Outline
Fundamentals of Single and Multiple Row
Detector Computed Tomography
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Mahadevappa Mahesh, Ph.D.
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•
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The Russell H. Morgan Department of Radiology
and Radiological Science
The Johns Hopkins University
Baltimore, MD. USA
Introduction
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Single row detector helical CT
Multiple row detector helical CT
Four section/rotation scanners
Scanners with >4 sections/rotation
X-ray tube issues
Relationship between pitch, dose,
noise and section thickness
Conventional XX-ray Imaging
A recent survey* of internists rates CT among top 5
major medical innovations over the past 30 years
Non-uniform beam
Nonexits opposite surface
with intensity pattern
due to differential
attenuation of rays
along different paths
through patient
Two major evolutionary leaps occurred during last
decade, spiral or helical CT in early 90’s and
multiple--row detector CT late 90s to present
multiple
CT has evolved considerably since its invention in
1972, the progression might be characterized as
search toward the 3D radiograph
X -Ray
Tube
Uniform xx-ray beam
enters patient
Image receptor
captures intensity
pattern
*Decisions in Imaging Economics, Nov 2001
The Problem
2D Images of 3D Anatomy from Single Projection
Image due to
differences in
x-ray
attenuation
along different
paths through
the patient
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
•
•
Resolution >5 lp
lp/mm
/mm
Acquisition time <<1 s (stops physiologic
motion)
But in 2D images of 3D anatomy
• Tissues are superimposed
• Poor contrast resolution due to high
scatter acceptance by image receptor
1
Computed Tomography
Ultimate Goal: 3D Radiography
•
Resolution as good as conventional
radiography in all planes
•
Method for acquiring and reconstructing an
image of a thin crosscross -section of an object
•
•
High contrast sensitivity (no scatter)
•
Based on measurements of x -ray
attenuation through the section plane using
many different projections
•
Can CT get us there?
Fast acquisition times to stop
physiologic motion
Basic data acquisition in CT
CT Gantry
X-ray Tube
X-ray Beam
Gantry Opening
(60--70 cm dia
(60
dia.)
.)
Sampled region
where attenuation
measurements are
made (50 –55 cm
diameter)
y
z
CT Table
x
Detectors
The CT Image
Pixel
Voxel
w
Pixel value is measure of
x -ray attenuation in
corresponding volume
element (voxel
(voxel )
• Depth dimension of voxel equal to section
thickness (1 -10 mm)
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
Field of View (FOV):
user selectable
location and diameter
within sampled region
where reconstruction
matrix is located
Limitations of Conventional CT
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•
Scan plane resolution is ~1~1-2 lp
lp/mm
/mm
Poor zz-axis resolution
−
Section thickness ranges 1 to 10 mm
−
Volumes underunder-sampled with abutted slices
•
Inter-scan delay due to stopInterstop-start action necessary
for table translation and cable unwinding
•
Section-to
Sectionto--section misregistration due to variation
in patient respiratory motion
2
Progress toward true 3D imaging
“Possible”
•
•
•
True 3D not yet practical!
“Step-like” contours
“StepLarge temporal lag
between sections
Not useful with
physiologic motion
Technological Advances That Led To:
Helical (Spiral) Acquisition
Surface rendition
of early 3D
reconstruction of
lower limbs using
10 mm abutted
sections
Slip--Ring Technology
Slip
•
• Slip
Slip--Ring gantry
• High power xx-ray tubes
• Interpolation algorithms
Permits continuous
rotation of tube and
detectors while
maintaining electrical
contact with stationary
components
Projection data
Power supply
Slip--Ring Technology
Slip
Helical/Spiral CT - Principles
•
Segment of slip-ring
Sliding Contactors
Patient is transported
continuously through
gantry while data are
acquired continuously
during several 360360 -deg
rotations*
* Kalender WA, et.al. Radiology, 176(1):181176(1):181-3, 1990
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
3
Technology Advances
Helical Path of XX-Ray Beam on Patient
•
Interpolation algorithms
−
Projection data is no longer
in a crosscross -sectional plane
−
Interpolation of projection
data into plane of interest
prior to conventional
filtered backback-projection*
* Kalender WA, et.al. Radiology, 176(1):181176(1):181-3, 1990
