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.