Free water derived by multi‐shell diffusion MRI reflects tau/neuroinflammatory pathology in Alzheimer's disease

INTRODUCTION: Free‐water (FW) imaging, a new analysis method for diffusion magnetic resonance imaging (MRI), can indicate neuroinflammation and degeneration. We evaluated FW in Alzheimer's disease (AD) using tau/inflammatory and amyloid positron emission tomography (PET). METHODS: Seventy‐one participants underwent multi‐shell diffusion MRI, (18)F‐THK5351 PET, (11)C‐Pittsburgh compound B PET, and neuropsychological assessments. They were categorized into two groups: healthy controls (HCs) (n = 40) and AD‐spectrum group (AD‐S) (n = 31) using the Centiloid scale with amyloid PET and cognitive function. We analyzed group comparisons in FW and PET, correlations between FW and PET, and correlation analysis with neuropsychological scores. RESULTS: In AD‐S group, there was a significant positive correlation between FW and (18)F‐THK5351 in the temporal lobes. In addition, there were negative correlations between FW and cognitive function in the temporal lobe and cingulate gyrus, and negative correlations between (18)F‐THK5351 and cognitive function in the same regions. DISCUSSION: FW imaging could be a biomarker for tau in AD alongside clinical correlations.