### Linear Interpolation Fraction

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 **Lin. Interpolation **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 **Lin. Interpolation **model interpolates linearly between the measured fraction values F_{k}.

and multiplies with f_{p} representing the free fraction of authentic tracer in plasma, i.e. the fraction of tracer not bound to plasma proteins.

Outside the measured values the following interpolation rules apply:

- The fraction at time t=0 is assumed to equal 1.
- The fraction is assumed to remain constant after the last measured fraction. For incomplete fraction data it is therefore recommended to replace the tail of the measurements by a fitted tri-exponential model.

Note that fraction values >1 are not allowed.

Parameter Fitting

There is nothing to fit with the **Lin. Interpolation** model. The only input parameter is the free fraction **fp** which defaults to 1, as it is experimentally difficult to measure.