Abstract
Recently, more chest radiography are being taken by computed radiography (CR) using the energy subtraction method. It has, however, been found, that the quality of chest CR is affected by noise, and has more coarse graininess than conventional chest radiography. In the present study, chest radiography was observed by the one shot dual energy subtraction method in CR. The chest radiographs were taken with or without a grid (grid-ratios 8: 1, 12: 1, and 16: 1), to compare the effects of noise, and scattered radiation was measured in a water phantom by a fluorescence quality meter. The scatter fraction at 8.7 cm in thickness of the phantom (equal to chest of 20 cm in thickness) was 59% without a grid and tended to decrease with the higher grid ratios. Six chest computed radiographs of a person with chest of 20 cm in thickness were taken with and without grids. In all exposures, the radiation doses reaching the imaging plate (IP) or the skin were kept constantly. Evaluation of these six radiographs was done by utilizing the modified BRH method by 10 observers (5 radiologists and 5 radiological technicians) . The results were as follows: 1) The best evaluation (the highest score) was obtained in the radiograph with a grid ratio of 8: 1, when the exposure dose at the IP and the skin were constant. 2) The second best evaluation was the radiograph without a grid when the skin exposure dose was contant. 3) The third grade evaluation was the radiograph taken with a grid ratio of 16: 1, when the exposure dose at the IP was kept constantly.
