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A photographic technique used to localize a radioactive substance within a solid specimen; also known as radioautography.
A photographic emulsion is placed in contact with the object to be tested and is left for several hours, days, or weeks, depending on the suspected concentration of the radioactive material to be measured. The emulsion, which is a gel containing silver halide, is then developed, fixed, and washed as in the usual photographic process. At sites where the emulsion was close enough to the radioactive substance, it appears dark because of the presence of silver grains. When the number of grains is insufficient to darken the film to the unaided eye, the film may be examined with the aid of a microscope. The individual silver grains may then be seen. The pattern formed by the grains depends on the type of radiation and the nature of the photographic emulsion. Alpha particles produce short, straight rows or tracks of grains. Beta particles as well as x-rays and gamma rays, which affect film by producing beta particles, produce tortuous tracks whose lengths and grain densities depend on the energy of the beta particles. Low-energy particles produce shorter tracks with higher grain densities. Very low energy particles like those from tritium (3-hydrogen) may produce only a single grain very close to the site of decay.
Autoradiography can be used to detect, and measure semiquantitatively, the radioactive materials in almost any object that can be placed in contact with film or photographic emulsion in some form. However, in biological research the object may be (1) a whole plant or animal that can be flattened against a film; (2) the cut surface of a plant or animal, or one of its organs; (3) thin sections of tissues or cells; (4) squashed or otherwise flattened cells; (5) surface films produced by spreading on water the protein monolayers containing DNA or ribonucleic acid (RNA) that are picked up on grids for electron microscopy; (6) sheets of paper or other materials on which radioactive substances have been separated by chromatography or electrophoresis; or (7) acrylamide gels in which DNA, RNA, or proteins have been separated by electrophoresis.
radioautography, a method used to study the distribution of radioactive substances in a given object. The method consists in covering the object with a photographic emulsion that is sensitive to radioactive emissions. The radioactive substances in the object are in a sense photographing themselves—hence, the name. Autoradiography is widely used in physics, technology, biology, and medicine—in all cases where isotopic indicators are used.
After the photographic emulsion is developed and fixed, a representation of the distribution under investigation appears on it. Several methods exist for applying the photographic emulsion to the object. The photographic plate can be applied directly to the polished surface of the sample, or a warm liquid emulsion can be applied. The cooled emulsion forms a layer which adheres firmly to the sample and is examined after exposure and development. The distribution of the radioactive substances is studied by comparing the intensity of blackening on the film with that of the observed and standard samples (so-called macroradiography). The second method consists in counting the tracks formed by ionizing particles in the photographic emulsion. This is done by means of optical or electron microscopes (microradiography). The second method is much more accurate than the first. Slide and X-ray emulsions are used to obtain macroautographs; to obtain microautographs, special fine-grained emulsions are used. The photographic image of the distribution of radioactive substances in the observed object obtained through autoradiography is called an autoradiogram or radioautograph.
Autoradiography can help establish the presence of radioactive elements in various ores, the distribution of natural radioactive elements in tissues of plant and animal organisms, and so on. The introduction into an organism of compounds labeled by radioisotopes—and the further observation of tissues and cells using autoradiography— makes it possible to gather accurate data describing in which cells or cell structures certain processes take place or certain substances are localized. This method also helps establish time parameters for various processes. Thus, for example, the application of radioactive phosphorus and autoradiography made it possible to observe very intensive metabolism in growing bone, the application of radioactive iodine and autoradiography permitted the ascertainment of the laws governing the activity of the thyroid gland, and the introduction of labeled compounds (the predecessors of proteins and nucleic acids) and autoradiography helped to explain the role these vitally important compounds play in the transformation of certain cell structures. Autoradiography not only permits the definition of the radioisotope’s localization in a biological object but also helps to find the quantity of the radioisotope, since the number of reduced silver grains in the emulsion is proportional to the number of particles reacting with the emulsion. Quantitative analysis of macroautographs is done with instruments usually used in photometry, while microautographs are analyzed by counting, under a microscope, the silver grains or the tracks which appear in the emulsion under the effect of the ionizing particles.
Autoradiography is beginning to be successfully combined with electron microscopy.
REFERENCESBoyd, D. A. Avtoradiografiia v biologii i meditsine. Moscow, 1957. (Translated from English.)
Zhinkin, L. N. “Primenenie radioaktivnykh izotopov v gistologii.” In Radioaktivnye indikatory v gistologii. Leningrad, 1959. Pages 5–33.
Perry, R. “Quantitative Autoradiography.” Methods in Cell Physiology, 1964, vol. 1. Ch. 15, pp. 305–26.
N. G. KHRUSHCHOV