The influence of the x-ray detector ofan automatic exposure control unit(AEC)on the image quality of digital chest radiography was evaluated by means of scatter fraction and receiver operating characteristic (ROC)analyses. The x-ray detector of the AEC was located to detect incident x-rays according to three experimental geometric arrangements:geometry A, in which the x-ray detector of the AEC was not used;geometry B, in which the x-ray detector was located in front of the antiscatter grids;and geometry C, in which the x-ray detector was located in the back of the anti-scatter grids. The scatter fraction was measured with metacryl plates of 0, 5, 10, 15, and 20 cm in thickness, and a phantom lung with simulated opacities(nodules)was used for ROC analyses and was exposed with a DR system(sampling distance: 175μm, 2, 048×2, 048 pixels, 12 bit)in geometric arrangements A, B, and C under the same radiographic conditions of exposure. ROC analyses of phantom images with nodular opacities were carried out by 14 radiological technicians, who examined the lung field(high-density area), mediastinum, area below the diaphragm(low-density area), and whole chest. In the low-density area, the scatter fraction of geometric arrangement C was approximately 2.0-3.0% greater, and the detection of simulated opacities was less than with the other geometric arrangements(p<0.05). In the high-density area, however, there were no statistically significant differences according to experimental geometric arrangement. In conclusion, in this study the x-ray detector of the AEC did not influence the image quality of digital chest radiography in the high-density area, but, when it was located at the back of the antiscatter grids, image quality decreased in the low-density area.
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