If a labeled metabolite of the tracer enters tissue, the additional signal has to be accounted for in the model. The current model introduced by Fujita et al. [1] includes a second input curve C_{M}(t) of a metabolite entering tissue and undergoing non-specific binding. C_{1} represents the non-displaceable compartment of the authentic ligand, C_{2} the specific binding of interest, and C_{3} metabolized ligand in tissue.

Differential Equation of the Mass Balance

with the input curves of authentic ligand and metabolite, C_{p}(t) and C_{M}(t), respectively. The input curve loading menu shows two corresponding entries as soon as the model is selected.

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 VS

The **3-Tissue Compartments with Metabolites, K1/k2, Vs **model variant uses the parameter ratios **K1p/k2p**, **K1m/k2m**, **K1p/K1m**, and **Vs**=K_{1p}/k_{2p}*k_{3}/k_{4} as fitting parameters instead of k_{2p}, K_{1m} , k_{2m}, and k_{4}. The advantage of this formulation is that the ratios may be fixed at known or assumed values, or estimated in a coupled fit. The other three fitting parameters **vB**, **K1p **and **k3 **are the same as for the **3-Tissue Compartments with Metabolites** model.

Reference

1. Fujita M, Seibyl JP, Verhoeff NP, Ichise M, Baldwin RM, Zoghbi SS, Burger C, Staley JK, Rajeevan N, Charney DS et al: Kinetic and equilibrium analyses of [(123)I]epidepride binding to striatal and extrastriatal dopamine D(2) receptors. Synapse 1999, 34(4):290-304.