A digital portal imaging (DPI) system for megavoltage photon beams was installed recently in our department. The purpose of this study is to evaluate the image quality of this system. We have analyzed the following properties of the system; relationship between measured dose-rate and pixel values of the DPI, spatial resolution, detectability of low-contrast objects and setup errors. The results were compared with those of conventional screen-film systems. As a result, the relationship between the measured dose-rate and the pixel value of the DPI was found to be linear in the dose-rate range between 100 and 400cGy/min. Spatial resolution was 1.25 and 0.5mm for the DPI and the screen-film systems, respectively. The slope of the contrast-detail curves differed between the DPI and the screen-film systems. The contrast thresholds were 0.6 and 0.3% for the DPI and the screen-film systems, respectively. The detectability of a setup error of 1mm and 2mm for the DPI was lower than that by the screen-film systems, although the difference was not very significant. In conclusion, the image quality of the DPI at present time is slightly inferior to the conventional screen-film systems. However, notable advantages of the DPI system are that any positional changes in patients during irradiation can be detected very quickly, and that quantitative analysis of the setup variation can be obtained. The image quality of the DPI will be improved as the technology regarding advances. Therefore, this verification system using the DPI device, is expected to be used for clinical radiation therapy in the future.
The detection performance of human observers has been used widely in the evaluation of imaging systems. In observer performance experiments, the detectability is affected by various factors such as object size, viewing distance, illumination, selected observer, order of reading samples and so forth. For the viewing distance, there are two situations where firstly observers are allowed to choose a suitable viewing distance and secondly they are not allowed to do so and must use a fixed viewing distance. The purpose of this paper is to assess whether the detectability of observer performance experiments is dependent on the viewing distance. The contrast-detail (C-D) diagram described the relationship between the threshold contrast in visually detectable by a human observers and viewing distance (20-200cm) using four different radiographic systems. Our results indicated that detectability by detection performance increased when observers adopted a short viewing distance for small objects and long viewing distance for large objects. It is possible that the selected viewing distance changes the evaluation of screen/film systems. Therefore, it is better that the designer of the experiment does not institute the fixed viewing distance.