Helical SingleSingle -Section Mode
Helical Pitch
Helical
Helical Trajectory
Trajectory
Pitch =
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Translation
z (mm)
t (s)
Table increment per rotation (mm)
Beam collimation (mm)
Typical Pitch Ratio - 0.5, 1.0, 1.5, 2.0
Pitch <1 implies overlapping and higher patient dose
Pitch >1 implies extended imaging and reduced
patient dose
Interpolation
Interpolation using
using samples
samples from
from single
single row
row detector
detector ring
ring
Capabilities of Single Row Detector CT
(SDCT)
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•
Large tissue volumes scanned in short times
Inter--scan delay eliminated
Inter
Arbitrary section position within scanned
volume permits overover-sampling without
increased dose
Z axis resolution improved by overover-sampling
•
Up to ~ 2 lp
lp/cm
/cm (best case), usually 0.5 to 1.0 lp
lp/cm
/cm
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
Limitations of SDCT
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•
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Large volume scan in short duration is limited
Near isotropic resolution only over small volume
Poor utilization of XX-ray tube
•
Multiple row detector CT (MDCT) offers
substantial improvement in volume coverage,
scan speed with efficient use of x -ray tube
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SDCT versus MDCT
Multiple Row Detector Helical CT (MDCT)
X-ray Tube
Tube Collimator
•
Single row of
detectors replaced
with multiple rows
Collimated Slice
Detector
Collimator
Single multi -element module
1-Row Detector
8-Row Detector
Multiple row detector CT scanner
Single row detector CT Scanner
*Mahesh M, RadioGraphics
RadioGraphics,, 22: 949949-962, 2002
Uniform Element Arrays
Non--Uniform Element Arrays
Non
Possible section widths
2
4
4
4
4
2
2
16 x 1.25 mm
20 mm
Z-axis
x
x
x
x
x
x
x
0.63 mm
1.25 mm
2.5 mm
3.75 mm
5 mm
7.5 mm
10 mm
Possible section widths
5
2.5 1.5 1 1 1.5 2.5
5
20 mm
Z-axis
x 0.5 mm
x 1 mm
x 2.5 mm
x 5 mm
x 8 mm
X 10 mm
Volume Zoom, Siemens Medical Systems
Lightspeed , GE Medical Systems
Hybrid Element Arrays
2
4
4
4
2
2
MDCT: Detector Element Arrays
16 x 1.25 mm
Possible section widths
GE
4 x 0.5
15 mm
15 mm
32 mm
4
4
4
4
4
4
2
x 0.5 mm
x 1 mm
x 2 mm
x 3 mm
x 5 mm
x 8 mm
x 10 mm
Z-axis
Acquilion,, Toshiba Medical Systems
Acquilion
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
20 mm
5
2.5 1.5 1 1 1.5 2.5
5
20 mm
Siemens &
Philips
4 x 0.5
Toshiba
15 mm
15 mm
32 mm
Z-axis
5
How are detector elements used in MDCT?
Detector Configuration: For 4 x 1.25 mm
X
-ray Tube
X-ray
Tube
Focal
Focal Spot
Spot
X
-ray Beam
X-ray
Beam
Collimator
Collimator
4 x 1.25 mm
Detectors
Detectors
20 mm
Switching
Switching Array
Array
4 x 1.25 mm
Detector
Detector
4- section scanners collect four
simultaneous channels of data
Switching
Switching Array
Array
Detector Configuration: For 4 x 2.5 mm
20 mm
MDCT: Detector Configurations
X
-ray Tube
X-ray
Tube
Focal
Focal Spot
Spot
X
-ray Beam
X-ray
Beam
Collimator
Collimator
4 x 2.5 mm
Detector
Detector
*GE LightSpeed , GE Medical Systems
Switching
Switching Array
Array
*Volume Zoom, Siemens Medical System
20 mm
Helical Multiple Section Mode
4 helical trajectories
The Detector’s Evolution…
Translation
z
t
Interpolation using samples of ALL detector rings
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
6
DAS channels: Four versus Eight
Detector Evolution: 4 vs 16 sections per rotation
4 x 1.25 mm
5
Detector
Detector
2.5
1.5
1 1 1.5
2.5
2X0.5, 4x1, 4x2.5, 4x5
2X8, 2x10
5
20 mm
20 mm
Switching
Switching Array
Array
16 x 0.75
8 x 1.25 mm
16x0.75, 16x1.5, 8x3,
4x6…
4 x 1.5
4 x 1.5
24 mm
Z-axis
GE Medical Systems
Siemens & Philips Medical Systems
Detector Evolution: 4 vs. 16 sections per rotation
4 x 0.5
4x0.5, 4X1, 4x2, 4x3
up to 4x8
15 mm
15 mm
MDCT Episode II: Attack of the Cones
32 mm
16 x 0.5
16x0.5, 16X1, 16x2,
up to 8x4
12 x 1
12 x 1
32 mm
Z-axis
Toshiba Medical Systems
Image Reconstruction:
Key Problem is Cone Angle
Cone Beam Geometry
Section
thickness
Focus
cone angle
fan angle
Z-axis
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In MDCT, widening
beam aperture in zzdirection increases
cone angle, that
results in significant
cone beam artifacts
Section
width ‘S’
Scan FOV
Detector Array
‘’Section blurring’’
dS
Scan direction
4 -section 8 -section
• Up to 4 sections dS = S, cone angle may be neglected
• What about more than 4 sections?
