CT
Multi-Slice CT
Third Generation CT
Single or Multislice
Z-axis orientation
perpendicular to page
Patient
Single Slice Thickness Determined by Collimation
Z
Detector
Single-Slice Detectors
 Many detectors rotate around
patient
 Single row in z-direction
 Slice thickness determined by
collimation
Z-Axis
Single Slice CT: Slice Thickness
Collimated Beam Thickness
Collimated Beam Thickness
Thin
slice
Thick
slice
Z-Axis
Z-Axis
Multi-slice CT
Tube
Post-collimator
Detectors
What’s Different for Multislice CT?
Multislice CT
 Multiple rows of
detectors
 Open collimators
in “Z” direction
34
2
1
http://www.veterinaryimaging.com/images/MSS_CT.gif
Multi-slice CT
 Developed in late
1990’s
 Detector array
segmented in zdirection
 Simultaneous
acquisition of
multiple slices
http://www.ctisus.com/gallery/images/m
ultidetector/multislice_ct.jpg
Single Slice vs. Multislice Detector
Collimated Beam Thickness
Z-Axis
Single slice
detector
Multislice
detector
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
Multi-slice CT
 Size & distribution of detectors in non-axial
direction similar to previous CT’s
 Similar spatial & contrast resolution
Distribution of detectors in axial direction
varies with manufacturer
All detectors
same width
“Z”
Direction
Variable widths
Multi-slice CT
Uniform Detector Thickness
 Multiple detectors in axial
“z” direction
direction
 Size must accommodate
thinnest slice
1 2 3 4
 Detector signals can be used
 Individually
 In groups
Four thin slices
1
2
3
Four thicker slices
4
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
Detectors vs. Channels Example
 16 detectors
 4 channels
Multi-Slice Detector Example
16 Detector Rows, 4 Channels
Detectors vs. Channels
4 X 1.25 mm
 Beam collimated to 4
detector rows
 1 detector row per DAS
channel
Effective
Detector
Detectors vs. Channels
4 X 2.5 mm
 Beam collimated to 8
detector rows
 2 detector rows per DAS
channel
Effective
Detector
Detectors vs. Channels
4 X 3.75 mm
 Beam collimated to 12
detector rows
 3 detector rows per DAS
channel
Effective
Detector
Detectors vs. Channels
4 X 5 mm
 Beam collimated to 16
detector rows
 4 detector rows per DAS
channel
Effective
Detector
Capture Efficiency
 Fraction of detector area
that is active detector
Equal-width Detectors
Disadvantage
 Many gaps
 Gaps are dead space
 Reduce capture
efficiency
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
Postcollimators
1 2 3 4
Four thinner slices
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
Single Slice Pitch Definition
table motion during one rotation
Slice Pitch = --------------------------------------slice thickness
Beam Pitch
Defined only for Multi-slice scanners
table motion during one rotation
Beam Pitch = --------------------------------------Beam thickness
Beam thickness
Beam Pitch
Defined only for Multi-slice scanners
Beam Pitch > 1
Beam Pitch = 1
CT Beam Pitch
Example
5
table motion during one rotation
Beam Pitch = --------------------------------------Beam thickness
mm slices
 4 simultaneous
slices
 Beam pitch = 1
 1 revolution / sec.
 Beam thickness?
 Table speed?
Beam Thickness
5
table motion during one rotation
Beam Pitch = --------------------------------------Beam thickness
mm slices
 4 simultaneous
slices
 Beam pitch = 1
 1 revolution / sec.
Beam Thickness = 5 X 4 = 20 mm
Table Speed
5
table motion during one rotation
Beam Pitch = --------------------------------------Beam thickness
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
Slice Thickness Defined at
Rotational Center
Rotational
Center
Tube
Detector Field must be Larger than Slice
Thickness at Rotational Center
Rotational
Center
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
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
Other Reasons for High CT Doses
 Repeat Exams
 No adjustment of technique factors for different size
patients
 No adjustment for different areas of body
Multislice CT Advantage?
Speed!
Single slice / Multislice Images about the
same!
Speed = Power
 Speed enables new applications
How do we spend our new speed?
Multi-slice CT Imaging
Clinical Advantages
1.
2.
3.
Thinner slices for improved z-direction resolution
Same acquisition in shorter time
Scan larger volumes in same time
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
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
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
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
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
Organ Coverage Time Depends On
1.
2.
3.
Beam Pitch
Rotation Time
Detector
Acquisition Length
64 Slice CT Typical Coverage Times
 Heart & coronary arteries / brain / lungs
 5 seconds
 Whole body coverage for blood clot search
 30 seconds
Philips
64-Slice Commercial Cardiac CT
IGE Philips
Siemens
(1 tube)
Siemens
(2 tube)
Toshiba
Min. Rotation Time (s)
.35
.53
.33
.33
.4
Detector length (mm)
40
40
19.2
19.2
32
Time to cover heart
(120 mm) (s)
5.3
6.3
10.3
5.1
7.5
www.impactscan.org
What’s Next?
Slice Wars
Philips
256 Slicer
Toshiba
320 Slicer
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
The Future
 More slices
 Flat panel area detectors
 ???
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
Acknowledgement
 Many drawings obtained from www.impactscan.org
website