Many people with Parkinson’s develop cognitive problems and, eventually, dementia. We need to identify suitable objective tools that measure the underlying brain changes associate with cognitive decline. Such tools are important clinically and for assessing new preventative treatments. Here, two novel brain imaging approaches will be used. First, we will measure accumulation in the brain of an abnormal protein, tau, which is associated with the development of Parkinson’s dementia. Second, we will use a new brain imaging technique to measure blood flow in the brain. Both measures will be acquired in 25 healthy control participants and 70 people with Parkinson’s disease with varying levels of cognitive impairment, including dementia, to show how tau burden and blood flow in the brain reflect degree of cognitive decline.