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Fix Parent/Plasma 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).
Operational Model Curve
The Fix model assumes that the concentration of authentic tracer in plasma is in constant proportion to the concentration of total tracer in plasma. The fraction fparent(t) of authentic tracer to total tracer in plasma is therefore given by
with fp representing the free fraction of authentic tracer in plasma, i.e. the fraction of tracer not bound to plasma proteins.
Note: The Fix model is the default as soon as blood data is loaded. This is adquate if a metabolite correction is not necessary or has been performed in a prior step, for instance externally before loading the plasma activity.
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.