A PET scan shows how active tissues are, not just what they look like — and because activity often changes before structure does, PET can reveal disease early. Many cancers are very active and 'light up', so PET is widely used to detect and assess cancer, and also to investigate epilepsy and some brain and heart conditions. It's usually combined with a CT (PET-CT) so a finding's activity and exact location are seen together.
Key takeaways
- PET shows how active tissues are — a clue to disease.
- Activity often changes before structure does, so PET can reveal disease early.
- It's used for cancer, epilepsy, and some brain and heart conditions.
- It's usually combined with a CT, as PET-CT.
A PET scan shows how active different tissues are.[1] That’s what makes it useful for diagnosis: many diseases change how active a tissue is before its structure looks different — so PET can reveal things a structural scan might miss.
Why activity matters for diagnosis
The common PET tracer, FDG, behaves like glucose (sugar), which the body’s most active cells use the most.[1] Because many cancers are very metabolically active, they take up more tracer and stand out on the scan. That principle lets PET help:
- find a cancer and show whether it has spread;
- investigate epilepsy — locating the area of brain responsible for seizures, especially before surgery;
- assess some brain conditions (such as certain dementias) and heart conditions, by showing how those organs are functioning.
A clue, not a verdict
Activity isn’t unique to cancer — infection and inflammation are active too, and normally busy organs (brain, heart, bladder) always take up tracer.[1] So a bright area on PET is a clue interpreted in context, not an automatic diagnosis. A specialist reads the whole pattern and often correlates it with the CT.
PET-CT: activity plus location
PET is usually done with a CT in the same machine. The PET shows activity and the CT shows detailed anatomy, so a finding can be matched to an exact location.[1] A whole-body PET-CT gives a higher radiation dose than many scans (around 14 mSv), balanced against the information it provides.
Frequently asked questions
What makes PET different from a CT or MRI?
CT and MRI mainly show structure (what something looks like); PET shows function (how active it is). They’re often combined for a fuller picture.[1]
Can PET diagnose things other than cancer?
Yes — it’s also used for epilepsy (finding the seizure focus), some forms of dementia, heart muscle assessment, and tracking down hidden infection or inflammation.[1]
Sources
- RadiologyInfo.org (RSNA & ACR) — General Nuclear Medicine (PET) — www.radiologyinfo.org/en/info/gennuclear
- RANZCR / InsideRadiology — Nuclear medicine (Cain) — www.insideradiology.com.au/nuclear-medicine/
