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1. Multi-Slice CT

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

3. Single Slice Thickness Determined by Collimation
Detector
Z

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
Z-Axis
Collimated Beam Thickness
Z-Axis
Collimated Beam Thickness
Thin
slice
Thick
slice

6. Multi-slice CT
Detectors
Post-collimator
Tube

7. What’s Different for Multislice CT?

8. Multi-slice CT
Developed in late
1990’s
Multiple rows of
detectors
Detector array
segmented in z-
direction
Simultaneous
acquisition of
multiple slices

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

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

12. Multi-slice CT
Uniform Detector Thickness
Multiple detectors in axial
direction
Size must accommodate
thinnest slice
Detector signals can be
used
Individually
In groups
1234
Four thin slices
1 2 3 4
Four thicker slices
“z” direction

13. 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

14. Detectors vs. Channels Example
16 detectors
4 channels

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

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

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

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

19. Detectors vs. Channels
4 X 5 mm
Beam collimated to 16
detector rows
4 detector rows per
DAS channel
Effective
Detector

20. Capture Efficiency
Fraction of detector area
that is active detector

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

22. 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
123
Three thicker slices
124
Four thinner slices
3
Post-
collimators

23. 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

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

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

26. Beam Pitch
Defined only for Multi-slice scanners
Beam Pitch = 1Beam Pitch > 1

27. 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?

28. 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

29. 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

30. Slice Thickness Defined at
Rotational Center
Tube
Rotational
Center

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

32. 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

33. 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

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

35. Multislice CT Advantage?
Speed!

36. Single slice / Multislice Images about the
same!

37. Multi-slice CT Imaging
Clinical Advantages
1.Thinner slices for improved z-direction resolution
2.Same acquisition in shorter time
3.Scan larger volumes in same time

38. 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

39. 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

40. 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

41. 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

42. Multi-Slice Compared to
Single-slice helical
Much Faster
No significant image quality differences
Equivalent Patient Dose

43. Organ Coverage Time Depends On
1.Beam Pitch
2.Rotation Time
3.Detector
Acquisition
Length

44. 64 Slice CT Typical Coverage Times
Heart & coronary arteries / brain / lungs
5 seconds
Whole body coverage for blood clot search
30 seconds

45. 64-Slice Commercial Cardiac CT
IGEPhilipsSiemens
(1 tube)
Siemens
(2 tube)
Toshiba
Min. Rotation Time (s).35.53 .33 .33 .4
Detector length (mm)4040 19.2 19.2 32
Time to cover heart
(120 mm) (s)
5.36.3 10.3 5.1 7.5

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

47. 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

48. The Future
More slices
Flat panel area detectors
???

49. 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

50. Acknowledgement
Many drawings obtained from
www.impactscan.org website