This is a general compartment model for ligands which have recirculating metabolites entering tissue and contributing to tissue activity. Tissue kinetics of both the authentic ligand (C1 and C2) as well as the metabolites (C3 and C4) are described by a 2-tissue compartment model, with a transfer ktrans constant linking the two exchangeable compartments which represents metabolites generated in tissue.
Differential Equation of the Mass Balance
Two input curves are required: the authentic ligand in plasma Cp(t), and the metabolites exchanging with tissue CM(t). The input curve loading menu shows two corresponding entries as soon as the model is selected.
Operational Model Curve
with the concentration CB(t) of tracer in whole blood, and the blood volume fraction vB.
Parameter Fitting
The model includes more parameters than can be reliably fitted at once. Therefore it has been implemented with K1/k2 and K1m/k2m as fitting parameters to allow fixing the distribution volume of the exchangeable compartment, or to use them as common parameters in a coupled fit.