There is increasing evidence that the usage and delivery of energy to the brain, known as brain energetics, is altered in people with Parkinson's disease (PD). This project will explore whether exercise has a positive impact on brain energetics using functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET) brain scanning in PD subjects. PD subjects will be scanned before and after a supervised exercise program to investigate the effect of exercise on brain energetics. In a separate study, the investigators will also scan healthy volunteers to compare brain energetics in the healthy brain with the brain energetics data in PD subjects acquired in this study.