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Positron emission tomography (PET) is a little different to other medical imaging techniques because it requires patients to be injected with a radioactive substance.

The most common radioactive substance used is fluorodeoxyglucose (FDG) – a sugar with fluorine attached to it (radioactive fluorine-18). The FDG has a half life of just under two hours. This means in two hours its radioactivity has halved, and in a further two hours, it has halved again.

When the radioactive fluorine starts to decay, it releases positrons. These positrons encounter electrons within the body and produce a pair of photons, which are detected by the scanner and used to form an image.

This image shows where in the body the radioactive glucose has been transported. This means a PET image is a picture of what is happening metabolically in the body compared to other medical imaging techniques that show the structure or parts of the body.

PET scans are often used alongside MRI or CT scans to help give a complete picture of what is going on in a patient's body. It is possible to fit a PET system to a CT scanner so both lots of data can be obtained at the same time.

PET scans are used to detect cancers and to check on the effects of cancer treatment. They can also be used for detecting heart disease and looking at brains for tumours or to work out what is going on in memory or seizure disorders.

PET scans are quite expensive and, in the past, have not been available in New Zealand – patients requiring a PET scan were flown to Australia. PET scans are now available through a private radiology practice in Wellington, which flies the radioactive glucose in from Melbourne.

It is argued that PET scans can save money on cancer operations but they are also expensive. A cyclotron (the machine used to make the radioactive sugar) is worth about $5 million. Prices for setting up a system are around $15–20 million. The Ministry of Health has been investigating whether PET scanners should be available in the public health system.

Useful links

Time magazine named the PET/CT scanner the medical instrument of 2000. For the full story users will have to log in or subscribe to the Time magazine.
A winning combination

    Published 23 July 2007