The blood and plasma activity concentrations require interpolation procedures for kinetic modeling, as described above.

Operational Model Curve

The **3 Exponentials **model allows replacing the tail of the blood activity curve by a fitted sum of up to three exponentials. The **Begin **parameter defines at what time the models switches from linear to tri-exponential interpolation. The exponentials are each defined by an **Amplitude **and a **Halftime **[min] of the decay. The number of exponentials can easily be reduced by fixing one or two of the amplitudes at a value of 0.

Parameter Fitting

Per default only two exponentials are enabled, and the **Begin **is initialized to the time of the blood peak. Use the **Fit plasma activity** or **Fit whole blood** button to fit the two exponentials to the measurements after the specified **Begin**. If the shape cannot be described well enough, enable the third exponential by enabling the fit boxes and set **Amplitude 3** to a positive number, then **Fit **again. Possibly, **Begin** has to be set to a later time for obtaining the signal tail.

The **Delay** parameter serves for correcting a timing offset between tissue and blood data. Positive delays correspond to delayed blood information and hence shift the blood curve to earlier times (to the left). The **Delay** is only relevant during the fitting of the tissue* *model where it can be fitted as an additional parameter to the parameters of the kinetic model.