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
7
Cone Beam Geometry:
Alternate Reconstruction Algorithms
Key Problem: Cone Angle
• What happens, if the cone angle of the rays is neglected ?
4*1mm, pitch 1.5 (6)
8*1mm, pitch 1.5 (12)
12*1mm , pitch 1.5 (18)
16*1mm , pitch 1.5 (24)
12 mm/sec
24 mm/sec
36 mm/sec
48 mm/sec
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Advanced Single Slice
Rebinning (ASSR)
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Adaptive Multiple Plane
Reconstruction (AMPR)
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Pi,, Pi
Pi
Pi--Slant and 33-Pi methods
Helical Feldkamp with
weighting function (HFK)
• Image results for > 4 sections are clinically unacceptable !
Kohler T, et al., Medical Physics, 29 (1): 5151-64, 2002
Courtesy Siemens Medical Systems
Cone Beam Reconstruction
(e.g. Adaptive Multiple Plane Reconstruction)
4*1mm, pitch 1.5 (6)
12 mm/sec
Standard Reconstruction
16*1mm , pitch 1.0 (16)
32 mm/sec
Standard Reconstruction
16*1mm , pitch 1.0 (16)
32 mm/sec
AMPR Reconstruction
Helical Pitch - Reality vs. Myth
Definition, Confusion…
Courtesy Siemens Medical Systems
Pitch redefined for MDCT
Beam Pitch vs. Detector Pitch
T
Beam Pitch =
W
T
Detector Pitch =
W=D
T
D
Detector Pitch
Beam Pitch =
N
Single Row Detector Array
T - Table travel (mm)/rotation
W - Beam width (mm)
Table Speed
(mm/rotn
(mm/
rotn))
Detector
combi
Detector
Pitch
Beam
Pitch
3.75
4 x 1.25
3
0.75
7.5
4 x 1.25
6
1.50
7.5
4x3
2.5
0.625
13.5
4x3
4.5
1.125
16.5
4x3
5.5
1.375
T
W
D
Multiple Row Detector Array
D - Single DAS channel width (mm)
N - Number of active DAS channels
Data for fourfour -section MDCT
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
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Example of Beam Pitch Options
Beam Pitch
•
Beam Pitch >1 implies extended imaging and
reduced patient dose with lower axial resolution
Z
•
Beam Pitch <1 implies overlapping and higher
patient dose with higher axial resolution
dd
3d
3d
Detector
Detector Pitch
Pitch 33
Beam
Beam Pitch
Pitch 0.75
0.75
dd
6d
6d
Detector
Detector Pitch
Pitch 66
Beam
Beam Pitch
Pitch 1.5
1.5
Lightspeed , GE Medical Systems
Beam Pitch vs. Volume Coverage
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Dose in Helical CT varies as:
Increase in pitch implies faster acquisition and
larger volume coverage
Dose
•
Lower pitch implies slower table speed with
overlapping of tissue (for P<1) and smaller
scanned volume
∝ Beam Pitch
1
(mAs/rotation)
Beam Pitch vs. Dose
•
Varying pitch results in increase or
decrease of radiation dose to patient
•
However in some MDCT scanner, image
noise is maintained constant by varying
tube current (“effective mAs
mAs”),
”), resulting in
patient dose independent of pitch*
pitch*
High Power XX-ray Tubes
*Mahesh M, et.al., AJR, 177: 12731273-1275, 2001
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
9
X-ray Tubes
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In helical CT, ZZ-axis resolution and scan
volume place huge demands on tube
•
Several technical advances have been
made to achieve power levels and deal
with problems of heat generation, storage
and dissipation
X-ray tubes used for Spiral CT
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Larger anode disks allow higher tube currents
Anodes of graphite based body with tungstentungstenrhenium or tungstentungsten-zircon
zircon--molybdenum
molybdenum**
layer deposited by sintering or chemical or
physical vapor process
* Ammann E, et al., BJR, 70, S1 -S9, 1997
X-ray tubes used for spiral CT
•
X-ray tube used in Spiral CT
Ceramic insulators
Metal envelopes with ceramic insulators
provide higher heat storage capacity
•
Spiral groove bearings improve heat
dissipation requiring shorter cooling periods
and therefore allow continuous rotation with
