The blood and plasma activity concentrations require interpolation procedures for kinetic modeling, as described above.

Operational Model Curve

This **Gamma + 2 Exponentials** model is very similar to the Compartment Model with 3 Eigenvalues. However, it includes a power term u^{B} which allows better modeling of sharp peaks.

Parameter Fitting

The **Gamma + 2 Exponentials** model includes a **Begin **parameter, three **Amplitude**s (A_{1}, A_{2}, A_{3})** **and a **Halftime**s (T_{1}, T_{2}, T_{3}), and a **Power Exponent **(B). This functional form supports a peak rising from time **Begin** followed by an exponentially decreasing tail. The initial default of the **Power Exponent** is a fixed value of 1 which corresponds to the **Compartment Model with 3 Eigenvalues**. Use the **Fit plasma activity** or **Fit whole blood** button to fit the operational model curve to the measurements. Then, for better fitting of the peak, enable the **Power Exponent** and fit again. The form of the curve reacts very sensitively to changes of the parameters. Note that all samples are considered, not only the ones after **Begin**.

The **Delay** parameter serves for correcting a timing offset between tissue and blood data. Positive delays correspond to delayed blood information and hence shift the blood curve to earlier times (to the left). The **Delay** is only relevant during the fitting of the tissue* *model where it can be fitted as an additional parameter to the parameters of the kinetic model.