In this study, we characterized the image quality of two types of indirect-conversion flat-panel detectors: an X-ray
incident-side photo-detection system (IS) and an X-ray penetration-side photo-detection system (PS). These detectors
consist of a Gd2O2S:Tb (GOS) scintillator coupled with a photodiode thin film transistor (PD-TFT) array on a glass
substrate. The detectors have different X-ray incident directions, glass substrates, and scintillators. We also characterized
the effects of layered scintillator structures on the image quality by using a single-layered scintillator containing large
phosphor grains and a double-layered scintillator consisting of a layer of large phosphor grains and a layer of small
phosphor grains. The IS system consistently demonstrated a higher MTF than the PS system for a scintillator of the same
thickness. Moreover, the IS system showed a higher DQE than the PS system when a thick scintillator was used. While
the double-layered scintillators were useful for improving the MTF in both systems, a thick single-layered scintillator
was preferable for obtaining a high DQE when the IS system was applied. These results indicate that an IS system can
efficiently utilize the light emitted from the phosphor at the far side of the PD without the occurrence of blurring. The
use of IS systems makes it possible to increase the thickness of the scintillator layer for improving the sensitivity without
reducing the MTF, which increases the DQE. The DQE of the IS system was 1.2 times that of the PS system, despite the
absorption of X-rays at the glass substrate before entering the phosphor.
For Computed Radiography (CR) systems that use a columnar phosphor plate (CPP) and a powder phosphor plate (PPP), we designed the systems to obtain the best image quality. To determine the optimum phosphor layer thickness for each phosphor plate, the relationship between the intensity and spatial spread of photo-stimulated luminescence (PSL), and the phosphor layer thickness of the phosphor plate is quantitatively clarified. Next, to determine the stimulation light intensity, we measured PSL, modulation transfer function (MTF) and detective quantum efficiency (DQE) by varying the stimulation light intensity, using the determined optimum phosphor layer thickness. We also investigated the noise components of each phosphor plate. Results show that, compared to the PPP, the CPP is more favorable in allowing thicker phosphor layer without reduction in MTF. As the result of the relationship between the layer thickness and the PSL, noise analysis, it was confirmed that the CPP could detect PSL in the deep region of the phosphor layer without reducing the intensity of PSL. This suggests that in comparison to the PPP, the CPP can make efficient use of X-ray information, thereby promising to enhance image quality and to reduce exposure dose.
In X-ray-to-light conversion digital radiography, we compared the image quality of a system in which photodetection is done from the X-ray incident surface (hereafter referred to as a front exposure system) and a system in which photodetection is done from the back side opposite the X-ray incident surface (hereafter referred to as a back exposure system). Modulation transfer function (MTF) and detective quantum efficiency (DQE) measurements were performed using the method IEC prescribes. Both MTF and DQE were higher with the front exposure system than with the back exposure system, with the former delivering better image quality. This difference can be accounted for by differences in the distribution of absorbed X-ray doses in the phosphor layer, the readout efficiency, which varies as a function of depth in the phosphor layer, and depth-dependent blurs of light. Furthermore, we determined changes in image quality incurred by varying the quality of X-rays, the thickness of the phosphor layer and the crystal structure of phosphors. The advantage of the front exposure system becomes more pronounced with decreasing X-ray tube voltage, increasing phosphor layer thickness, and the use of phosphors in powder form.
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