time

43. 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. 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

45. Multi-slice CT Imaging Clinical 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 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. 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. 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
IGE
Philips
Siemens (1 tube)
Siemens (2 tube)
Toshiba
Min. Rotation Time (s)
.35
.53
.33 .4
Detector length (mm) 40
19.2 32
Time to cover heart (120 mm) (s)
5.3
6.3
10.3
5.1
7.5

51. What’s Next? Slice Wars
Toshiba 320 Slicer
Philips 256 Slicer

52. Implications of 256+ Slices
Full organ coverage in single rotation
12-16 cm coverage
Improved temporal resolution
Reduced artifacts
Whole-organ function (perfusion) studies
Functional data perfectly registered to anatomic data

53. The Future
More slices
Flat panel area detectors
???

54. Multi-slice challenges: More Slices
Computer issues
More archival capacity
Requires faster computer systems
Requires faster communications for remote viewing
Radiologist responsible for all images

55. Acknowledgement
Many drawings obtained from website