Some participants may be asked to undergo a PET scan on a separate visit. A PET scanner is a doughnut-shaped machine similar in appearance to a CT (computed tomography) scanner. PET scans detect radioactivity used to provide information on brain activity. Before the test begins, subjects are given a dose of carbidopa-a medicine that increases the amount of levodopa in the brain. A catheter (thin, plastic tube) is then inserted into an arm or wrist vein, and a radioactive form of levodopa called 18Fluorodopa is injected through the catheter. A moldable plastic mask with large openings for eyes, nose, and mouth is placed on the face to help keep the head still during scanning. The total scan time is 2 hours or less.