- PET scanning: PET uses small amounts of a radioactive chemical called a tracer that "labels" active areas of the brain. The tracer used in this study is [11C]PBR28. For the procedure, the subject lies on the scanner bed. A special mask is fitted to the head and attached to the bed to help keep the head still during the scan so the images will be clear. A brief scan is done just before the radioactive tracer is injected to provide measures of the brain that will help to precisely calculate information from subsequent scans. After the tracer is injected through a catheter (plastic tube) placed in the arm, pictures are taken for 2 to 3 hours, during which the subject lies still on the scanner bed. Subjects return to the clinic the following day for more blood and urine sample collections.