A study of optimum radiographic conditions on chest examinations with computed radiology (CR) in conjunction with the offset balance of patient dose and image quality]

In a report of a nationwide survey on radiographic conditions of chest radiography in Japan, it was pointed out that the average entrance surface dose (ESD) of the computed radiography (CR) system was higher than that of the film-screen system. It seemed important that an objective index and criteria be established for dose reduction without a loss of image quality that would interfere with diagnostic observation. For this purpose, we investigated the properties of root of mean square (RMS) granularity, since it is a dominant factor in CR image quality and strongly depends on dose. The results indicated that RMS granularity showed little dependence on tube voltage when relative exposure was kept constant and that it decreased with the increment of exposure and approached a finite minimum value in a very high exposure region. For the most frequently used radiographic conditions in Japan (120 kV, 2.5 mmAl, 200 cm SID, 10: 1 grid), the decrement in RMS granularity from 6 to 16 mAs was 0.0276 to 0.0253 (9.1%). This finding suggested that exposure exceeding 6 mAs did not improve image quality, i.e., exposure reduction down to 6 mAs would not cause a significant loss of image quality. It was therefore concluded that RMS granularity was a useful objective index by which to determine the upper limit of exposure. Use of the most frequent conditions with 6 mAs seemed to be recommendable as an initial condition for the technical optimization of CR chest radiography, since ESD under this condition was 0.265 mGy, which was approximately equal to the value of the ESD distribution of a total chest radiogram in Japan.