In this study, the investigators want to examine the different mechanisms of the accumulation and the clearance of Aß- deposits with imaging methods. One focus of the study is an improved characterisation of a blood-brain-barrier disorder (which seems to have an impact on the Aß-accumulation). Another main aim is to provide an improved mechanistic clearance model, which integrates crucial components such as the recently proposed cerebral glymphatic and lymphatic pathways, and which addresses the interaction between the different components and their individual contribution to Aβ removal from the brain. A possible connection between sleep and an altered transport mechanism will be analysed. The prospective study cohort (N ~60) will include patients with Mild Cognitive Impairment, mild clinical AD and Subjective Cognitive Decline. All study participants will undergo a detailed clinical and neuropsychological assessment according to a standardised protocol (i.a. MRI, PET, CSF, actigraphy). Follow-up assessments will not be performed in the present project, but are planned in a subsequent study, pending further funding.