A PET scan is used to detect cellular reactions to sugar. Abnormal cells tend to react and "light up" on the scan, thus helping physicians diagnose a variety of conditions. For the PET scan, a harmless chemical, called a radiotracer, is injected into your blood stream. Once it has had time to move through your body, you will lie on a table while a scanner follows the radiotracer and sends three-dimensional images to a computer screen. Patients are generally asked to wear comfortable clothing and refrain from eating for 4 hours before the scan. Tell your doctor if you are pregnant or breastfeeding. Patients with diabetes should discuss diet guidelines with their physician for the hours leading up to the scan.