Monte Carlo Modeling and Analysis of Structured CsI Scintillator -coupled Pixel Detectors

1. SCINT 2007 Monte Carlo Modeling and Analysis of Structured CsIScintillator-coupled Pixel Detectors 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

2. 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: DETECT2000TM Biomedical Mechatronics Lab

3. Monte Carlo model • Spectral function • Pencil beam MCNPXTM DETECT2000TM 512 x 512 CsI columns in rectangular array format with a 10-mm-pitch Tally rcsi Depth of interaction Strong absorber Reflection tcsi Absorption tsio2 tsi Refraction z Polish surface origin x y Detection plane SiO2 layer Si layer Biomedical Mechatronics Lab

4. Simulation parameters Biomedical Mechatronics Lab

5. Spectral source sampling Input energy condition for MCNPX simulation Filter : 2 mm Al Target material : Tungsten (W) Source : 70kVp spectrum energy Biomedical Mechatronics Lab

6. 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 a d a d Biomedical Mechatronics Lab

7. z= depth position of CsI column I(i, j)= a pixel value after sampling & Depth of Interaction AED(zi), w(zi) PSF(z1) PSF(z2) PSF(zi) PSF(zN) • • • • • • • • LSF 1D Extraction 2D FFT MTF NNPS Biomedical Mechatronics Lab

8. X-ray response Incidence of pencil-beam with spectrum y x z 100mm CsI 0.24% 1.44% 0.91% 0.92% 1.46% 0.24% 1.45% 0.24% 89.55% 0.93% 1.47% 0.92% 0.24% • Lateral dispersion is very small compared with the center of CsI column (~90%) → Neglecting lateral dispersion Biomedical Mechatronics Lab

9. Depth of interactions Absorbed energy distribution along a single column of CsI (mm) 0.5 z = 5 z = 55 z = 95 Collected optical photon distribution at the Si detector plane 0 -0.5 -0.5 0 0.5 Biomedical Mechatronics Lab

10. 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. Biomedical Mechatronics Lab

11. Modulation-transfer function MTF as a function of depth position 1-D FFT of LSF (line-spread function) where Biomedical Mechatronics Lab

12. Simulated MTFs were quantified by Lorenz fitting Biomedical Mechatronics Lab

13. Noise-power spectrum Normalized NPS = 2-D FFT of Biomedical Mechatronics Lab

14. 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 Biomedical Mechatronics Lab

15. Effect of pixel fill factor d=100 mm (mm) 1.5 g = 20 % g = 60 % g = 40 % g = 80 % g = 100 % 0 -1.5 0 1.5 -1.5 Biomedical Mechatronics Lab

16. 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