The transformation of the plasma activity curve into the input curve is governed by the configuration on the Metabolites tab. Two cases are to be distinguished.
Measured Parent Fraction Data is available
If a parent fraction curve has been loaded, it is shown in the curve area. Initially, the parent fraction model is set to Lin. Interpolation so that linear interpolation between the samples is used to calculated intermediate parent fractions. The model selection choice offers smoother models, and they can be fitted to the measurements using the Fit plasma fraction button as illustrated below.
To the right of the Fit fraction button, there is an option button for changing the current data.
▪Replace fraction data: This function replaces the data samples by the values of the current model function at the original sampling times.
▪Clear fraction data: This function discards the loaded parent fraction data. In that case the assumption is that an entirely analytical parent fraction function will be applied to the plasma activity curve in order to derive the parent activity.
No Parent Fraction Data available
If no data parent fraction data have been loaded, the parent fraction model is set to Fix with a free parent fraction fp=1. This is adequate for all tracers which do not require metabolite correction, and for cases when the metabolite correction has already been applied to the plasma data prior to loading them with Load Plasma Activity.
In the case of a tracer which shows a characteristic buildup of the metabolites it may be possible to derive an average parent fraction curve and fit a model to it. To apply a metabolite correction the user can then simply select the appropriate model function and enter the demographic parameter values, as illustrated below. To apply a demographic correction, it must be ensured that time zero of plasma data always represents the time of tracer injection. Naturally, no fit will be possible in this situation.
The available models are described in the corresponding reference section.
Note: The metabolite correction is applied to the plasma curve using the original plasma sampling times. The assumption is that the parent fraction and the plasma curve have a common time scale. Only after the metabolite correction has been applied is the input curve shifted by the blood sampling time delay.