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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