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

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 instead of k2

The 2-Tissue Compartment Model, K1/k2 model variant 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. The other four fitting parameters vB, K1, k3, k4 are the same as for the 2-Tissue Compartment Model.