The 2-tissue compartment model separates tracer in tissue into two exchanging compartments C_{1} and C_{2} with a serial structure. The arterial plasma exchanges with the first tissue compartment C_{1}, which in turn exchanges with the second tissue compartment C_{2}. K_{1} [ml/ccm/min] and k_{2} [1/min] are the uptake and clearance rate constants, whereas k_{3} [1/min] and k_{4} [1/min] describe the exchange between the tissue compartments. The usual interpretation is that C_{1} represents free and non-specifically bound tracer in tissue (non-displaceable compartment), and C_{2} specifically bound tracer.

Differential Equation of the Mass Balance

with input curve C_{p}(t).

Operational Model Curve

with the concentration C_{B}(t) of tracer in whole blood, and the blood volume fraction v_{B}.

Parameter Fitting using K_{1}/k_{2} and Vt

This **2-Tissue Compartment Model, K1/k2, Vt **model variant uses **K1/k2** and **Vt** as fitting parameters instead of k_{2} and k_{4}. V_{t} (=K_{1}/k_{2}*(1+k_{3}/k_{4})) represents the total distribution volume of tracer in tissue. This formulation has particular advantages for the generation of synthetic data with predefined distribution volumes. The other three fitting parameters v_{B}, K_{1}, and k_{4} are the same as for the **2-Tissue Compartment Model**.