Modeling granular phosphor screens by Monte Carlo methods Panagiotis F. Liaparinos Department of Medical Physics, Faculty of Medicine, University of Patras, 265 00 Patras, Greece Ioannis S. Kandarakis and Dionisis A. Cavouras Department of Medical Instruments Technology, Technological Educational Institute, 122 10 Athens, Greece Harry B. Delis and George S. Panayiotakis a Department of Medical Physics, Faculty of Medicine, University of Patras, 265 00 Patras, Greece Received 9 December 2005; revised 30 September 2006; accepted for publication 2 October 2006; published 8 November 2006 The intrinsic phosphor properties are of significant importance for the performance of phosphor screens used in medical imaging systems. In previous analytical-theoretical and Monte Carlo stud- ies on granular phosphor materials, values of optical properties, and light interaction cross sections were found by fitting to experimental data. These values were then employed for the assessment of phosphor screen imaging performance. However, it was found that, depending on the experimental technique and fitting methodology, the optical parameters of a specific phosphor material varied within a wide range of values, i.e., variations of light scattering with respect to light absorption coefficients were often observed for the same phosphor material. In this study, x-ray and light transport within granular phosphor materials was studied by developing a computational model using Monte Carlo methods. The model was based on the intrinsic physical characteristics of the phosphor. Input values required to feed the model can be easily obtained from tabulated data. The complex refractive index was introduced and microscopic probabilities for light interactions were produced, using Mie scattering theory. Model validation was carried out by comparing model results on x-ray and light parameters x-ray absorption, statistical fluctuations in the x-ray to light conversion process, number of emitted light photons, output light spatial distribution with previous published experimental data on Gd 2 O 2 S : Tb phosphor material Kodak Min-R screen. Results showed the dependence of the modulation transfer function MTF on phosphor grain size and material packing density. It was predicted that granular Gd 2 O 2 S : Tb screens of high packing density and small grain size may exhibit considerably better resolution and light emission properties than the conventional Gd 2 O 2 S : Tb screens, under similar conditions x-ray incident energy, screen thickness.© 2006 American Association of Physicists in Medicine. DOI: 10.1118/1.2372217 Key words: x-ray imaging, phosphor screens, Monte Carlo, modeling, MTF I. INTRODUCTION Luminescent materials are employed as radiation to light converters in detectors of medical imaging systems. 1–3 In x-ray projection imaging, a large number of such materials have been employed in the form of granular screens, 4 often referred to as phosphor screens, consisting of a large number of grains embedded within a binding material. The x-ray detection and imaging performance of phosphor screens are affected by intrinsic physical properties, related to x-ray and light photon transport through the material. These properties have been previously investigated by experimental, 5–11 theoretical, 11–15 and Monte Carlo methods, 16–22 and have been taken into account in the design of commercial imaging systems. 23–25 Phosphor screens have been previously 12,14,15,26–30 mod- elled as a series of superimposed x-ray absorbing, light cre- ating, and light attenuating elementary thin layers within the framework of cascaded linear systems analysis. Analytical methods, based on photon diffusion equations or on one- dimensional light transport considerations, 14,15,26–29 have been preferably employed to investigate phosphor intrinsic properties. Analytical and recursive methods have also been used to model phosphor screens as an ensemble of grains of specific size. 31,32 On the other hand, most Monte Carlo studies 20,21,33–35 have focused on the simulation of x-ray in- teractions within the phosphor’s mass, mainly using avail- able Monte Carlo simulation packages. 33–35 Only a few Monte Carlo studies have investigated or have taken into consideration the effects of phosphor screen light transport properties. Morlotti 36 has employed Monte Carlo methods to evaluate the emission efficiency and the modulation transfer function of phosphor screens by taking into consideration light propagation effects. Radcliffe et al. 37 and Kausch et al. 38 have studied light transport in phosphor screens for portal imaging applications. They have used a Monte Carlo code, based on Fresnell reflection at phosphor grain bound- aries. Most recently, Badano et al. 39 have used light attenu- ation scattering and absorption coefficients to study, via the DETECT II Monte Carlo simulation code, the signal and noise transfer and the so-called Lubberts effect in columnar and granular screens. In all these studies, light propagation has been examined using optical parameters determined ei- ther by empirical methods or by fitting to experimental 4502 4502 Med. Phys. 33 „12…, December 2006 0094-2405/2006/33„12…/4502/13/$23.00 © 2006 Am. Assoc. Phys. Med.