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

The model includes the 5 fitable parameters **vB**, **K1**, **k2**, **k3**, **k4**. Usually, v_{B} is fixed at a physiologic value of 3-5% to reduce the number of fitted parameters. An alternative is to fit v_{B} with a TAC from a big representative VOI, and fix it for the TACs from smaller VOIs. For irreversible tracers with trapping in the second compartment, k_{4} is fixed: **k4**=0. Please inspect the **%SE** standard error to get information about the reliability of the parameter estimates.

Alternative Models with VND and VS Fit Parameters

There are two alternative models available which have the same structure but use distribution volumes as fit parameters. The advantage of the reformulation is that in some situations distribution volumes can be assumed to be common among regions and estimated by a coupled fit, or they might already be known and can thus be fixed at a prior value.