Fig. 1.

Schematic diagram of the model that includes a tissue metabolite compartment. BBB represents the blood–brain barrier; P^Parent and P^Met are tracer plasma radioactivity concentrations (Bq ml⁻¹) of parent and labeled metabolites, respectively; T is the radioactivity concentration in tissue, F+NS^Parent that of free and non-specifically bound parent in tissue, S^Parent that of bound parent in tissue; and F+NS^Met that of labeled metabolites in tissue (Bq ml⁻¹); K₁ (ml·cm⁻³·min⁻¹) and k₂ (min⁻¹) are rate constants, describing exchange of parent between plasma and tissue; k₃ and k₄ are rate constants (min⁻¹), describing exchange of parent between free and bound compartments; K_1m (ml·cm⁻³·min⁻¹) and k_2m (min⁻¹) are rate constants, describing exchange of labeled metabolites between plasma and tissue.