Input Curve (AIF)

PKIN supports four types of blood data which together allow computing the input curve. All of them are functions of the acquisition time which are represented as curves in the display window.

1. The tracer activity concentration in whole blood. This information is used for the spillover term in the compartment models as well as in combination with the plasma fraction.
2. The total activity concentration of tracer in plasma.
3. The plasma fraction, the ratio of tracer in plasma to tracer in whole blood. The plasma fraction is multiplied with the whole blood concentration to calculate the total concentration of tracer in plasma.
4. The parent fraction (ratio of unchanged tracer to total tracer in plasma). The parent fraction is multiplied with the total plasma concentration to calculate the input curve.

For the evaluation of the kinetic models, the input curve needs to be available at any arbitrary time. It is calculated by multiplying the interpolated plasma concentration with the interpolated parent fraction at any particular time.

Note that the plasma options 2. and 3. above are excluding each other. If plasma activity has been loaded, the plasma fraction option can not be used. On the other hand, if the plasma fraction has been loaded, the plasma activity option can not be used.

The different possibilities to calculate the input curve work as follows:

• As soon as a whole blood curve is loaded and no other information is available, the plasma mode is set to "fractional" and a Fix model is applied with a factor of 1. This means in practice that the plasma activity is set equal to the whole blood activity.
• As soon as a plasma activity curve is loaded any plasma fraction data is discarded, and the plasma mode set to "activity". The activity interpolation mode defaults to Measured. The parent fraction is not changed.
• As soon as a plasma fraction curve is loaded any plasma activity data is discarded, and the plasma mode set to "fractional". The fraction interpolation mode defaults to Measured. The parent fraction is not changed.
• A parent fraction equal to 1 is applied as long as no parent fraction curve is loaded, or no analytical parent fraction model selected.

The different blood-related informations are readily available for inspection in the curve area of the Tissue tab as illustrated below.