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FIGURE 1. The 3- and 4-compartment model. The 4 compartments correspond to the radioactivity concentration of unchanged radioligand in plasma (CP), unbound radioligand in brain (CF), nonspecifically bound radioligand in brain (CNS), and radioligand specifically bound to receptors (CB). With the assumption that CF and CNS rapidly coalesce, they can be substituted with one compartment representing nondisplaceable radioligand (CN, dotted line) (A). In the 2-compartment model, CB and CN are substituted by one tissue compartment (CT) (B).
