The imaging performance of compact powdered phosphor screens: Monte Carlo simulation for applications in mammography

Abstract

In medical mammographic imaging systems, one type of detector configuration, often referred to as indirect detectors, is based on a scintillator layer (phosphor screen) that converts the x-ray radiation into optical signal. The indirect detector performance may be optimized either by improving the structural parameters of the screen or by employing new phosphor materials with improved physical characteristics (e.g., x-ray absorption efficiency, intrinsic conversion efficiency, emitted light spectrum). Lu2O3:Eu is a relatively new phosphor material that exhibits improved scintillating properties indicating a promising material for mammographic applications. In this article, a custom validated Monte Carlo program was used in order to examine the performance of compact Lu2O3:Eu powdered phosphor screens under diagnostic mammography conditions (x-ray spectra: 28 kV Mo, 0.030 mm Mo and 32 kV W, 0.050 mm Rh). Lu2O3:Eu screens of coating weight in the range between 20 and 40 mg/cm2 were examined. The Monte Carlo code was based on a model using Mie-scattering theory for the description of light propagation within the phosphor. The overall performance of Lu2O3:Eu powdered phosphor screens was investigated in terms of the (i) quantum detection efficiency, (ii) luminescence efficiency, (iii) compatibility with optical sensors, (iv) modulation transfer function, (v) the Swank factor, and (vi) zero-frequency detective quantum efficiency. Results were compared to the traditional rare-earth Gd2O2S:Tb phosphor material. The increased packing density and therefore the light extinction properties of Lu2O3:Eu phosphor were found to improve the x-ray absorption (approximately up to 21% and 16% at 40 mg/cm2 for Mo and W x-ray spectra, respectively), the spatial resolution (approximately 2.6 and 2.4 cycles/mm at 40 mg/cm2 for Mo and W x-ray spectra, respectively), as well as the zero-frequency detective quantum efficiency (approximately up to 8% and 18% at 20 mg/cm2 for Mo and W x-ray spectra, respectively) of the screens in comparison to the Gd2O2S: Tb screens. Data obtained by the simulations indicate that certain optical properties of Lu2O3:Eu make this material a promising phosphor which, under appropriate conditions, could be considered for use in x-ray mammography imagers.