Suspecting Non-Alzheimer's Pathologies and Mixed Pathologies: A Comparative Study Between Brain Metabolism and Tau Images

Background: Alzheimer's disease (AD) pathology can be disclosed in vivo using amyloid and tau imaging, unlike non-AD neuropathologies for which no specific markers exist. Objective: We aimed to compare brain hypometabolism and tauopathy to unveil non-AD pathologies. Methods: Sixty-one patients presenting cognitive complaints (age 48-90), including 32 with positive AD biomarkers (52%), performed [F-18]-Fluorodeoxyglucose (FDG)-PET (brain metabolism) and [F-18]-MK-6240-PET (tau). We normalized these images using data from clinically normal individuals (n = 30), resulting in comparable FDG and tau z-scores. We computed between-patients correlations to evaluate regional associations. For each patient, a predominant biomarker (i.e., Hypometabolism > Tauopathy or Hypometabolism <= Tauopathy) was determined in the temporal and frontoparietal lobes. We computed within-patient correlations between tau and metabolism and investigated their associations with demographics, cognition, cardiovascular risk factors (CVRF), CSF biomarkers, and white matter hypointensities (WMH). Results: We observed negative associations between tau and FDG in 37 of the 68 cortical regions-of-interest (average Pearson's r = -0.25), mainly in the temporal lobe. Thirteen patients (21%) had Hypometabolism > Tauopathy whereas twenty-five patients (41%) had Hypometabolism <= Tauopathy. Tau-predominant patients were more frequently females and had greater amyloid burden. Twenty-three patients (38%) had Hypometabolism <= Tauopathy in the temporal lobe, but Hypometabolism > Tauopathy in the frontoparietal lobe. This group was older and had higher CVRF than Tau-predominant patients. Patients with more negative associations between tau and metabolism were younger, had worse cognition, and greater amyloid and WMH burdens. Conclusions: Tau-FDG comparison can help suspect non-AD pathologies in patients presenting cognitive complaints. Stronger Tau-FDG correlations are associated with younger age, worse cognition, and greater amyloid and WMH burdens.