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.