A simplified three-compartment model that is more practically implemented for PET and SPECT experiments: Ca is the total tracer concentration in arterial plasma; C2 is the intracerebral tracer concentration, including free and nonspecifically bound tracer; C3 is the concentration of receptor-bound tracer; K1 is the rate of extraction of tracer from the plasma into the brain; k2–k4 are fractional rate constants governing tracer exchange between compartments. The current model describes the behavior of a reversible ligand. Under conditions of ligand irreversibility, k4, the dissociation rate constant, is considered negligible and is set equal to zero.