Biomedical Mechatronics Lab
MONTE CARLO MODELING AND ANALYSIS OF STRUCTURED CSI SCINTILLATOR-COUPLED PIXEL
DETECTORS
Chang Hwy Lim, Ho Kyung Kim*
School of Mechanical Engineering
Pusan National University, Republic of Korea
Hyosung ChoDepartment of Radiological Science
Yonsei University, Republic of Korea
SCINT 2007
Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Objectives• Columnar-structured CsI layer is commonly used in DR
system.▫ High sensitivity without much spreading of light compared with the
conventional granular phosphor screens▫ Good spectral matching with the photodiode array
• Investigating, theoretically, imaging performance of columnar-structured CsI scintillator▫ Using Monte Carlo simulation▫ Considering depth of interactions: Lubberts effect▫ Including optical device in the simulation by simply modeling as
SiO2/Si layers
▫ Investigating MTF and NPS
• X-ray transport: MCNPXTM
• Optical photon transport: DETECT2000TMBiomedical Mechatronics Lab 2
Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Monte Carlo model
Detection plane
SiO2
layer
Reflection
Depth of interaction
z
yx
Polish surface
Strong absorber
Tally
origin
rcsi
tcsi
tsio2
tsi
MCNPXTM
DETECT2000T
M
• Spectral function
• Pencil beam
Si layer
Absorption
Refraction
Symbol value
rcsi 4μm
tsio2 2.5μm
tsi 1μm
512 x 512 CsI columnsin rectangular array formatwith a 10-m-pitch
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Simulation parameters
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Spectral source sampling• Input energy condition for MCNPX simulation
▫ Filter : 2 mm Al▫ Target material : Tungsten (W)▫ Source : 70kVp spectrum energy
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Post-data processing• To mimic digital sampling process in a digital X-ray imager• Assuming squared pixel geometry;
▫ d = pixel pitch in one direction▫ a = aperture size in one direction
da
a
d
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
▫ z = depth position of CsI column▫ I(i, j) = a pixel value after sampling
Depth of InteractionAED(zi), w(zi)
PSF(z1) PSF(z2) PSF(zi) PSF(zN)• • • • • • • •
LSF
1D Extraction
MTF
2D FFT
NNPS
2}/),({ IIjiI &
)(
)(MTF)(MTF
i
iieff z
zzz
)(
)(NNPS)(NNPS
i
iieff z
zzz
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
X-ray response• Incidence of pencil-beam with spectrum
89.55% 1.45%
1.47%
1.46%
1.44%0.91% 0.92%
0.93% 0.92%
0.24%
0.24%
0.24%0.24%
x
y
z
100
m
CsI
• Lateral dispersion is very small compared with the center of CsI column (~90%) → Neglecting lateral dispersion
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Depth of interactions
Absorbed energy distribution along a single column of CsI
z = 95
0
0.5
-0.5
(mm)
z = 5 z = 55
0.5-0.5 0
Collected optical photon distribution at the Si detector plane
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Light collection efficiency
• In this simulation, loss due to the detector materials, such as SiO2 and Si, has been considered.
• Optical photon transport is mostly terminated between columns because of the refraction through the detector materials.
• Columns pattern is apparently shown.
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Modulation-transfer function• MTF as a function of depth position• 1-D FFT of LSF (line-spread function)
zz
zzz
i
iieff )(
)(MTF)(MTF )(
)()( i
ii z
W
zEz
where
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
• Simulated MTFs were quantified by Lorenz fitting
21
1)(MTF
kff
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Noise-power spectrum
• Normalized NPS = 2-D FFT of
•
zz
zzz
i
iieff )(
)(NNPS)(NNPS
2),(
I
IjiI
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Effect of number of bursts of x rays
1 burst of x ray 5 bursts of x rays 10 bursts of x rays 50 bursts of x rays 100 bursts of x rays
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Effect of pixel fill factor
0
1.5
-1.51.5-1.5 0
(mm) d=100 m
= 20 % = 40 % = 60 % = 80 % = 100 %
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Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors , SCINT 2007
Conclusion• Columnar structure may give rise to a fixed pattern noise.• Insufficient optical photons;
▫ Overestimate MTF;▫ Degrading NPS
• MTF degrades as the pixel fill factor decreases.
• The developed simulation procedure will be helpful for the better understanding of the underlying physics in imaging with scintillation materials and for the better design of the imaging detector.
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