- Positron emission tomography (PET) PET is a diagnostic test that is based on differences in how cells take up and use glucose (sugar), one of the body s main fuels. The patient is given an injection of radioactive glucose. A special camera surrounding the patient detects the radiation emitted by the radioactive material and produces images that show how much glucose is being used by various tissues. Fast-growing cells, such as tumors, take up and use more glucose than normal cells do, and therefore, the scan might indicate the overall activity or aggressiveness of the tumor. The procedure takes about an hour.