Dual-energy CT Physics
Dual-energy X-ray CT with
Fast-kVp Switch
•
Concept proposed in the 70’s.
•
Two x-ray / matter interaction: photoelectric & Compton.
•
Mass attenuation coefficient is composed of the
Photoelectric effect component, αp, and the Compton
scatter component, αc.
µ
( E) = α p f p (E) + α c fc ( E)
ρ
Also be expressed as a linear combination of the mass
attenuation coefficient of two materials.
µ
µ
(E ) = β A
ρ
ρ
GE Healthcare Technologies
(E ) + β B
A
µ
ρ
(E)
B
% interaction
•
Jiang Hsieh, Naveen Chandra, David Langan,
Mary-sue Kulpins, Xiaoye Wu, Paul Licato
1
Compton
photoelectric
energy, keV
2
Equivalent-density Images
Material Basis
• Non-basis materials are mapped to both.
• For CT, we can select water-iodine pair or softtissue-bone pair.
• Equivalent-density images are not in HU, but in g/cm3
µbone = 0.88µ water + 0.18µiodine
Water
80kVp
100000
attenuation coefficient
10000
1000
Non-linear
mapping
iodine
100
bone
10
140kVp
Iodine
soft tissue
1
0.1
0
30
60
90
120
150
energy (keV)
3
4
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Monochromatic Imaging
Image Quality Optimization
• The material basis pair can be mapped to produce a
synthesized monochromatic project.
µ
µ ( r , E0 )ds =
ρ
µ
( E0 ) ψ A ( r )ds +
ρ
A
• Mass attenuation coefficient decreases with energy.
• Low-contrast decreases with increased energy.
• Optimal LCD and Noise performance at certain keV.
( E0 ) ψ B ( r )ds
B
• The reconstructed mono-images are in HU.
80 kVp
140 kVp
Monochromatic
40 keV
•More contrast
•More noise
•Less contrast
•Less noise
75 keV
140 keV
A hybrid image optimizing image
qualities of 80/140 kVp
5
6
Detector Requirement
Fast kV Switching
•
High Power Tube Fast Generator
• Change kVp setting on
a view by view basis.
• High- and low-kV are toggled
every view
• Little patient motion
• Allow projection space
processing
140kV
80kV
140kV
Key Performance Parameters of Gemstone Spectral Imaging
• Primary speed and afterglow
• Stability
Stable in boiling water
Primary speed
0.03µ
µs
3 µs
XYZ
1
Gemstone
0.75
• Require fast generator
response.
• Require fast
scintillator response.
Scintillator
Gemstone
Intensity
XYZ
4.5cm
Fast Scintillator High-speed DAS
high-precision molding
0.5
0.25
0
0
5
10
15
Time (microsec)
Pulsed x-ray 30kvp, 0.5A, ~1ns pulse
7
8
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Projection Based Material Decomposition
GSI Data Acquisition
Interleaved High- and
Low-kVp Projections
Beam Hardening Effect on Lesion ROI Values
Attenuation-to-material density transformation
Single Energy Imaging Not Consistently Accurate
2
2
iodine P1(i) = α1(i)Plow(i ) + β1(i )Phigh(i ) + χ1Plow
(i ) + δ1Phigh
(i) + ε1Plow(i)Phigh(i) + ....
2
2
water P2 (i ) = α2 (i )Plow(i ) + β2 (i )Phigh(i ) + χ 2Plow(i ) + δ2 Phigh(i ) + ε 2Plow(i)Phigh(i) + ....
Iodine Projections
split
Low kVp
Projections
Water Projections
Image Reconstruction
High kVp
Projections
Image reconstruction
MD Iodine
MD Water
Monochromatic
Generation
140kVp QC Image
10
70 keV
Low Density, Hypo-Enhancing Hepatic Masses
Beam hardening reduction
Known Malignant (MET)
140 kVp
MD Iodine
Hypo-Enhanced
(with structure)
MD Water
Close to Normal Liver
Known Benign (Cyst)
Gemstone Spectral
Imaging’s ability to
reduce beam
hardening artifact due
to dense bone in the
posterior fossa is
demonstrated in the
Spectral image.
80 kVp Source Image
Spectral 80keV image
Hypo-Enhanced
11
Black like Background
Mayo Clinic: Dr. Amy Hara, MD
12
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Linear Discriminate Analysis
Renal Cyst with Calcification
Tissue is Characterized by Combining Water and Iodine Basis Density
Require accurate density measurement
Li
ne
•
•
•
•
Enhanced lesion indicates potential malignancy.
Calcification appearance: bright on water density image.
LD
A
Pr
oj
ec
tio
n
Normal Liver
(Enhanced by Iodine)
(Enhanced)
cyst
(Un-enhanced)
calcification
original
MD Iodine
MD Water
Images Courtesy of Dr. Amy Hara, Mayo Clinic, Scottsdale, AZ
13
Mayo Clinic: Dr. Amy Hara, MD
14
Kidney Stone Experiment
Kidney Stone Identification
Kidney stone can be easily identified in the water-density image.
• Kidney stones
placed inside. potato
phantom
• Potential
differentiation with
GSI
Low kVp
High kVp
Ca Oxolate
Brushite
Cystine
Uric acid
Potato
• Data-base to set up
75 keV
• Similar approaches
to other applications
such as cardiac
kidney stone
MD Water
MD Iodine
Mayo Clinic: Dr. Amy Hara, MD
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Recent Research:
•
Cardiac Spectral Imaging*
Cardiac motion phantom study
•
•
•
• Delineation of different material in the moving vessels
• No motion artifacts
•
Recent Research: Spectral Projection Imaging*
Separation of bone and soft-tissue
No mis-registration due to motion
Same time and orientation
Applications
Fast-kV Acquisition
• Re-stenosis
• Complex calcification
Dynamic Heart Phantom…
Pavlicek, Mayo Clinic
Fast Switching
Dual KVP Cardiac
*Technology-in-development
regular scout
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soft-tissue scout
*Technology-in-development
bone scout
18
Conclusion
• Gemstone spectral imaging provides an
advanced platform to move CT beyond the
pure anatomical modality.
• Provides additional information for aiding in
tumor characterization, material differentiation,
• Artifact reduction
• Current research on GSI for cardiac imaging
and super low-dose spectral projection
imaging.
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