Fig. 1.

A schematic diagram of the compartment model used to fit fluorescent tracer kinetics. The top targeted tracer reaches three compartments with four kinetic rate parameters, while the lower nontargeted tracer is chosen to have matched $K_1$ and $k_2$ parameters, but does not bind and, therefore, only requires two compartments. By combining the known values, the nontargeted tracer provides a reference, which can allow estimation of the $k_3$ and $k_4$ parameters for the targeted tracer. The ratio $k_3/k_4$ is known as the binding potential (BP) for that tracer to the receptor.