Biomedical Mechatronics Lab M ONTE C ARLO M ODELING AND A NALYSIS OF S TRUCTURED C S I S CINTILLATOR...

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 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

3Biomedical Mechatronics Lab


Simulation parameters



Spectral source sampling• Input energy condition for MCNPX simulation

▫ Filter : 2 mm Al▫ Target material : Tungsten (W)▫ Source : 70kVp spectrum energy



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



▫ 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



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



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



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.



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



• Simulated MTFs were quantified by Lorenz fitting

21

1)(MTF

kff



Noise-power spectrum

• Normalized NPS = 2-D FFT of

•

zz

zzz

i

iieff )(

)(NNPS)(NNPS

2),(

I

IjiI



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



Effect of pixel fill factor

0

1.5

-1.51.5-1.5 0

(mm) d=100 m

= 20 % = 40 % = 60 % = 80 % = 100 %



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.


Biomedical Mechatronics Lab M ONTE C ARLO M ODELING AND A NALYSIS OF S TRUCTURED C S I S CINTILLATOR...

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