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## 2-Tissue Compartment Model, K1/k2, Vt |

The 2-tissue compartment model separates tracer in tissue into two exchanging compartments C1 and C2 with a serial structure. The arterial plasma exchanges with the first tissue compartment C1, which in turn exchanges with the second tissue compartment C2. K1 [ml/ccm/min] and k2 [1/min] are the uptake and clearance rate constants, whereas k3 [1/min] and k4 [1/min] describe the exchange between the tissue compartments. The usual interpretation is that C1 represents free and non-specifically bound tracer in tissue (non-displaceable compartment), and C2 specifically bound tracer.

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.

Parameter Fitting using K1/k2 and Vt

This 2-Tissue Compartment Model, K1/k2, Vt model variant uses K1/k2 and Vt as fitting parameters instead of k2 and k4. Vt (=K1/k2*(1+k3/k4)) represents the total distribution volume of tracer in tissue. This formulation has particular advantages for the generation of synthetic data with predefined distribution volumes. The other three fitting parameters vB, K1, and k4 are the same as for the 2-Tissue Compartment Model.