The sequential steps for converting blood measurements to an input curve are explained in a dedicated section. The current plasma fraction model serves for converting a whole-blood activity curve into a plasma-activity curve.

It is assumed that a curve representing the measured fractions of plasma/whole-blood activity has been loaded with **Load Plasma/WB Fraction**.

Operational Model Curve

The **3 Exponentials **model allows replacing the tail of the fraction 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.

The illustration below shows an example with the exponentials fitted following 2 minutes (**Begin** = 120sec). Note that the linear interpolation assumes a fraction of 1 at t=0.

Parameter Fitting

The model supports the fitting of the parameters **Begin**, **Amplitude 1**, **2**, **3** (=A_{1}, A_{2}, A_{3}) and **Halftime 1**, **2**, **3** (=T_{1}, T_{2}, T_{3})**.** Fitting is started with the **Fit plasma fraction** button, which only takes the samples after the **Begin **time into consideration.

Appropriate default parameters for FDG scans:

- Rat experiments [1]: Plasma/whole_blood(t) = 0.51 e
^{(-ln2/4.79 t)}+0.3 e^{(-ln2/337 t)}+0.8 - Mouse Experiments [2]: Plasma/whole_blood(t) = 0.386*e
_{(-0.191*t)}+ 1.165

Use without Measured Fraction

If no measured plasma/whole-blood fraction data is available, the **3 Exponentials **model can still be applied. In this case the **Begin **time is disregarded, and the functional defined by the entered parameters applied.

References

- Weber B, Burger C, Biro P, Buck A: A femoral arteriovenous shunt facilitates arterial whole blood sampling in animals. Eur J Nucl Med Mol Imaging 2002, 29(3):319-323.
- Ferl GZ, Zhang X, Wu HM, Kreissl MC, Huang SC: Estimation of the 18F-FDG input function in mice by use of dynamic small-animal PET and minimal blood sample data. J Nucl Med 2007, 48(12):2037-2045. DOI