Positron emission tomography (PET) scan is not recommended for certain types of patients, including pregnant or nursing women. Fetuses are still undergoing organ development, which makes them more sensitive to the effects of radiation. Nursing mothers who receive PET scans may expose their infants to the radionuclide material through breast milk.

Patients are urged to inform their physician of any medications or supplements they are taking, some of which may interfere with the radionuclides used in the scan. Patients should also report any recent testing using any other radioactive materials or a contrast medium (dye) that has been injected or taken orally or rectally.

In addition, patients are urged to tell their physician about any medical conditions they have. For example, patients with diabetes may be asked to alter their diet to control glucose levels during the day of the test. Blood sugar and blood insulin levels must be in a certain range if a PET scan is to be accurate.

Patients should consult their physician about necessary preparatory steps. The preparation necessary before a PET scan vary depending on the type of tissue to be studied. For example, patients preparing to undergo gallbladder, liver or thyroid scans may be instructed not to eat or drink for between two and 12 hours before testing. Other patients may be asked to take laxative or have an enema (a colon-cleansing procedure) prior to testing, or to avoid taking certain medications.

In many cases, patients are asked not eat for several hours prior to the PET scan. This helps ensure that blood sugar levels are at the optimal level to achieve accurate results during testing. Patients also may be urged to drink specific amounts of water prior to testing. 

Prior to testing, patients should remove jewelry, dentures and any other metal that can block gamma rays. Patients likely will be asked to wear comfortable, loose-fitting clothes or may be given a hospital gown.