2003-07-23 Vesa Oikonen
There are compartmental models that are specific to a certain tracer. This text handles only computation of the common models that are applicable to several tracers.
The usage of these models requires that there exists a reference region with no specific binding (k3=0). As in all models applying reference tissue, also here it is assumed that K1/k2 is the same in all (brain) regions. First, two- or three-compartment model (K1 and k2, or K1, k2, k5 and k6) is fitted to the reference tissue curve. Then, three-compartment model is fitted to the other regions, constraining the K1/k2-ratio to the value estimated from the reference region.
The input to these models is always the metabolite corrected plasma curve. The unit of model parameters K1-k4 is min-1.
Programs fitk3 and fitk4 can be used to fit three-compartment models to the PET TACs. With fitk3 the irreversible model with parameters K1, k2 and k3 is fitted (k4=0), and with program fitk4 the reversible model with parameters K1, k2, k3 and k4 is fitted.
These programs do not fit fit the volume fraction of vascular blood, but they can automatically subtract a specified VB.
Any or all of of the following steps can be done in Solaris terminal window or MS Windows command prompt window on either SUN or PC platform.
The procedure is dependent on the tracer and study protocol. Detailed instructions on the preparation of input curves in given elsewhere. In short, if on-line detector was used to collect the blood curve during the early phase of the study, the blood curve must be corrected and converted to plasma TAC, and then combined to the manually sampled plasma curve. Fractions of metabolites must be corrected.
Usually the blood TAC is not measured separately. If it is needed for fitting the vascular volume fraction, it can be computed from the plasma TAC, which is not corrected for metabolites(!), using tracer specific programs.
Delay correction can be done for plasma and blood TACs at the same time using program fitdelay, if the countrate datafile (*.cr) is available, or if the tissue data was collected with short frames in the initial phase.
This is explained in detail elsewhere. In short: draw ROIs and calculate regional TACs from dynamic images, and calculate averages over planes and regions if required. If you have regional TACs in old format (*.roi.kbq), convert those to DFT format using program nci2dft.
Weights can be added to tissue data file using program dftweigh. Weights can be extracted either from a SIF file (*dy1.img.sif), which is originally copied to the same place as the dynamic image, or automatically from the average tissue curves. The weights are not absolute, but only relational to each time frame in the TAC.
If weights are extracted from the SIF file, the command could be e.g.:
dftweigh ua0268.dft ua0268dy1.img.sif C-11
If SIF file is not used, the command would be:
dftweigh ua0268.dft
Note that weights need to be added to the DFT datafile only once, and they may affect the results of other calculation programs. The weights can be removed using the same program, dftweigh, with option -rm.
The programs fitk3 and fitk4 can be run with at least the following command line parameters:
In addition, the fitted TACs can be saved by specifying filename for it as an extra parameter. This file can then be read into e.g. Excel or Origin, together with the original datafile, for drawing the fitted curves.