CT in NM Imaging
Presented by:
Donna Stevens, M.S.
Senior Medical Physicist
UT M. D. Anderson Cancer Center
Houston, TX USA
Part 2
CT Image Formation
Objectives
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X-ray for CT imaging
Back projection method of image reconstruction
Effects of various reconstruction filters on the
images
Scan parameters affecting image quality
Defining the concept of pitch in helical CT
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CT Image Formation
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X-ray for CT imaging
Image Reconstruction
Image Quality
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CT System
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Transmission imaging
Narrow fan beam
Rotate around patient
Hundreds of projections
Produce transaxial images
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Computerized Axial Tomography
One CT projection
CT image from >700
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projections
without permission of author.
Axial Platforms
first generation
second generation
third generation
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Axial and Helical Platforms
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Helical or Spiral CT
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Slip-ring gantry allows continuous rotation
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Reduction of interscan delays
Constant table motion during scanning
Special reconstruction methods developed
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Pitch for Single-Slice CT
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Image and beam width are same for
conventional CT
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Pitch = table travel ÷ beam width
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Typical pitch values are 0.7 to 2.0
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For Pitch < 1
Pitch
Spirals overlap
Dose is increased
(at same mAs)
For Pitch > 1
Spirals are stretched
Dose is decreased
(at same
Slides mAs)
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Helical Interpolation
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Early helical used linear interpolation to
estimate data for the “missing” projections
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Rebinning the collected data permits
estimation of a data set 180° (opposite) from
collected data
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Interpolation between the collected and
“rebinned” projections reduces the scan
length required to reconstruct each image
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Reduces the “helical” artifact
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Helical Interpolation
Collect data (black)
Rebin to estimate the
180° data (red)
Interpolate to estimate
points between collected
and rebinned data
(blue)
Helical CT needs fast
computers
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Beam Filtration
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Removes low energy x-rays from beam
Low E photons contrib to dose, not image
„ Filter reduces beam-hardening artifacts
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Shapes energy distribution across beam
Removes more low energy from edges
„ Results in more uniform beam hardening after
passing through filter and patient
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Beam Quality
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More filtration (“hard”)
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Avg energy higher
Lower skin dose
Better penetration
Loss of contrast
Less filtration (“soft”)
Half-Value Layer
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kVp
Mm Al needed to reduce exposure-in-air to one-half
Typical HVL for x-ray: 2.5 to 5.0 mm Al
Typical HVL for CT: 5.0 to 9.0 mm Al
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Beam Hardening
Light and dark variations in
regions having high beam
attenuation:
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Shoulders (apex of lungs)
Skull
Pelvis
Low energy photons more
readily absorbed
Exit beam has higher average
energy (is “harder”)
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Beam Hardening
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Beam Filtration
X-ray fan beam
Filter
Shaped beam
Patient
More uniform output
Pre-patient filter called “bowtie”
Hardens and “shapes” beam for head or body imaging
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Detector Characteristics
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X-ray absorption efficiency
Response time
Dynamic range
High reproducibility
Electronic stability and sampling rate
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Solid State Detectors
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Photodiode
multipliers (no PMT)
CdWO4 crystals (old)
99% conversion and
capture efficiency
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Ceramics (modern)
99% absorption, 3X
conversion
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Conventional CT Detectors
Image width determined by
beam thickness
Pitch = table mm / beam mm
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MSCT Detectors
Single element – Image
thickness determined by
physical collimators
Multiple element – Image
thickness determined
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electronically by
withoutchannels
permission of author.
MSCT detectors
64 x 0.625 mm
Table direction
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Multi-Detector Concept
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Acquisition of multiple images per scan
Electronic post-patient collimation
Faster volume acquisition times
Better bolus tracking and thin slices for 3D
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Binning of Detector Elements
1 1.5
2.5
5.0
4-channel system
Variable element, total 20mm
16-channel system
4 @ 1.5, 16 @ 0.75, 4 @ 1.5, total 24mm
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3rd Generation Platform
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Detector arc with ~1000
segments subdivided in zdirection
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Due to 8,000 to > 30,000
detector elements and assoc
electronics, 4th gen MSCT
platform not feasible
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CT Image Formation
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X-ray for CT imaging
Image Reconstruction
Image Quality
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CT Image Reconstruction
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Detector signal represents attenuation by all material
along a path
µ value (attenuation coefficient) calculated for each
path
Infinite number of paths needed for perfect
reproduction of object
Realistic limitations on number of paths
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Sampling rate of electronics
Dose to patient
Heat loading on system
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Reconstruction Filters
1 Object
2 Projection data
3 Recon filter
1
2
3
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4 Backprojection
of filtered data
5 Backprojection
of filtered data
for two angles
5
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Filtered Backprojection
Filtered profile
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Filtered Backprojection
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Ideal
“Standard”
“Edge”
Various filters produce different effects
in the structure of the noise.
