Assessment of simplified ratio-based approaches for quantification of PET [¹¹C]PBR28 data

Kinetic modelling with metabolite-corrected arterial plasma is considered the gold standard for quantification of [¹¹C]PBR28 binding to the translocator protein (TSPO), since there is no brain region devoid of TSPO that can serve as reference. The high variability in binding observed using this method has motivated the use of simplified ratio-based approaches such as standardised uptake value ratios (SUVRs) and distribution volume (V_T) ratios (DVRs); however, the reliability of these measures and their relationship to V_T have not been sufficiently evaluated.

Data from a previously published [¹¹C]PBR28 test-retest study in 12 healthy subjects were reanalysed. V_T was estimated using a two-tissue compartment model. SUVR and DVR values for the frontal cortex were calculated using the whole brain and cerebellum as denominators. Test-retest reliability was assessed for all measures. Interregional correlations were performed for SUV and V_T, and principal component analysis (PCA) was applied. Lastly, correlations between ratio-based outcomes and V_T were assessed.

Reliability was high for V_T, moderate to high for SUV and SUVR, and poor for DVR. Very high interregional correlations were observed for both V_T and SUV (all R ² > 85%). The PCA showed that almost all variance (>98%) was explained by a single component. Ratio-based methods correlated poorly with V_T (all R ² < 34%, divided by genotype).

The reliability was good for SUVR, but poor for DVR. Both outcomes showed little to no association with V_T, questioning their validity. The high interregional correlations for V_T and SUV suggest that after dividing by a denominator region, most of the biologically relevant signal is lost. These observations imply that results from TSPO PET studies using SUVR or DVR estimates should be interpreted with caution.