"Risks and Contraindications
A PET scan is painless and poses few risks. The scanner itself does not emit radiation, and the amount of radiotracer used for the imaging is so small as to not require the use of standard radiation precautions.
Since the radiotracer is essentially glucose with a radioactive isotope attached, the drug half-life is extremely short. Some of the agents have a half-life as short as two minutes (such as oxygen-15), while others may be active for up to two hours (such as with FDG). In most cases, the drug will be in and out of your system within a day.
While the injection itself may cause localized pain and swelling, allergic reactions are rare, and there are no outright contraindications to the procedure, including pregnancy.
The only other concern—and, in some ways, the most significant—is the risk of claustrophobia. If being placed inside the tube-like device makes you nervous, let your doctor know in advance. In extreme cases, the doctor may prescribe a mild sedative, such as low-dose Valium (diazepam) or Ativan (lorazepam), to help reduce anxiety."