A traditional limitation with radiation therapy portal images is low image contrast, due in part to the low attenuation of the exposing radiation by the tissues being imaged, and the contrast capabilities of the image receptor. We have developed, and have clinically evaluated, a cassette-screen-film combination for portal localization imaging, which features a copper front screen plus Gd2O2S:Tb fluorescent screens and a slow-speed, fine grain, film emulsion with inherently high contrast coated on both sides of a 7 mil Estar base. The film can be processed in a conventional rapid-process film processor. Sensitometric data indicate that the film contrast (average gradient) for the new combination is approximately 3.5 times higher than the conventional portal localization systems in current use. The new combination has been clinically compared with two conventional systems. The required monitor unit settings were found to be similar. Initial clinical results indicate portal images made with the new combination are superior to those obtained with the conventional combinations. The images have much higher contrast, subjective impressions of lower noise, show clearer definition of structures, and are much easier to read.

A study of the objective imaging characteristics of a new asymmetric screen-film system is presented herein. The system is characterized by high x-ray absorption asymmetric screens, and a low-noise, high-contrast asymmetric film having near-zero crossover. Comparisons are made with the imaging characteristics of two widely used conventional screen-film systems. Sensitometry, modulation transfer function, and noise power spectra were measured using standard methods. Granularity, noise equivalent quanta, and detective quantum efficiency were computed from these. The new screen-film system has an average gradient at lung-field densities between the two conventional systems studied, while the mediastinum contrast exceeds both conventional systems. The lung-field modulation transfer function half bandwidth of the new asymmetric system exceeds that of both conventional systems by 60%. At mid exposures the detective quantum efficiency of the new asymmetric system is comparable to those of the conventional systems studied. However, the exposure range over which detective quantum efficiency remains high is substantially wider.

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