Lubberts effect in columnar phosphors

Abstract

Noise transfer in granular x-ray imaging phosphor screens is not proportional to the square of the magnitude of the signal transfer when the transfer properties are considered for the entire screen thickness, unless appropriately weighted at each depth of interaction. This property, known as the Lubberts effect, has not yet been studied in columnar structured screens because of a lack of a generalized description of the depth-dependent light transport. In this paper, we investigate the signal and noise transfer characteristics of columnar phosphors used in digital mammography detectors using DETECT-II, an optical Monte Carlo light transport simulation code. We first validate our choice of optical parameters for the description of granular and columnar screens using published normalized modulation transfer (MTF) experimental data. Our calculations of MTF match empirically measured MTFs for a granular film/screen analog system, and for an indirect x-ray digital imaging system with CsI:Tl screen representative of digital mammography systems. Using the depth-dependent spread functions and collection efficiencies, we calculate the signal and noise transfer functions and the Lubberts fraction, which is the ratio of the signal transfer function to the noise transfer function, for different screen thicknesses of granular and columnar phosphors. We find that the Lubberts fraction of a 85 microm granular screen model corresponding to a Gd2O2S:Tb screen is similar to the fraction for a 100 microm columnar CsI:Tl screen.

Analysis of the imaging performance in indirect digital mammography detectors by linear systems and signal detection models

“…Physical and scintillating properties of granular Gd 2 O 2 S:Tb and columnar CsI:Tl phosphor materials: (i) material's density, r [9], (ii) packing density,r p [14,54], (iii) intrinsic efficiency, Z c [14,15] and (iv) optical parameters s, t, r 0 , r 1 (present study). physical wide sense stationary (WSS) process that governs the transfer of signal from the input to the output [15,29].…”

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

“…We assume w ¼ 17 [27] and 3.6 eV [28] for the Gd 2 O 2 S:Tb phosphor and the Si photodiode, respectively. For the purposes of the calculations, it is assumed the optical collection efficiency is h scn ¼ 0.1 and charge collection from direct interactions is h pd ¼ 1 for both front and rear detectors.…”

Signal and Noise Characteristics Induced by Unattenuated X Rays from a Scintillator in Indirect-Conversion CMOS Photodiode Array Detectors

“…Mean free paths of absorption and scattering were taken to be 1 and cm, respectively [4]. The side surface of a phosphor screen was defined as a metal surface with a reflection coefficient of zero, which implies that the optical photon escaping through the side surface never comes back and its simulation is terminated.…”

“…In thick phosphors, the Lubberts' effect suggests that the NPS may fall off with frequency more slowly than MTF because of depth-dependent interaction of x-rays and light collection [3]. Badano et al [4] demonstrated the Lubberts' effect in a phosphor screen by estimating the Lubberts fraction, , based on a Monte Carlo simulation. However, our own studies using a phosphor screen having a thickness similar to that of Badano showed a greater deviation from the simple MTF model than predicted by the Lubberts' effect, suggesting another mechanism is causing NPS degradation.…”