In addition to uptake and washout the model handles two exchanging compartments C_{1} and C_{2} in tissue, as well as an efflux from C_{2}.

Differential Equation of the Mass Balance

with input curve C_{p}(t), concentration C_{1}(t) in the first and concentration C_{2}(t) in the second compartment. 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 typical interpretation is that C_{1} represents free and non-specifically bound tracer in tissue (non-displaceable compartment), and C_{2} represents specifically bound tracer.

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 6 fitable parameters **vB**, **K1**, **k2**, **k3**, **k4**, **k5**. Usually, vB is fixed at a physiologic value of 3-5% to reduce the number of fitted parameters. Per default, **k4**=0 is fixed to model an efflux from the specific compartment rather than an exchange with the non-displaceable compartment.