Deriving physiological information from PET images: from SUV to compartmental modelling

Abstract

Positron emission tomography (PET) imaging has made it possible to detect the in vivo concentration of positron-emitting compounds accurately and non-invasively. In order to relate the radioactivity concentration measured using PET to the underlying physiological or biochemical processes, the application of mathematical models to describe tracer kinetics within a particular region of interest is necessary. Image analysis can be performed both by visual interpretation and quantitative assessment and, depending on the ultimate purposes of the analysis, several alternatives are available. In clinical practice, PET quantification is routinely performed using the standard uptake value (SUV), a semiquantitative index in use since the 1980s. Its computation is very simple since it requires only the PET measure at a prefixed sample time and the injected dose normalised to some anthropometric characteristic of the subject (generally body weight or body surface area). An alternative to the SUV is the tissue-to-plasma ratio (ratio). As its name indicates, this index is computed as the ratio between the tracer activity measured in the tissue and in the plasma pool within a prefixed time window. Moving from static to more informative dynamic PET acquisition, three model classes represent the most frequently used approaches: compartmental models, the spectral analysis modelling approach, and graphical methods. These approaches differ in terms of application assumptions (e.g. reversibility of tracer uptake, model structure, etc.) and computational complexity. They also produce different information about the system under study: from a macro-description of tracer uptake to a full quantitative characterisation of the physiological processes in which the tracer is involved. The application of these approaches to clinical routine is restricted by the need for invasive blood sampling. In order to avoid arterial cannulation and blood sample management, different alternative approaches have been developed for quantification of PET kinetics, including reference tissue methods. Although these approaches are appealing, the results obtained with several tracers are questionable. This review provides a complete overview of the semi-quantitative and quantitative methods used in PET analysis. The pros and cons of each method are evaluated and discussed.

Quantification of Dynamic [18F]FDG Pet Studies in Acute Lung Injury

“…In addition, for each of the model kinetic components, a distribution volume Vi (ml/cm 3 ) can be computed [11,31]. Since normalization in a standard reference space is not possible for lung imaging, a regionbased comparison of these maps was performed between groups.…”

“…Kinetic modeling allows a quantitative description of tracer behavior by taking into account the blood flow and the transportation rate constants between the blood and the tissue [11], which is particularly important in lung studies. For instance, the fraction of blood volume is significantly higher in the lungs compared to other tissues [12,13], and its exclusion from the quantification model might bias the results when analyzing data both at the region of interest (ROI) and the voxel levels [14].…”

Quantification of [11C]PIB PET for Imaging Myelin in the Human Brain: A Test—Retest Reproducibility Study in High-Resolution Research Tomography

“…9 A reliable measure of the tissue myelin content would allow us to significantly improve our understanding of the physiopathological mechanisms underlying these disorders and would offer the opportunity to evaluate the effects of neuroprotective or pro-myelinating therapies. [10][11][12] Positron emission tomography (PET) represents a wellestablished tool for imaging the CNS at the molecular level, 13 and therefore it has the potential for detecting ongoing demyelination and remyelination processes in vivo in human. For this purpose, PET tracers such as [ 11 C]BMB, 14 a Congo red derivative, [ 11 C]PIB, 15 a Thioflavin derivative, [ 11 C]CIC, 16 and [ 11 C] MeDAS, 17 two stilbene derivatives, have been explored as myelin imaging biomarkers.…”

Spectral Analysis of Dynamic PET Studies: A Review of 20 Years of Method Developments and Applications

“…For the numerical quantification of PET data several solutions are available [1]; methods range from the calculation of the simple concentration of the tracer in a region of interest up to the complete description of the exchange of the radioactive molecules in the tissue of interest through a compartmental model. The choice of the quantification method is strictly dependent on the purpose of the PET study and on the experimental settings.…”