1. Biomedical Mechatronics Lab M ONTE C ARLO M ODELING AND A NALYSIS OF S TRUCTURED C S I S CINTILLATOR - COUPLED P IXEL D ETECTORS Chang Hwy Lim, Ho Kyung Kim * School of Mechanical Engineering Pusan National University, Republic of Korea Hyosung Cho Department of Radiological Science Yonsei University, Republic of Korea SCINT 2007

2. 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 SiO 2 /Si layers ▫ Investigating MTF and NPS X-ray transport: MCNPX TM Optical photon transport: DETECT2000 TM Biomedical Mechatronics Lab 2

3. Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors, SCINT 2007 Monte Carlo model Detection plane SiO 2 layer Reflection Depth of interaction z y x Polish surface Strong absorber Tally origin r csi t csi t sio2 t si MCNPX TM DETECT2000 TM Spectral function Pencil beam Si layer Absorption Refraction Symbolvalue r csi 4μm4μm t sio2 2.5μm t si 1μm1μm 512 x 512 CsI columns in rectangular array format with a 10-  m-pitch 3 Biomedical Mechatronics Lab

4. Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors, SCINT 2007 Simulation parameters 4 Biomedical Mechatronics Lab

5. 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 5 Biomedical Mechatronics Lab

6. 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 d a a d 6 Biomedical Mechatronics Lab

7. 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 Interaction AED(z i ), w(z i ) PSF(z 1 ) PSF(z 2 )PSF(z i )PSF(z N ) LSF 1D Extraction MTF 2D FFT NNPS & 7 Biomedical Mechatronics Lab

8. 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% x y z 100  m CsI Lateral dispersion is very small compared with the center of CsI column (~90%) → Neglecting lateral dispersion 8 Biomedical Mechatronics Lab

9. 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 0.5-0.50 Collected optical photon distribution at the Si detector plane 9 Biomedical Mechatronics Lab

10. 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 SiO 2 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. 10 Biomedical Mechatronics Lab

11. 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) where 11 Biomedical Mechatronics Lab

12. Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors, SCINT 2007 Simulated MTFs were quantified by Lorenz fitting 12 Biomedical Mechatronics Lab

13. Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors, SCINT 2007 Noise-power spectrum Normalized NPS = 2-D FFT of 13 Biomedical Mechatronics Lab

14. 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 ray5 bursts of x rays10 bursts of x rays50 bursts of x rays100 bursts of x rays 14 Biomedical Mechatronics Lab

15. Monte Carlo modeling and analysis of structured CsI scintillator-coupled pixel detectors, SCINT 2007 Effect of pixel fill factor 0 1.5 -1.5 1.5 -1.5 0 (mm) d=100  m  = 20 %  = 40 %  = 60 %  = 80 %  = 100 % 15 Biomedical Mechatronics Lab

16. 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. 16 Biomedical Mechatronics Lab