The saturable receptor-ligand model includes two tissue compartments. C1 represents the non-displaceable tissue compartment with free and non-specifically bound tracer, and C2 tracer bound to the target receptors. The model accounts for a saturation of receptor sites due to low specific activity Sact of the injected tracer. In this situation, k3 is given by kon(Bmax-C2(t)/Sact), and is therefore time-dependent. Bmax is the total receptor concentration, kon the bi-molecular association rate, and k4 the dissociation rate constant koff.
Differential Equation of the Mass Balance
with input curve Cp(t).
Operational Model Curve
with the concentration CB(t) of tracer in whole blood, and the blood volume fraction vB.
The model includes the 6 fitable parameters vB, K1, k2, k3on, k4, Bmax. Due to the high number of parameters, it is difficult to use the model for actual fitting. However, it may be valuable for generating synthetic data and assessing the effect of model simplification.
Alternative Model with K1/k2 Fit Parameter
There is an alternative 2-Tissue Compartment Model, K1/k2, Bmax model available which has the same structure but uses K1/k2 as a fitting parameter instead of k2. K1/k2 represents the distribution volume VND of the non-displaceable compartment C1. The advantage of this formulation is that in some situations VND can be assumed to be common among regions and estimated by a coupled fit. Alternatively, VND might already be known and can thus be fixed at a prior value.