The scintillation counter as an instrument for in vivo determination of thyroid weight.

Abstract

The scintillation counter—a new name in the rapidly expanding nomenclature of medical science—has been used by other branches of science for only a short time, although the principle was employed by Rutherford and his coworkers at an early stage of their fundamental investigations. It is being applied by physicists to the detection and measurement of cosmic rays, and of alpha particles and beta particles. Now, for the first time, scintillation counting is being developed for the detection and measurement of gamma radiation in an application designed principally for use in clinical medicine. The gamma scintillation counter serves essentially the same function in medicine as the more familiar Geiger Müller tube or counter, namely, the detection of gamma radiation. Just as the Ceiger-Müller counter gets its name from the Geiger-Müller tube employed in its construction, so the scintillation counter gets its name from a property of the synthetic crystal or luminescent phosphor that enters into its construction. These luminescent phosphors, when exposed to different types of radiation, give off visible light and are thus said to scintillate. The phenomenon is a familiar one in medical science, for it is this that makes possible the fluoroscopic examination of patients. X-rays from the hot cathode tube strike the calcium tungstate or cadmium tungstate coating of intensifying screens or the zinccadmium coating of a fluoroscopic screen, and visible light is given off. In the scintillation counter, the same phenomenon occurs but it is applied in a somewhat different manner. As a detector of gamma radiation, the scintillation counter has many advantages over the conventional Geiger-Müller counter in clinical medicine. It is many times more sensitive to gamma radiation, depending on the phosphor used. The calcium tungstate scintillation counter now in use at Wadsworth Veterans Administration Hospital is over one hundred times as sensitive as the end-window (3.5 mg./sq. cm.) Geiger-Müller tube. By virtue of this increased sensitivity to gamma radiation, it has become possible, in the study of thyroid function with radioiodine, to lower the tracer dose of 8-day I131 to harmless levels, such that the thyroid gland is exposed to no more radiation than is received by the skin from a routine survey chest x-ray examination and only onetwenty-eighth that from a routine gastrointestinal series. A second advantage offered by the increased sensitivity of the scintillation counter is that, with specially designed equipment, it is now possible to determine the weight of the thyroid gland in vivo, which is essential for the proper calculation of the dose of radioiodine to be used in the treatment of thyrotoxicosis.