Thyroid uptake measurements: the influence of gland depth, gland mass and lobe separation

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Abstract
Phantom measurements were carried out to define the dependence of standard [human] thyroid uptake measurements on gland depth, gland mass and lobe separation. In the case of 99mTc and 131I, depth can cuase a variation of up to a factor of 2 in the apparent uptake, while in the case of 125I the variation due to depth is a factor of about 4. By contrast, gland mass and lobe separation cause only small errors, of the order of 2 and 10%, respectively, for medium energy isotopes. These errors are particularly important when the uptake value is used to guide prescription of 131I for therapy of thyrotoxicosis.