Anisotropic optical distribution of powder phosphor materials applied in medical imaging instrumentation

Abstract

Image quality for medical purposes is related to the useful diagnostic information that can be extracted from an image. The performance of indirect X-ray detectors, which in turn affects the quality of the medical image, can be significantly influenced by the characteristics of the phosphor, employed to convert incident radiation into emitted light. Given the technological and medical importance of phosphor materials, understanding the fundamental effects of optical anisotropy is crucial. The purpose of the present paper was to examine the influence of optical anisotropy in optical diffusion within the powder phosphor-based X-ray detectors. The present investigation was based on Mie scattering theory and Monte Carlo simulation techniques. The variation of the anisotropy factor was examined for: (1) light wavelengths in the range 400–700 nm, (2) particle refractive index between 1.5 and 2 and (3) three regions of particle sizes: nanoscale (from 10 up to 100 nm), submicron scale (from 100 nm up to 1 μm), and microscale (from 1 up to 10 μm). In addition, optical diffusion performance was carried out considering: (a) anisotropy factor values 0.2, 0.5, 0.8 which represent different aspects of light propagation after scattering and (b) phosphors of different layer thickness, 100 (thin layer) and 300 μm (thick layer), respectively. Results showed that the highest variation on the anisotropy factor was observed in the submicron scale, and, in particular, for grain diameters between 100 and 600 nm (increase from 0.1 up to 0.8). In addition, Monte Carlo simulations showed that the spread of light photons decreases (i.e., high spatial resolution) with the decrease in the anisotropy factor. In particular, the FWHM was found to decrease with the anisotropy factor: (1) 11.4 % at 100 μm and 4.2 %, at 300 μm layer thickness, for light extinction coefficient 0.217 μm⁻¹ and (2) 1.9 % at 100 μm and 2.0 %, at 300 μm layer thickness, for light extinction coefficient 3 μm⁻¹. The present work indicated that lateral spreading is affected by the anisotropy factor. However, this effect is more dominant for low values of light extinction coefficient of the material.