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} instead of k_{2}

The **2-Tissue Compartment Model, K1/k2 **model variant uses **K1/k2** as a fitting parameter instead of **k2**. K_{1}/k_{2} represents the distribution volume V_{ND} of the non-displaceable compartment C_{1}. The advantage of this formulation is that in some situations V_{ND} can be assumed to be common among regions and estimated by a coupled fit. Alternatively, V_{ND} might already be known and can thus be fixed at a prior value. The other four fitting parameters **vB**, **K1**, **k3**, **k4** are the same as for the **2-Tissue Compartment Model**.