minimal wear
Direct oil
cooling of
spiral groove
bearing
Unique
200 mm
anode disk
Compact, all metal envelope
Courtesy Philips Medical Systems
Noise
Modern CT XX-Ray Tubes
• Heat storage capacity exceeds >3>3-8 MHU
• No longer the limitations for studies
demanding higher speed and larger
volume coverage
Noise
1
∝ vno. of photons
Tube current
Scan time
Section width
• Double the tube current, reduces noise by √2
• Halve the section width, increases noise by √ 2
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
10
Noise vs. Pitch
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For SDCT, noise is independent of pitch for
constant mAs and section width
•
However on most MDCT scanners, system
software automatically adjust scanmA
scan mA per
Effective Section Thickness
protocol to obtain comparable image noise as
user alters acquisition parameters
Section and Beam Collimation
•
SDCT: Both are same, influences zz-axis
coverage per gantry rotation
•
MDCT: Section thickness*
thickness * is total beam
collimation divided by number of active
detector channels
−
Section Thickness
•
True thickness of the
reconstructed image,
measured as full width at
half maximum (FWHM) of
slice sensitivity profile
•
Same as beam collimation in
Slice Sensitivity Profiles:
conventional scanning but
conventional and spiral
acquisition
different in spiral scanning
e.g., 10 mm / 4 channels = 4 x 2.5 mm
*defined at center of rotation
Effective Section Thickness
•
•
Measure of slice sensitivity profile at FWHM
•
In SDCT user selects section thickness, but true
width of reconstructed section is influenced by
pitch and interpolation algorithm (180° vs. 360°)
•
Affected by beam collimation, pitch and
interpolation algorithm
In MDCT user selects beam collimation in
combination with desired section width which is
affected by pitch, interpolation algorithm & ZZ-filter
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
Pitch vs. Effective Section Thickness
•
Increasing pitch
broadens effective
section thickness
•
Structures outside
nominal section
thickness will
contribute to image
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Evolution of Isotropic Voxel
MDCT Advantages
Acquisition of same region in shorter scan time or
larger region in same scan time
−
Thinner slices yielding higher zz -axis resolution
−
Better tube utilization
−
Greater coverage per breath hold
−
Better use of contrast agents
1.0 mm
0.5 mm
Approaching Isotropic Resolution!
Conventional
Helical: SDCT → MDCT
Speed of Volume Acquisition
Region
Head
Neck
Chest
Abdomen
Pelvis
0.2 mm
0.2 mm
Increased coverage per rotation
0.5 mm
•
1.0 mm
−
5 mm
•
Compared to SDCT
10 mm
•
Radiography (200 µ m)
CT Timeline
Distance
(cm)
Section
Thickness
(mm)
20
15
30
20
20
8
5
8
8
8
16.7
20.0
25.0
16.7
16.7
2.1
2.5
3.1
2.1
2.1
Total
95.1
11.9
Total scan time (sec)
SDCT†
MDCT‡
Slip -ring technology
Slipone second scan
Half second scan
Sub--second scan
Sub
72 . . . 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02
Invention of CT Spiral CT
Twin detector CT
†
1 sec scan, pitch 1.5
sec scan, pitch ~ 1.5 for 44-section MDCT
‡ 0.5
Future Directions
•
•
Partial rotation scan times ~150 ms possible!
•
Extended zz-axis coverage to cover most
organs in one or two gantry rotations should
be possible with large area detectors or flat
panel detectors
PET-CT
PET16+ sections CT
Conclusions
•
Cone beam reconstruction algorithms for 16,
40 and 64 row detectors are available
Mahadevappa Mahesh, Ph.D.
Johns Hopkins University, Baltimore, MD
MDCT
•
CT technology has evolved to level where
large 3D volumes can be imaged with:
−
isotropic resolution
−
acquisitions independent of most physiologic
motion
3D imaging of 3D anatomy - the 3D
radiograph - is becoming a reality!
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