The sequential steps for converting blood measurements to an input curve are explained in a dedicated section. The current parent fraction model serves for converting a plasma activity curve into an input curve (authentic, unchanged tracer in plasma).

The **Power-Damped Exp **model assumes that the concentration ratio of authentic tracer to total tracer in plasma has been determined at certain times during the acquisition and loaded with the **Load Fraction** entry in the menu.

Operational Model Curve

The general **Power-Damped Exp **model for the parent fraction handles the situation where the parent fraction starts at a low value and then increases, before dropping. It has the following functional form

The calculated fraction is multiplied with f_{p}, the free fraction of authentic tracer in plasma, i.e. the fraction of tracer not bound to plasma proteins.

Parameter Fitting

The free fraction **fp** is an input parameter which has no impact on fitting the model curve to the fraction data. The default for **fp** is 1 as it is experimentally difficult to measure.

The model supports the fitting of the parameters **alpha**, **Amplitude 1**, **2**, **3** (=A_{1}, A_{2}, A_{3}) and **Halftime 1**, **2**, **3** (=T_{1}, T_{2}, T_{3}). The three **Amplitude** parameters are restricted to the range [0,1] per default. However, the calculated fractions are *not *restricted to a certain value range.

Use without Measured Parent Fraction

The **Power-Damped Exp **parent fraction model can be applied even if no parent fraction measurements were loaded. In this case the user has to specify a parameter set which establishes a representative metabolite correction for the used tracer.

Reference

1. Parsey RV, Ojha A, Ogden RT, Erlandsson K, Kumar D, Landgrebe M, Van Heertum R, Mann JJ: Metabolite considerations in the in vivo quantification of serotonin transporters using 11C-DASB and PET in humans. J Nucl Med 2006, 47(11):1796-1802. PDF