Alzheimer's disease (AD) and vascular dementia (VaD) are the most common forms of dementia. Yet, the cause of these diseases is still unknown. A potentially important initiating factor is a disrupted blood-brain barrier. This can initiate cerebral microangiopathy, which has frequently been associated with VaD. Nevertheless, also in most AD patients a substantial increase of vascular damage has been observed. The present study investigates the correlation between blood-brain-barrier breakdown and cognitive decline in AD and VaD. An innovative dynamic contrast-enhanced MRI scan that has recently been developed and tested at our institute, will be used to measure blood-brain barrier permeability.