Estimation of skeletal burden of 'bone-seeking' radionuclides in man from in vivo scintillation measurements of the head

A method for estimating the skeletal burden of various bone-seeking low-energy photon-emitting radionuclides, e.g. 210Pb, 241Am, 238Pu, and 239Pu, has been developed utilizing the technique of in vivo scintillation measurement of photon activity from the skull bones. In this procedure the head (skull), which represents a structure comprising more than 12% of the skeletal mass, is surrounded by three NaI(Tl)CsI(Tl) dual crystal scintillation detectors to maximize counting efficiency and minimize detection variability. In addition to measuring the low-energy photons ('less than' 100 keV) of important radionuclides known to deposit in or on the surfaces of bone, these detectors are employed to simultaneously monitor the higher energy gamma-ray spectra of natural radioactivity such as 40K. From this information it has been possible to optimize the accuracy of background correction, thereby minimizing the detectable levels of a bone-seeking contaminant. In addition, considerations of accurate phantom head construction and the precision of head counting methodology have been investigated. Once the amount of activity deposited in the skull bones has been determined, it is possible to estimate the contribution from the nuclide in the thoracic skeleton to in vivo measurements of activity in the lungs. (author)