2. Relative and absolute measures in molecular imaging: an overview

COEX 

PET analysis involves both semi-quantitative and fully quantitative assessment of measured radioactivity concentrations.
For data generated in clinical settings, the widely used standard uptake value (SUV) and its ratio version (SUVr) are simple semi-quantitative indices derived from the ratio of PET measurements and injected dose normalized by subject characteristics and, for SUVr, by a reference region. However, SUV's specific biological interpretation is limited due to various factors, encompassing both physiological and technical aspects.
Transitioning from static to dynamic PET data, most commonly generated in specialized research settings, essential mathematical models are required to extract additional information. Each of these models provides distinct biological significance and reliability, utilizing different units of measurement, rendering them non-comparable and non-interchangeable. For instance, when analyzing PET data related to radioligand binding to protein targets, it becomes possible to quantify various aspects such as the total concentration of receptors (Bmax, [pmol/mg]), binding potential (BPF, [ml/cm3]) as a product of Bmax and tracer affinity, and the total volume of distribution (VT, [ml/cm3]) calculated as the ratio of radioligand concentration in the tissue to that in plasma. Each of these measures holds a unique biological meaning and cannot be readily substituted.
This presentation aims to deliver a comprehensive overview of semi-quantitative and quantitative PET measures, critically evaluating their respective biological meanings and usability in a multimodal research environment.