Evaluation Study of Abnormal Detectability with Thurstone and Scheffé (Nakaya) of Paired Comparison Method Using Chest Phantom

Monitors are increasingly being used as diagnostic imaging devices. In this study, using an all-purpose liquid-crystal display (LCD), the rate of detection of abnormalities was investigated using Thurstone's and Scheffé's (Nakaya) paired comparison methods. A chest phantom was prepared as a test sample with acryl and aluminum plates and intensities suggesting small adenocarcinomas. For the acquisition conditions for computed radiography, after setting the baseline at a dose at which the film density of the standard screen-film system at the same as those for the lung, costal bone, and mediastinum, 5 steps of 2-fold serial doses were then set: 1/4, 1/2, 1, 2, and 4. The test sample was observed by 10 students. On the Thurstone scale, detectability decreased with a decrease in the dose in the lung, costal bone, and mediastinum. When the significance of differences between the values at adjacent doses was investigated using the yardstick method, using Scheffé's method revealed a significant difference between the 4- and 2-fold doses and between the 1/2 and 1/4 doses in the pulmonary region. A significant difference was also noted between the baseline and 1/2 doses in the mediastinum. Changes in the order of the scale values may not occur in the intervals in which significant differences were noted using Scheffé's methods.