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Effects of Recon Filters on Resolution
Std Recon
Soft Recon
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Effects of Recon Filters on Resolution
Std Recon
Bone Recon
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Effects of Recon Filters on Resolution
Std Recon
Detail Recon
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Effects of Recon Filters on Resolution
Std Recon
Edge Recon
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Effects of Recon Filters on Noise
Recon Filter
Soft
Standard
Lung
Detail
Bone
Edge
Bone Plus
Std Dev Water
Img
3.8
4.7
19.6
6.5
18.8
35.8
27.0
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Reconstruction “Filters”
“Standard” recon filter
“Bone” recon filter
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Dx vs Attenuation CT
Standard recon
Attenuation correction
recon
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Pitch
Pitch = table travel ÷ beam width
Typical pitch values are 0.7 to 2.0
For Pitch < 1
For Pitch > 1
Spirals overlap
Spirals are stretched
Dose is increased
(at same mAs)
Dose is decreased
(at same mAs)
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without permission of author.
Helical Interpolation
Collect data (black)
Rebin to estimate the
180° data (red)
Interpolate to estimate
points between collected
and rebinned data
(blue)
Helical CT needs fast
computers
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without permission of author.
MSCT Image Recon
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Axial system uses 360° data for each image
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Conventional helical uses < 360° data,
depending on pitch, then interpolates
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MSCT uses small arcs (<90°) from each
element, plus interpolation, to “build” 360° of
data
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MSCT Pitch
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Pitch = table travel ÷ beam width
Beam width due to pre-patient collimators not image
width
• Pitch relates to dose
Beam width
and effective mAs
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MSCT Image Recon
Image 1
Table direction
A
D
B C
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Lesion moves from Channel A to D in one rotation
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MSCT Image Reconstruction
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To get Image 1
Sample Channel A for 1st qtr rotation
Channel B for 2nd qtr rotation
Channel C for 3rd qtr rotation
Channel D for 4th qtr rotation
Apply all that data to Image 1
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4-channel Recon, Pitch = 1.0
Image 5
Image 1
A
D
B C
Table direction
Qtr A
arc
1st Im1
2nd
3rd
4th
5th
B
C
D
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4-channel Recon, Pitch = 1.0
Image 5
Image 1
A
D
B C
Table direction
Qtr
arc
1st
2nd
3rd
4th
5th
A
B
C
D
Im1
Im2 Im1
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4-channel Recon, Pitch = 1.0
Image 5
Image 1
A
D
B C
Table direction
Qtr
arc
1st
2nd
3rd
4th
5th
A
B
C
D
Im1
Im2 Im1
Im3 Im2 Im1
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4-channel Recon, Pitch = 1.0
Image 5
Image 1
A
D
B C
Table direction
Qtr
arc
1st
2nd
3rd
4th
5th
A
B
C
D
Im1
Im2 Im1
Im3 Im2 Im1
Im4 Im3 Im2 Im1
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4-channel Recon, Pitch = 1.0
Image 5
A
Image 1
D
B C
Table direction
Qtr
arc
1st
2nd
3rd
4th
5th
A
B
C
D
Im1
Im2 Im1
Im3 Im2 Im1
Im4 Im3 Im2 Im1
Im5 Im4 Im3 Im2
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CT Image Formation
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X-ray for CT imaging
Image Reconstruction
Image Quality
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Image Quality Parameters
kVp
pitch
mA
effective mAs
Rotation time
recon filters
mAs
image thickness
focal spot size
display field size
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Effective mAs
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To compare protocols, need effective mAs
Eff mAs = mA • time ÷ pitch
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To compare if kVp’s are different
Correct effective mAs by kVp2
eff mAsold • (kVpnew2 / kVpold2)
then compare to eff mAsnew
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Comparing Protocols w/ Eff. mAs
Scanner
4-slice
Image
7.5
Detector 4 x 3.75
Pitch
0.75
kVp
120
mA
200
Rotation
0.8
Eff. mAs
213
8-slice
7.5
8 x 2.5
.625
120
200
0.8
16-slice
7.5
16 x 1.25
1.375
120
320
0.5
256
116
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What it does
When to change
kVp
contrast,
penetration
very lg or
very sm patient
mAs
# x-rays
“noise” in image
display FOV
image spatial
resolution
patient body size
Parameter
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Parameter
What it does
When to change
image thickness
resolution,
noise
reformats fine
structure
recon filters
smooth,
edge enhance
depends on reason
for study
focal spot size
spatial resolution
user cannot control
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Summary
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Detectors
Binning for multi-row
Pitch
Filtered back projection
Image Quality Parameters
Effective mAs
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Thank You!
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