Determination of spatially distributed iodine thyroidal activity using coincidence counting

The two-probe coincidence technique has recently been discussed by Burns and Peggie (ibid., vol.25, p.445, 1980) for the quantification of ¹²⁵I in the thyroid. Their derivation of the activity in terms of the singles and coincidence count rates, like that of earlier work on ¹²³I coincidence counting (Herman et al., 1974), assumes a point source. The error introduced when the source is distributed within a finite volume has been studied experimentally for a few geometric configurations for ¹²⁵I by Burns and Peggie (1980) and for ¹²³I by Mpanias et al (1976). A more general derivation is presented which demonstrates theoretically that the coincidence method is applicable to certain classes of distributed sources. This derivation suggests that a probe configuration different from the one used by previous authors will lead to improved accuracy for thyroid measurements. Measurements made in the present authors' laboratory using ¹²³I confirm this prediction. The discussion is given in terms of ¹²⁵I, but it would be essentially the same for ¹²³I, ¹¹¹In, etc.