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The blood and plasma activity concentrations require interpolation procedures for kinetic modeling, as described above.
Operational Model Curve
The Compart, 3 EV model has been developed for the FDG tracer [1]. It is the result from modeling the distribution and delivery of FDG in the circulatory system by a compartment model and results in an analytical solution with 3 eigenvalues (EV):
Parameter Fitting
Similar to the 3 Exponentials model it includes a Begin parameter and three Amplitudes (A1, A2, A3) and a Halftimes (T1, T2, T3). However, the functional form supports a peak rising from time Begin followed by an exponentially decreasing tail. Use the Fit plasma activity or Fit whole blood button to fit the operational model curve to the measurements. Note that all samples are considered, not only the ones after Begin.
The form of the curve reacts very sensitively to changes of the parameters. The example above illustrates a peak which could not fully reach the maximum value. This model may be more suited for simulations rather than actual data processing.
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.
Reference
1. Feng D, Huang SC, Wang X: Models for computer simulation studies of input functions for tracer kinetic modeling with positron emission tomography. International journal of bio-medical computing 1993, 32(2):95-110.