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
Multi-slice CT
Detector
Tube
Tube
Detectors

6. What’s Different for Multislice CT?
Your text here

7. Multislice CT Multiple rows of detectors
Open collimators in “Z” direction 4 3 2 1

8. Multi-slice CT Developed in late 1990’s Wider beam in z-direction
Detector array segmented in z-direction
Simultaneous acquisition of multiple slices

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

10. 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

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

12. Detectors & Slice Thickness 4 X 1.25 mm
Beam collimated to 4 detector rows
1 detector row per slice
Effective Detector

13. Detectors & Slice Thickness 4 X 2.5 mm
Beam collimated to 8 detector rows
2 detector rows per slice
Effective Detector

14. Detectors & Slice Thickness 3.75 mm
Beam collimated to 12 detector rows
3 detector rows per slice
Effective Detector

15. Detectors & Slice Thickness 4 X 5 mm
Beam collimated to 16 detector rows
4 detector rows per slice
Effective Detector

16. Capture Efficiency
Fraction of detector area that is active detector

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

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

19. Beam Pitch (defined only for Multi-slice scanners)
table motion during one rotation Beam Pitch = Beam thickness
Beam thickness

20. Beam Pitch Defined only for Multi-slice scanners

21. CT Beam Pitch (of course patient moves in z-direction, not tube)

22. Gap or No Gap? Like single slice, no gaps in data acquisition
slice can be reconstructed for any axial position

23. 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

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

25. Table Motion in One Rotation
5 mm slices
4 simultaneous slices
Beam pitch = 1
1 rotation / sec.
table motion during one rotation Beam Pitch = Beam thickness
Table motion during one rotation = Beam Pitch X Beam thickness
Table motion in during one rotation = 1 X 20 mm / rotation = 20 mm / rotation

26. Table Speed 5 mm slices 4 simultaneous slices Beam pitch = 1
1 rotation / sec.
20 mm beam thickness
Table speed= Table motion during one rotation X Rotations/sec
20 mm rotation
Table speed = X = 20 mm / sec
1 rotation sec

27. If Pitch Reduced
Table moves less during one rotation
If rotation time is the same, table moves slower
Beam Pitch table motion during one rotation = Beam thickness

28. If Pitch Increased
Table moves more during one rotation
If rotation time is the same, table moves faster
Beam Pitch table motion during one rotation = Beam thickness

29. Pitch vs. Table Speed If all else remains the same
Lower pitch  Slower table speed
Higher pitch  Higher table speed

30. Slice Thickness Defined at Rotational Center
Detector Field must be Larger than Slice Thickness at Rotational Center
Defined slice thickness
Rotational
Center
Tube
Diverging Beam

31. Beam Divergence More of a Problem for Multi-Slice
Rays diverge
No longer parallel
Leads to Cone Angle Artifact
Significant for 16, 32, 64 … data channels
Special algorithms used to compensate

32. Multislice CT Doses Dose inversely proportional to beam pitch
Can be 10-30% higher than for single slice units (ICRP #47) depending on implementation
Other considerations
Tendency to cover more volume (anatomy)
Better availability of equipment because of greater speed

33. Other Reasons for High CT Doses
Repeat Exams
No adjustment of technique factors for different size patients
No adjustment for different areas of body

34. Multislice CT Advantage?
Speed!

35. Single slice / Multislice Images about the same!

36. Multi-slice CT Imaging Clinical Advantages
Thinner slices for improved z-direction resolution
Tradeoff: “z” axis resolution vs. image noise
Same acquisition in shorter time
Scan larger volumes in same time

37. 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