Imaging
PET Scan
A PET scan uses a small amount of radioactive tracer to show how tissues are working, not just their shape. It is especially useful in cancer, heart and brain conditions.
Quick answer
PET Scan: what it is and what the results mean
A PET (positron emission tomography) scan uses a mildly radioactive tracer, usually a form of sugar, that collects in active tissues. A scanner detects it to show how organs and tissues are functioning, often combined with a CT scan (PET-CT).
- Why it is done: It is used mainly in cancer — to stage disease, find where it has spread and check response to treatment — and sometimes to assess the heart and brain, where activity rather than structure matters.
- Understanding results: Areas that take up more tracer ("hot spots") are more active, which can indicate cancer or inflammation, while combined CT images show the exact location.
What it is
A PET (positron emission tomography) scan uses a mildly radioactive tracer, usually a form of sugar, that collects in active tissues. A scanner detects it to show how organs and tissues are functioning, often combined with a CT scan (PET-CT).
Why it is done
It is used mainly in cancer — to stage disease, find where it has spread and check response to treatment — and sometimes to assess the heart and brain, where activity rather than structure matters.
What to expect
A tracer is injected and you rest quietly while it spreads, then you lie still through a ring-shaped scanner. The whole visit can take a couple of hours, though the scan itself is shorter.
Understanding the results
Areas that take up more tracer ("hot spots") are more active, which can indicate cancer or inflammation, while combined CT images show the exact location. A specialist interprets the findings alongside other tests.
Good to know
Risks and limitations
The radiation dose is higher than a plain X-ray but used when the information justifies it. The tracer is very short-lived. Tell staff if you are or might be pregnant, breastfeeding or diabetic, as preparation may differ.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
PET Scan: frequently asked questions
Is a PET scan radioactive?
It uses a small amount of short-lived radioactive tracer, which fades quickly. The dose is used only when the benefit clearly outweighs the small risk. Staff will advise on any brief precautions afterwards.
What does a PET scan show that a CT does not?
A CT shows detailed structure, while a PET shows how active tissues are. Combined as PET-CT, they reveal both where something is and how metabolically active it is — useful in cancer.
Related tests
Other imaging investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Royal College of Radiologists / relevant professional body
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