A new method of calculating absolute thyroid activity in intravenous I-123 uptake tests

A technique for determining the absolute activity of I-123 in early thyroidal uptake tests is presented. The method is independent of geometry, tissue attenuation, an extrathyroidal neck activity (ENA). It utilizes the efficiency factors (ExT and E gamma T) derivable for the thyroid gland using coincidence counting techniques, and a quantity, alpha, characteristics of the vascularity of the individual patient. Although ENA is not determined in absolute terms (e.g., percent of injected dose), count rates due to ENA are determined. After subtracting the ENA from the total count rates thyroidal uptake can be determined as absolute activity by means of the efficiency factors. The validity and usefulness of the technique have been demonstrated in phantom and patient studies. Thyroid uptakes of less than 0.1% of injected dose can be measured accurately in our patient studies.