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FIGURE 1. Three-compartment model used to fit SPECT data. This model contains 2 components for labeled and unlabeled ligands with individual parameters for IMZ and FMZ. Compartments Cp*(t) and Cp(t) represent unchanged plasma 123I-IMZ and FMZ, respectively; Mf+ns(t) represents free and nonspecifically bound ligand, and Ms(t) represents specifically bound ligand. Parameters K1 and k'2 are associated with exchanges between plasma and free/nonspecifically bound ligand compartment. B'max represents concentration of receptors available for binding and is defined as the common parameter of the model; kon and koff are association and dissociation rate constants, respectively; and VR is volume of the reaction, which accounts for tissue inhomogeneity (26). Consequently, only the apparent equilibrium dissociation constant, KdVR, can be estimated. Kd is equilibrium dissociation constant, defined as the ratio of koff to kon. Parameter FV represents the fraction of blood present in tissue volume and was fixed at 0.04 (13,27,28).
