Tau PET. PET imaging of tau aggregates will be done in all the included cohorts at baseline. In the present study, Tau PET imaging will be performed using 18F-RO6958948 developed by Hoffmann-La Roche that will provide the precursor for this PET ligand. This tau PET imaging agent has been shown to accurately detect tau pathology in cases with AD when compared to controls. We will perform a 20-30 min PET scan approximately 60 min post intravenous injection of 18F-RO6958948. The impact of the investigation on clinical diagnostic accuarcy and patient care will be investigated. 18F-RO6958948 has not yet been approved for use in clinical routine practice in Sweden, and can only be used in research studies, such as the present study.