It has been long recognized that radiation arising from selectively deposited radioelements may be used to record the spatial distribution of these elements so as to reflect the configuration and position of an organ structure. Although it had been shown previously that some concept of the general deposition of radioelements in the body could be established by making static counts of the accumulated radioactivity along a grid system, the later development of the automatic scintillation scanner (1, 2, 6) demonstrated the possibilities of graphic illustration of radioisotope distribution. In its original concept, the automatic scanner was designed to delineate the regions of the thyroid gland or of thyroid-functioning sites by virtue of the avidity of thyroid tissue for radioactive iodine. Thus the problem was concerned mainly with delineating structures in which a steep differential in radioactivity existed between the region in question and its environment. In the application of scanning systems to the visualization of large organs, however, several problems arose which were not encountered in the original thyroid-scanning procedures. Since the delineation of any specific organ is dependent upon obtaining a differential uptake of some radioisotope, scanning technics have in general been concerned with maximizing this differential. In the problem of visualization of the thyroid gland by administration of I131, for example, a greater differential may be obtained simply by increasing the dose of radioiodine. When, however, two sites are to be demarcated by some variation in structure (such as difference in vascularity), the differential is inherent in the anatomical configuration alone and cannot be made greater by administration of larger amounts of radioactive material. Similarly, the finding of a defect or absence of radioactivity which is surrounded by tissue with normal uptake is dependent entirely upon the ratio of the normal tissue to the abnormal tissue as subtended by the detector. Wherever visualization depends upon the differentiation of minor variations in the uptake of the radioactive material, we have been primarily concerned with technics that would make readily perceptible small differences in the counting rates of the scanned area. Technics Two systems have been devised to accentuate small differences between the radioisotope uptakes of adjacent areas of a scanned region. The first has involved the introduction of a counting rate controlled cut-off circuit that permits the operator to select arbitrarily a counting rate below which no points are recorded. Any radiation reaching the counter above this predetermined level is recorded at its actual counting rate, while radiation reaching the counter below this level is completely eliminated. Small variations may now assume an infinite ratio of contrast with the surrounding environment, since counting rates below the selected level are not recorded.