Elevated tau in the piriform cortex in Alzheimer's but not Parkinson's disease using PET‐MR.

INTRODUCTION: Olfactory dysfunction can be an early sign of Alzheimer's disease (AD). We used tau positron emission tomography‐magnetic resonance (PET‐MR) to analyze a key region of the olfactory circuit, the piriform cortex, in comparison to the adjacent medial temporal lobe. METHODS: Using co‐registered magnetic resonance imaging (MRI) and 18F‐PI‐2620 tau PET‐MR scans in 94 older adults, we computed tau uptake in the piriform‐periamygdaloid cortex, amygdala, entorhinal‐perirhinal cortices, and hippocampus. RESULTS: We found an ordinal cross‐sectional increase in piriform cortex tau uptake with increasing disease severity (amyloid‐negative controls, amyloid‐positive controls, mild cognitive impairment [MCI] and AD), comparable to entorhinal‐perirhinal cortex. Amyloid‐positive controls showed significantly greater tau uptake than amyloid‐negative controls. Negative correlations were present between memory performance and piriform uptake. Piriform uptake was not elevated in cognitively unimpaired Parkinson's disease. DISCUSSION: Cross‐sectionally, there is an early increase in tau uptake in the piriform cortex in AD but not in Parkinson's disease. Highlights: Positron emission tomography–magnetic resonance (PET‐MR) analysis of the piriform cortex sheds light on its role as a potential early region affected by neurodegenerative disorders underlying olfactory dysfunction.Uptake of tau tracer was elevated in the piriform cortex in Alzheimer's disease (AD) and mild cognitive impairment (MCI) but not in Parkinson's disease (PD).Memory performance was worse with greater piriform uptake. [ABSTRACT FROM AUTHOR]