Post on 03-Aug-2020
Co-moderators:
Norbert J. Pelc, Sc.D. Stanford University
Cynthia McCollough, Ph.D.
Mayo Clinic
Multi-Energy CT: Current status and recent innovations
Multi-Energy CT: Current status and recent innovations
Basic concepts & current implementations
Norbert Pelc
Data and image analysis methods
Lifeng Yu
Clinical applications
Cynthia McCullough
Future directions
Taly Gilat Schmidt
Panel Discussion
Norbert J. Pelc, Sc.D.
Departments of Bioengineering and Radiology
Stanford University
Multi-Energy CT: Basic concepts & current
implementations
Acknowledgements
Uri Shreter and Robert Senzig: GE Healthcare
Thomas Flohr, Bernhard Schmidt and Karl Krzymyk: Siemens Healthcare
Ami Altman, Alex Ganin, Andy Mack, Efrat Shefer: Philips Healthcare
Cynthia McCollough: Mayo Clinic
Research support:
GE Healthcare
Philips Healthcare,
Samsung Electronics
Motivation
www.uhrad.com/
ctarc/ct153b2.jpg
Lower density or lower atomic number?
Motivation
www.uhrad.com/
ctarc/ct153b2.jpg
Is the HU increase due to underlying tissue
property or contrast agent?
Motivation
TN Sheth, et al, CARJ 56, 15, 2006.
Can we separate iodine and calcium,
or reduce blooming?
Attenuation coefficients depend on photon energy
0.1
1
10
100
35 55 75 95 115 135
bone
water
iodine
atte
nuat
ion c
oef
fici
ent
photon energy
use CT measurements at multiple energies
to add material specificity
Motivation
“Two pictures are taken of the same slice, one
at 100 kV and the other at 140 kV...so that
areas of high atomic numbers can be
enhanced... Tests carried out to date have
shown that iodine (Z=53) can be readily
differentiated from calcium (Z=20)”.
G.N. Hounsfield, BJR 46, 1016-22, 1973.
OUTLINE
• Physical principles of multi-energy x-ray
measurements
• Implementations
achieving spectral selectivity
obtaining multi-energy measurements
• Summary
attenuation and material identification
T
I0
I = I0 e-T
T = ln(I0/ I)
measures product of & T
not very material specific
unknown thickness two known materials
energy E1
I1
tb
I1 = I01 e -(w1tw+ b1tb)
I01
water
bone
10 100 20 50 200
cortical bone
water
photon energy (keV)
lin
ear
atte
nu
atio
n c
oef
ficie
nt
(cm
-1)
0.1
1.0
10.0
100.0
I2
E2
I02
I2 = I02 e -(w2tw+ b2tb)
E2 E1
solve for tw and/or tb
dual energy x-ray absorptiometry (DEXA)
2 energies 2 materials
• 2 energies 2 materials
material analysis with absorptiometry
• can we generalize this? N energies for N
materials?
• limitation: two strong interaction mechanisms
Compton scattering and photoelectric absorption
Barring a K-edge in the spectrum, the energy
dependence of each is the same for all elements!!
• Attenuation of any material is ~ a weighted sum of photoelectric and Compton functions
• Any material can be modeled as a weighted sum of two other materials (basis materials)
basis material decomposition:
Basis material decomposition
0.1
1
10
100
1000
0 20 40 60 80 100 120 140
O
Ca
Cu
Ca'
Cu
.61*O + .04*Cu
Ca
O
Basis material decomposition
M grams of Ca .04 M grams of Cu
.61 M grams of O
I
I0 I0
I
=
Indistinguishable at any x-ray energy above their K-edge
Common “basis functions”:
iodine and water, aluminum and plastic
photoelectric and Compton
material analysis with absorptiometry
• 1 energy
constant thickness
2 known materials
(constant thickness is a constraint)
• 2 energies
unknown thickness
2 known materials
• N energies
unknown thickness
still only 2 known materials
(only 2 independent functions)
• 2 energies
constant thickness
3 known materials
(constant thickness (CT voxel) is a constraint)
K-edges
10 100 20 50 200
iodine cortical bone
water
photon energy (keV)
linea
r at
tenuat
ion c
oef
fici
ent
(cm
-1)
0.1
1.0
10.0
100.0
Very specific material information
iodine image
SNR=3.4
Noise
low energy high energy
water image
SNR=37
~ a2 low + b2 high
Iodine data ~
a • Datalow - b • Datahigh
depends on:
- specific energies
- allocation of dose to
the two measurements
iodine image
SNR=1.3
iodine image
SNR=3.4
Noise depends on dose allocation
water image
SNR=34
water image
SNR=37
with 80/140 kVp dose
allocation that maximizes
iodine SNR
80 kVp dose, 140 kVp dose
same total dose
Spectral separation and control
• very critical for SNR efficiency, separation
robustness, etc.
