The purpose of this research is to evaluate possible reasons why people with Parkinson's disease tend to fall more often. Parkinson's disease is a disease where persons have slowness in movements and walking as well as symptoms of shaking. You are being asked to participate in this study because you previously participated as a person with Parkinson's or as a normal comparison subject in a similar imaging study about 3-4 years ago. The previous research study evaluated functions of memory, thinking, walking and how these relate to the measurement of certain chemicals (acetylcholine and dopamine) in the brain using an imaging procedure called positron emission tomography (PET). You may know that the brain chemical dopamine, a "neurotransmitter" substance (a chemical messenger that nerve cells need to communicate with each other), is important for the brain to control movements and that the brain chemical acetylcholine may have functions related to mental concentration and attention. You are now being asked for a new study as a follow-up about 3 years after the previous study. The main purpose of the follow-up study is to determine which of the two brain chemicals (dopamine, acetylcholine) or changes seen on brain MR called "small vessel disease" (which is commonly seen with aging) are related to development of balance problems. The study activities are very similar as the previous study you participated in.