Evaluation of powder/granular Gd2O2S:Pr scintillator screens in single photon counting mode under 140 keV excitation

Abstract

The aim of this paper is the evaluation of an alternative, low cost solution for the gamma detector in planar imaging. It is based on a powder scintillator, well established in X-ray imaging, and could be further exploited in simultaneous bimodal imaging systems. For this purpose, we have examined the performance of Gd2O2S:Pr powder scintillator, in the form of thick granular screens easily produced in the laboratory by commercially available Gd2O2S:Pr powder. The screen was coupled to a round position sensitive photomultiplier tube (R3292 PSPMT). The system's evaluation was performed in photon counting mode under Tc-99m excitation. In all measurements, a general purpose hexagonal parallel collimator was used. Different samples of screens with coating thickness varying from 0.1 g/cm(2) to 1.2 g/cm(2) were tested. The 0.6 g/cm(2) screen, corresponding to similar to 2mm actual thickness, was found most efficient under 140 keV irradiation. The system's performance with the proposed screen is reported with the modulation transfer function. Moreover sensitivity, spatial and energy resolution as well as the uniformity response using phantoms were measured. The performance of the proposed screen was compared with two CsI:Tl pixellated crystal arrays with 2x2x3 mm(3) and 3x3x5 mm(3)pixel size. A spatial resolution, of 3 mm FWHM, for a Tc-99m line source, was achieved at zero source to collimator distance. In addition, the Gd2O2S:Pr screen showed a slower degradation of the spatial resolution with increasing source to collimator distance e.g at 20 cm, the Gd2O2S:Pr screen showed aq spatial resolution of 8.4 mm while the spatial resolution of the pixellated crystals was 15 mm. Taking into account its easy production, its flexibility due to powder form, the very low cost and the good spatial resolution properties of the proposed alternative detector, powder scintillators could potentially be used for the construction of flexible detector geometries, such as ring type or gamma probes or as a low cost detector solution in educational photon counting imaging applications, complementary to standard X-ray imaging.