Modeling granular phosphor screens by Monte Carlo methods

Abstract

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 studies 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 Gd2O2S: 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 Gd2O2S: Tb screens of high packing density and small grain size may exhibit considerably better resolution and light emission properties than the conventional Gd2O2S: Tb screens, under similar conditions (x-ray incident energy, screen thickness).

Optical photon transport in powdered‐phosphor scintillators. Part 1. Multiple‐scattering and validity of the Boltzmann transport equation

“…The use of the BTE in a medium sparsely filled with scattering objects is 55 well-motivated. The validity of its use in a more densely-packed medium is less clear 21 and particularly relevant because of the typical packing factors in real screens 13 and some researchers modeling of very high packing factors. [13][14][15][16] In Part 1 of this, predictions of an explicit microscopic model (EMM) based on geometrical optics are compared to those of both the BTE and a model derived from Swank's well-known diffusion equation solution.…”

“…The validity of its use in a more densely-packed medium is less clear 21 and particularly relevant because of the typical packing factors in real screens 13 and some researchers modeling of very high packing factors. [13][14][15][16] In Part 1 of this, predictions of an explicit microscopic model (EMM) based on geometrical optics are compared to those of both the BTE and a model derived from Swank's well-known diffusion equation solution. 5 A relatively abstract approach is 60 taken: no attempt is made to model a particular real phosphor screen, but rather a range of ideal examples.…”

“…An attractive alternative is to treat a phosphor screen as an effective homogenous medium with associated scattering and absorption lengths. There is a long history of similar theoretical reductions 18 and the Boltzmann transport equation 6,9,[12][13][14][15][16] (BTE) or diffusion equation 5,7,17 may be used to treat the multiple-scattering problem. The use of the BTE in a medium sparsely filled with scattering objects is 55 well-motivated.…”

Investigation of two heavy element scintillators by Monte-Carlo methods

“…In the present work, a custom Monte Carlo simulation code was utilized [5][6][7]. The latter requires as input data, the x-ray energy or spectrum, the phosphor material's chemical composition and some standard reference tabulated data related to the phosphor material's physical parameters [8][9][10][11].…”

“…In the present study, a previously developed (in MATLAB) custom validated Monte Carlo simulation program [5][6][7] has been extended, in order to investigate the K-characteristic effect (in terms of absolute or relative values and in terms of spatial distribution) on scintillators with two K-absorption edges.…”