Intraneuronal tau accumulation is a hallmark of Alzheimer's disease. Currently, positron emission tomography with tau specific ligands (Tau-PET) is the only neuroimaging method for measuring tau in AD. In this study that combines Tau-PET with diffusion weighted MRS we show that the apparent diffusion coefficient of NAA, a neuronal metabolite, is significantly lower in AD patients compared to healthy controls in a brain region that shows elevated levels of uptake of the tau-PET ligand. Standard MRS data shows elevated level of glial metabolites, suggesting concomitant gliosis of neuroinflamation. We propose ADC(tNAA) as a putative marker for tau aggregation in AD.