Positron-Emission Tomography

How does it work? For a PET scan, a radioactive substance is attached, or tagged, to a natural body compound, most commonly glucose, and injected into the body. The image results from the emission of positrons​—positively charged particles—​from the tissues. PET scans operate on the principle that cancerous cells use more glucose than normal ones do, thus attracting a larger amount of the radioactive substance. As a result, diseased tissues emit a greater number of positrons, which register as a variation in color or degree of brightness on the final image.

Whereas CT scans and MRI scans reveal the shape and structure of organs and tissues, PET scans show how they are functioning, thus revealing changes at an earlier stage. PET scans can be performed in combination with CT scans, the superimposed image enhancing the detail. PET scans may give false results, however, if patients have eaten within a certain time prior to the scan or if their blood sugar levels, perhaps because of diabetes, are outside the acceptable range. Also, because the radioactivity is very short-lived, timing is important.

Risks: Because the amount of radioactive substance used is very low and its radioactivity short-lived, radiation exposure is low. Still, it can pose a risk to a developing fetus. Hence, women who may be pregnant should inform their doctor and the imaging staff. And women of childbearing age may be asked to give a blood or urine sample to test for pregnancy. If a PET scan is used in conjunction with a CT scan, then the risks associated with CT scans should also be taken into account.

Benefits: Because PET scans show not just the shape of organs and tissues but also how well they are working, these scans can uncover problems before changes in tissue structure can be seen with CT or MRI.

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PET

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Courtesy Alzheimer’s Disease Education and Referral Center, a service of the National Institute on Aging