A REVISED DOSIMETRIC MODEL OF THE ADULT HEAD AND BRAIN

During the last decade, several new radiopharmaceuticals have been int roduced for brain imaging. The marked differences of these tracers in tissue specificity within the brain and their increasing use for diagn ostic studies support the need for a more anthropomorphic model of the human brain and head. Brain and head models developed in the past hav e comprised only simplistic representations of this anatomic region. M ethods: A new brain model has been developed which includes eight subr egions: the caudate nucleus, the cerebellum, the cerebral cortex, the lateral ventricles, the lentiform nucleus, the thalamus, the third ven tricle and the white matter. This brain model has been included within a slightly modified version of the head model developed by Poston et al, in 1984. The head model, which includes both the thyroid and eyes, was modified in this work to include the cerebrospinal fluid within t he cranial and spinal regions. Results: Absorbed fractions of energy f or photon and electron sources located in thirteen source regions with in the new head model were calculated using the EGS4 Monte Carlo radia tion transport: code for radiations in the energy range 10 keV to 4 Me V. Conclusion: S-values were calculated for five radionuclides used in brain imaging (C-11, O-15, F-18, Tc-99m and I-123) and for three radi onuclides showing selective uptake in the thyroid (Tc-99m, I-123, and I-131). S-values were calculated using 100 discrete energy points in t he beta-emission spectrum of the different radionuclides.