What should you know about PET and SPECT scans?

A PET scan commonly uses FDG, a radiotracer related to glucose, to highlight areas using more of it. A SPECT scan uses a different radioactive tracer selected for the question being investigated. Both create images after the tracer travels through the body, while a CT or other image may add anatomical detail.

Doctors may use these tests to assess a known cancer, look for disease in other areas, or investigate findings when the primary site is unclear, including [Cancer of Unknown Primary](/conditions/cancer-of-unknown-primary/). An area of increased activity is not a diagnosis by itself; inflammation and healing can also alter tracer uptake.

Follow the department's instructions about food, drink, diabetes medicines, exercise, and arrival time. Tell staff if you are pregnant, breastfeeding, diabetic, have difficulty lying still, or recently had another scan. Wear comfortable clothing and remove metal items when asked.

After the tracer is given, you may rest quietly before imaging. The scanner is open at both ends, but you need to remain still. Drink fluids afterwards if the team recommends it. Ask whether you should limit close contact with a baby or pregnant person for a short period, depending on the tracer used. Learn more about the [FDG Radiotracer](/materials/fdg-radiotracer/) and the [PET-CT Scanner](/technology/pet-ct-scanner/).

Tell staff immediately if you develop itching, a rash, swelling, wheezing, faintness, or trouble breathing after the tracer. These reactions are uncommon but need prompt assessment. After leaving, seek urgent help for severe breathing difficulty or collapse. Contact the imaging team about persistent vomiting or any symptom that stops you drinking normally.

Ask which scan is being recommended and what decision it may inform. Confirm whether you need to fast, what to do with diabetes medicines, when results will be reviewed, and whether another test such as a biopsy is needed to explain an abnormal area. You can also ask about the [PET-CT Scan](/treatments/diagnostics-and-staging/pet-ct-scan/) process.