Correction of plasma TAC for metabolites
It is common that the PET tracer is metabolized in the liver, kidneys or other parts of the body already during the PET scan, and one or more of the metabolites is still carrying the isotope label. If labeled metabolites are found in the plasma in significant amounts, their proportion has to be subtracted from the plasma curve, because only the concentration of parent tracer can be used as input in quantitative data analysis.
Measured metabolite fractions
The fractions of authentic (parent) tracer in plasma must be written in an ASCII file
(fraction data).
A mathematical function can be fitted to these fractions.
Metabolite corrected plasma curve can
be calculated using metabcor.
Mathematical metabolite correction
For references, see Burger and Buck (1996), and Sanabria-Bohórquez et al. (2000).
Population based methods
Ideally, fractions of plasma metabolites should be measured for each person participating in a PET study. However, the measured fraction curves are sometimes noisy, or there are missing samples. One alternative is to calculate population average curve of the fractions of parent tracer in the plasma, if the inter-individual variation in the rate of metabolism is small. Population average must be determined from a group that is comparable to the study population by their age, sex, and body weight. For example, for rate of metabolism of [18F]FDPN a significant gender difference has been found (Henriksen et al., 2006).
The population average fraction curve can be fitted to a function, for example to the "Hill-type" or exponential functions, if there were only few samples or if the fraction curve must be extrapolated. In the fitting, use the weights that were written in the mean fraction curve.
