Effect of Scattered X Rays on Image Signal of Radiograph

For the optimization of the image quality of radiographs, the effect of the blur due to the scattered x rays on the image signal of radiographs has been ignored. The purpose of this study is to investigate the effect of the scattered x rays on the object sharpness of radiographs for tube voltages of 50 to 100 kV. For the purpose, using a phantom of the polymethyl methacrylate (PMMA) with an aluminum plate of low contrast object, the edge images including the blurs due to the x-ray focal spot, the screen-film system, and the scattered x rays were produced. However, it is considered that the shape of the overall modulation transfer function (MTF) derived from the edge image is not influenced by the scattered x rays. As a result of comparing the overall MTFs with the product of the MTFs of an x-ray focal spot and a screen-film system measured for various tube voltages in this study, we found that the scattered x rays gave different influence on the contrast at each spatial frequency for each tube voltage. To investigate the influence of the scattered x rays on the overall MTF in detail, considering the MTF of the x-ray focal spot, we developed a method for determining the primary and scatter components of the overall MTF, whose components are proportional to the MTFs of the screen-film system and the scattered x rays, respectively. Using the method, it was found that (1) the overall MTF depends on the MTF of scattered x rays as well as the MTFs of the focal spot and screen-film system; and (2) at frequencies near 0 mm 1, the influence of the blur due to the scattered x rays on the object sharpness was greater than that occurred in the intensifying screen and increased with the tube voltage. These results shows that the blur due to the scattered x rays should be considered for the optimization of the image quality.