It is thought that noise property and the shape of H-D curve of AD system (HGM/URI) are more suited for chest radiographs compared with conventional system (HR4/super HRS). Therefore, we studied the visual detectability of low contrast simulated nodules using chest radiographs taken by AD and HR4/super HRS systems. Film base was used as a simulated nodule. Fifty-six radiographs, with and without a nodule each, were employed for ovserber performance test. Reciever operating characteristic analysis (the method for continuously-distributed test results) was used for observer performance data. As a result, the total detectability of AD system was slightly superior to that of HR4/super HRS system. In the mediastinum area, however, AD system was greatly superior compared to HR4/super HRS system, and in the lung field both were almost equal.
We studied the characteristics of a new screen/film system (AD system : HGM/UR-1, Fuji) for chest radiography, that is, H-D curve, relative speed, cross-over effect, resolution and noise properties, X-ray attenuation rate and relative fluorescence. The shape of characteristic curve of UR-1 film is suitable for chest radiography, because the density of UR-1 is higher than that of super HRS film (sHRS) in low exposure range. However, the film gradient of UR-1 is a little lower compared to sHRS. The visibility of mediastnum area and lungs by AD system is superior and or equivalent, compared with conventional HR4 screen/sHRS film system. Relative speed of UR-1 film was approximately 50% sHRS. However, the speed of AD system was equivalent to HR4/s HRS system. Cross-over ratio of UR-1 film was 30% lower than that of sHRS film. X-ray attenuation rate and relative fluorescence of HGM screen were 15% and 120% higher than these of HR4, respectively. Modulation transfer function of AD system and HR4/sHRS system were equivalent. Wiener Spectral value of AD system was 50% lower than that of HR4/sHRS system in low spatial frequency, and 20% lower in high spatial frequency.
Field factor (F_A) for a small field is able to be obtained by film dosimetry with conception of monitor unit (MU) which provides constant density. Close agreement between F_A measured by film dosimetry and F_A measured by the ionization chamber and the semiconductor detector, could be obtained. In this method, F_A is able to be measured without converting film density into an irradiated dose. This method could also be applied to the measurement of F_A for infinitesimally small irradiated fields.
The methods for determining the margin of a radiation field using the opposed two fields irradiation technique are evaluated. Conventionally, the prescribed distance of the margin is determined by only one side projection of the two ports (method A). There is another method in which the gantry is rotated to the other side and the margins are determined for both sides projection (method B). Differences in the irradiated volumes are analyzed using a dose-volume histogram between method A and B. In the target, irradiated volumes of the 100% dose range are not significantly different for either method. In the phantom, however, irradiated volumes of each dose range differ according to the location of target by method A. In contrast, those in method B are not changed. It is considered that taking the exact distance from the target as a margin is theoretically acceptable for determining the size of the field.