- PET scan. This test uses a radioactive chemical called 18FDG, which is detected by the PET scanner to obtain images of the brain. The subject lies on a table with his or her head positioned in the scanner. A swimming cap with a small light reflector is placed on the head to monitor the position of the head during the scan. A catheter (plastic tube) is inserted into an artery at the wrist or elbow crease of the arm for obtaining blood samples during the scan, and a second catheter is placed in a vein in the other arm for injecting the 18FDG. The scan takes up to 2 hours. A second scan may be done over an additional 15 minutes.