A PET (positron emission tomography) scan uses a small radioactive tracer — usually a form of glucose called FDG — to show how active tissues are, revealing function that structural scans miss. Its biggest use is in cancer: detecting it, working out how far it has spread (staging), and checking how it responds to treatment. It's almost always done as a combined PET-CT, which gives about 14 mSv of radiation — justified by the information it provides.
Key takeaways
- PET shows how active tissues are, not just their structure.
- Its biggest use is detecting, staging and monitoring cancer.
- The tracer (usually FDG) collects where cells are most active.
- It's almost always combined with a CT as a PET-CT scan.
A PET scan is one of the most powerful tools in modern imaging. Instead of showing what tissues look like, it shows how active they are — which is often the first thing to change when disease is present.
What PET shows
PET stands for positron emission tomography. You’re given a small radioactive tracer — most often FDG, a form of glucose (sugar) — which collects in cells that are using a lot of energy.[1] A special scanner detects where the tracer concentrates, building a picture of activity across the body.
Because many cancers are very active and hungry for glucose, they take up more FDG and show as bright areas on the scan. But activity isn’t unique to cancer — infection and inflammation are active too — so a bright area is interpreted in context.[1]
What it’s used for
PET’s biggest role is in cancer, where it helps to:[4]
- Detect cancer and find where it started
- Work out how far it has spread (staging) — often across the whole body in one scan
- Check how well treatment is working
- Look for cancer that has come back (recurrence)
PET is also used for some heart conditions and brain disorders — such as investigating epilepsy or dementia — where it shows how those organs are functioning.[1]
PET-CT: function plus anatomy
Almost all PET scans today are done as a combined PET-CT in a single machine.[1] The PET shows the activity and the CT shows the anatomy, so doctors can see not just that something is active but exactly where it is. This “image fusion” makes the result far more precise.
The radiation
A whole-body PET-CT gives about 14 mSv of radiation — coming from both the tracer and the CT part, and roughly nine years of natural background radiation in Australia.[3] For someone being investigated or treated for cancer, the information it provides usually far outweighs the small long-term risk. The tracer’s radioactivity fades quickly and is mostly gone within a day.[2]
In Australia
Access to PET has expanded significantly, and Medicare funds PET-CT for a growing list of specified cancer indications when requested by an appropriate specialist. Whether a particular scan is bulk-billed or attracts a fee depends on the indication and the clinic, so it’s worth asking when you book. For the current, official list of funded indications, check MBS Online or ask your specialist.
Frequently asked questions
Does a bright spot on a PET scan mean cancer?
Not on its own. FDG collects wherever cells are active — which includes infection and inflammation, not just cancer. A bright area is a clue that’s interpreted alongside your history and other scans.[1]
Is a PET scan the same as a CT or MRI?
No. CT and MRI mainly show structure (what tissues look like); PET shows function (how active they are). PET is usually combined with CT to get both at once.[1]
Will Medicare cover my PET scan?
Medicare funds PET-CT for a defined list of cancer indications when referred appropriately. Coverage and any out-of-pocket cost depend on the indication and clinic — ask when you book, or check MBS Online.
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/
- ARPANSA — Having a scan? A guide for medical imaging (2024) — www.arpansa.gov.au/sites/default/files/documents/2024-05/patienthandout.pdf
- Cancer Council Australia — Tests for cancer — www.cancer.org.au/cancer-information/what-is-cancer/tests-for-cancer
