In addition to collecting structural MRI data, seven PPMI sites will also be performing diffusion tensor imaging (DTI), an MRI technique that analyzes the movement of water molecules in brain tissue. Several groups have reported DTI differences in PD patients that may be associated with disease progression. The seven PPMI sites that were selected for this PPMI sub-study all have similar MRI scanners and will implement the same scanning protocol. We expect to include approximately 140 PD patients and 70 control subjects. Not only is this one of the largest DTI studies ever done in PD patients but it may be the only such study that includes multiple MRI centers in both the United States and Europe. The images are being processed centrally at the Center for Imaging of Neurodegenerative Diseases at the VA Medical Center and the University of California, San Francisco. The raw and processed DTI data are stored in a database by LONI where the data can be downloaded and analyzed by interested scientists.