A PET scan detects metabolic activity in the body using a small radioactive tracer. It’s used to detect and stage cancers, find metastases, and monitor treatment response; evaluate brain disorders such as Alzheimer’s disease, dementia, and epilepsy; and assess heart function and blood flow to identify areas of reduced viability. It also helps guide biopsy and treatment planning.

PET scans detect areas of increased metabolic activity and are very useful for locating, staging and monitoring suspected cancer. They do not definitively confirm cancer because infections, inflammation and some benign conditions can also “light up,” and small or low‑metabolism tumors may be missed. Tissue biopsy or further imaging and clinical correlation are usually required for a conclusive diagnosis.

A CT scan uses X‑rays to create detailed cross‑sectional images of body anatomy—bones, organs and structural changes. A PET scan uses a small radioactive tracer to map metabolic or biochemical activity, highlighting areas of abnormal function such as cancer, inflammation, or altered brain activity. PET is often combined with CT (PET/CT) so functional signals can be precisely localized to anatomy. Both involve radiation.

A PET scan reveals metabolic and functional activity (useful for cancer, dementia and cardiac viability) by detecting radioactive tracers, while MRI provides high-resolution anatomical and soft-tissue detail without ionising radiation. MRI is preferred for structural brain, spine and joint evaluation; PET is better for assessing tumour activity or physiological function. Choice depends on the clinical question; sometimes both are combined for complementary information.

A positive PET scan shows abnormal areas of increased metabolic activity, suggesting possible cancer, infection, or inflammation. It doesn’t confirm diagnosis alone; doctors use results to guide further steps such as targeted imaging, biopsies, blood tests, and specialist consultations. Findings help with staging and treatment planning. False positives and incidental findings are possible, so decisions consider clinical context and additional tests.

Before an FDG‑PET scan, fast for at least 4–6 hours — only plain water is allowed. Avoid carbohydrate‑rich or sugary foods and drinks for 24 hours beforehand to reduce background uptake. Skip strenuous exercise for 24 hours. Take routine medications only if instructed; people with diabetes should follow clinic-specific preparation and inform staff about insulin or oral glucose‑lowering drugs.