Thyroidal radioactivity and scintillation counter response : a computational study of the various geometrical parameters for radio-iodine and technetium isotopes

The response, in terms of count rate, of a scintillation counter when unit radioactivity is uniformly distributed in the organ is calculated. The method of calculation used is valid for the organ which can be spherical or cylindrical in shape, varying in size, for varying distances between the skin surface and the surface of scintillator and for the varying effective depth of the organ. The calculations were done for the three most commonly used isotopes in thyroid studies, namely, 131I, 123I and sup(99m)Tc. Errors that may be introduced in thyroid uptake measurements on account of patient to patient variation are discussed. (M.G.B.)