Radioactive Microphotographs for Resolution Testing in Autoradiography

Radioactive resolution test charts were prepared by the chemical toning of the silver in minute photographic images. Conversion of the silver successively to AgBr and then to AgI could be effected with little loss of sharpness; the latter conversion was made with I131. As the silver images are confined to the emulsion surface and have a known distribution of radioactive material, examination of autoradiographs prepared from them permits a systematic experimental examination to be made of the factors influencing the resolving power of this technique. Tests on the influence of the type of the autoradiographic emulsion and its coating thickness are described. Moreover, the test charts can be supercoated with a gelatin layer of known thickness and then indicate how resolution is likely to suffer when the radioactivity is located below the surface of the histological specimen. Even a separation of 3μ produced a serious reduction in sharpness, while one of 10μ caused gross obliteration of fine detail.