PET scans and CAT scans explained

The paired images help clinicians locate areas with altered activity and relate them to organs and other structures. The scan can support cancer diagnosis, staging or treatment review, but its findings are not interpreted in isolation.

Before the scan, a small amount of radiotracer is given, commonly FDG radiotracer. FDG behaves similarly to glucose, so tissues that use more glucose may appear more active. Some cancers show increased uptake, but inflammation, infection and normal organs can also take up the tracer.

The CT component maps the body's structures, while the PET component maps tracer activity. A specialist reads both together, considering the reason for the scan, recent treatment, symptoms and previous images. A bright area is therefore a finding to interpret, not a diagnosis by itself.

Follow the imaging centre's instructions about food, drinks, exercise and medicines because these can affect tracer distribution. Tell the team if you have diabetes, might be pregnant, are breastfeeding, feel unable to lie flat or have severe claustrophobia. Mention recent infection, surgery, chemotherapy or radiotherapy as well.

Wear comfortable clothing and expect a quiet resting period after the injection before imaging begins. During the scan, remaining still helps prevent blurred images. Arrange follow-up with the referring clinician, who can explain the report in the context of your other tests.

Contact the imaging centre before attending if you become unwell, develop an infection or cannot follow the preparation instructions. The team can decide whether the appointment should proceed and can give specific guidance for diabetes or pregnancy.

After the injection, immediately tell staff about breathing difficulty, facial swelling, widespread rash, faintness or sudden severe discomfort. Serious reactions are uncommon, but these symptoms need urgent assessment. New or worsening cancer-related symptoms should be reported to the treating team rather than waiting for the scan result.

Confirm what clinical question the scan is intended to answer, whether diagnostic CT contrast is also planned and how medicines should be managed. Ask when and from whom you will receive the result, whether an uncertain area needs another scan or biopsy, and how the finding may affect treatment.