• implementations
different kVp and/or filtration
detectors with energy discrimination
Principle of Dual Energy CT
Tube 2
Tube 1
Mean Energy:
56 kV 76 kV
Data acquisition with different X-ray spectra: 80 kV / 140 kV
Different mean energies of the X-ray quanta Courtesy of B. Krauss, B. Schmidt, and
Th. Flohr, Siemens Medical Solutions
SOMATOM Definition Flash
SPS= Selective Photon Shield
syngo Dual Energy
- Principle of Dual Energy
S1: 80 kV
x 104 15
10
5
0 50 100
150
80 kV 140 kV
photon energy (keV)
nu
mb
er
of
qu
an
ta
S2: 140 kV S1: 80 kV
15
10
5
0 50 100
150
80 kV
photon energy (keV)
nu
mb
er
of
qu
an
ta
140 kV
S2: 140 kV
x 104
+ SPS
140 kV + SPS
Dual kVp, dual filtration
135 kVp
1.5 mm bronze
85 kVp
0.1 mm erbium
• switched filtration
improves separation
• different mA helps
apportion dose
Lehmann et al: Med Phys 8, 659-67, 1981.
Slide 6 – Learn More 1 Perfect alignment Simultaneous alignment in time and space
NanoPanel Prism
Top Scintillator
X-Rays
Side-looking photodiode array
Low Energy Raw data E1 image
Top scintillator
Full CT Image
Bottom Scintillator:
2-mm GOS+ High Energy Raw data
Combined Raw data
Effective atomic number small but does not sacrifice light output Thickness optimized for energy separation and low-energy image noise
Bottom scintillator absorbs 99.5% of high-energy spectrum
E2 image
CT image
The IQon Spectral CT is not yet CE Marked and not available for sale in all regions. The IQon Spectral CT is pending 510(k) and not available for sale in the USA.
Carmi R, Naveh G, and Altman A: IEEE
NSS M03-367, 1876-78, 2005
Layered detector
• relatively poorer spectral separation
• simultaneous dual energy sensing
Investigational device Confidential
Absorbed single X-ray photon
High
Voltage
Counter 3
Counter 2
Counter 1
Counter 4
Charge pulse
Pulse height proportional
to x-ray photon energy
Discriminating
thresholds
Stored counts of all energy windows,
for each reading time period
Direct conversion
material
Electronics
Pixelated electrodes
Photon energy
Counts
Photon Counting Spectral CT – Detector Principle
Direct- Conversion Detector efficiently translates X-ray photons into large electronic signals
These signals are binned according to their corresponding X-ray energies
(CZT crystal: Cadmium Zinc Telluride)
Spectral separation
Separation and control of the spectra is important
reduced noise
improved material characterization
improved dose efficiency
Achieved by
choice of kVp (and mAs allocation)
use of filtration
performance of energy discriminating detectors
• ideal photon counting with K-edge filter
• ideal photon counting with energy analysis
• different kVp and filtration
• different kVp
• layered detector
Spectral separation implementations
better
spectral
separation
and
dose
efficiency
Data acquisition implementations
• Sequential scans at different kVp
motion sensitivity > scan time
• Two sources at ~90º on the same gantry
SOMATOM Definition Flash
syngo Dual Energy
- Principle of Dual Energy
Two X-ray tubes, 1st: 80 or 100 kVp, 2nd: 140 kVp
Dual Source Challenge: Inconsistent scans
Ideal
Moving Phantom
Simulation
Dual Source system
Moving Objects
Does not see movement
Courtesy of R. Senzig, GE Healthcare
Data acquisition implementations
• Sequential scans at different kVp
motion sensitivity > scan time
• Two sources at ~90º on the same gantry
some motion sensitivity (~ 25% Trot)
• Switching kVp within a single scan1, 2
1. Lehmann et al: Med Phys 8, 659-67, 1981.
2. Kalender et al, Med Phys 13, 334, 1986.
Courtesy of Uri Shreter, GE Healthcare
Rapid kVp switching Dual energy CT
• Requires fast
generator and
detectors
• Dose allocation
controlled by dwell
time
• Difficult to switch
filters
Data acquisition implementations
• Sequential scans at different kVp
motion sensitivity > scan time
• Two sources at 90º on the same gantry
some motion sensitivity (~ 25% Trot)
• Switching kVp within a single scan
• Energy discriminating detectors
layered detector, photon counting
better
immunity
to
motion
Summary of commercial systems
• Siemens: two sources, different kVp (80 or 100
/140) and filtration, direct control of mA
• GE: single source with rapid switching, same
filter for both kVps, control mAs by dwell time
• Philips: dual-layer detector, usual kVp mAs
control
• Lots of R&D work, especially on photon
counting detectors