This model was developed for the analysis of renal PET data . It consists of two parallel tissue compartments, C1 representing non-specific uptake, and C2 specific binding.
Differential Equation of the Mass Balance
with input curve Cp(t), concentration C1(t) in the first and concentration C2(t) in the second compartment. K1 [ml/ccm/min] and k2 [1/min] are the uptake and clearance rate constants of C1(t), K3 [ml/ccm/min] and k4 [1/min] of C2(t) respectively. In the analysis of Gulaldi et al  C1 represents non-specifically bound tracer in tissue and C2 represents specifically bound tracer.
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, K3, k4. Usually, vB is fixed at a physiologic value to reduce the number of fitted parameters. In  vB was fixed at 0.15, which is used as the parameter default.
1.Gulaldi NC, Xia J, Feng T, Hong K, Mathews WB, Ruben D, Kamel IR, Tsui BM, Szabo Z: Modeling of the renal kinetics of the AT1 receptor specific PET radioligand [11C]KR31173. Biomed Res Int 2013, 2013:835859. DOI