Fig. 1.

An illustration of the theoretical relationship between the targeted and untargeted reporter uptake in a region of interest and the binding potential (BP: a correlate of receptor density) is shown. The two-tissue compartment model is depicted in (a) and represents the distribution of the targeted reporter in the blood, unbound in the tissue space, and specifically bound in the tissue space. The tracer is modeled to move between the compartments governed by the rate constants $K_1$, $K_2$, $K_3$, and $K_4$. The one-tissue compartment model is depicted in (b) and represents the distribution of the untargeted reporter in the blood and unbound in the tissue space (reference space). Assuming the plasma concentration is negligible and the unbound concentrations of the targeted and untargeted reporters are equivalent, it is possible to show that ratio on the left side of the expression is approximately equivalent to the binding potential (BP), which is directly proportional to receptor availability.