THE SCINTILLATION COUNTER IN CLINICAL STUDIES OF HUMAN THYROID PHYSIOLOGY USING I131*

Abstract

A new application of the principle of scintillation counting was applied to clinical medicine for the in vivo detection of gamma radiation in the human being. Two specially designed scintillation counters, each with a different clinical application, were described,using Ca tungstate as the inorganic phosphor and either the RCA 1P21 or 5819 as the photomultiplier tube. No refrigeration of the phototube or coincident circuitry was necessary to obtain a satisfactory noise to pulse ratio. By a system of rectangular coordinates using a directional scintillation counter, so called because of its high spatial resolving characteristics, it was possible very accurately to outline the thyroid gland, following admn. of 100-200 [mu]c of carrier-free I131, thereby offering a means of determining thyroid weight. The directional scintillation counter utilizes the 1P21 photo tube and was found to be 27 times more sensitive to gamma radiation than the conventional end-window Geiger-Mueller tube. The wide angle scintillation counter, so called because of the wide angle of approx. 70-90[degree] that incident quanta can subtend and still be detectable, employs the RCA 5819 photo tube. This wide angle scintillation counter was 94 times more sensitive to gamma radiation than the conventional end-window Geiger-Mueller tube and 56.8% of all incident quanta gave countable scintillations as compared to 1-2% efficiency of end-window Geiger-Mueller tubes. By virtue of this increased sensitivity to gamma radiation it was possible to lower the tracer sosage of I131 to relatively harmless levels when used in the study of thyroid dysfunction. Doses of I131 of the order of 1 to 10[mu]c were used and reliable counting rates over the thyroid of 3 to 7 times background were obtained. When such low tracer doses are used, thyroid tissue receives no more radiation than that received from many of the routine diagnostic X-ray produce.