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Positron Emission Tomography (PET)

By Hakan Ilaslan, MD

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PET, a type of radionuclide scanning, uses compounds containing radionuclides that decay by releasing a positron (the positively charged antimatter equivalent of an electron). The released positron combines with an electron and produces 2 photons whose paths are 180° apart. Ring detector systems encircling the positron-emitting source simultaneously detect the 2 photons to localize the source and to produce color tomographic images of the area. Because PET incorporates positron-emitting radionuclides into metabolically active compounds, it can provide information about tissue function. Standard uptake value (SUV) indicates metabolic activity of a lesion; typically intensity of color is increased with higher SUVs.

The most commonly used compound in clinical PET

  • Fluorine-18 [18F]–labeled deoxyglucose (FDG)

FDG is an analog of glucose, and its uptake is proportional to glucose metabolic rates. A patient’s relative glucose metabolic rate (SUV) is calculated: The amount of FDG taken up from the injected dose is divided by the patient’s body weight.

Uses of PET

PET has several clinical indications, such as

  • Cancer (eg, staging and evaluating specific types of cancer and evaluating response to treatment), which accounts for about 80% of PET usage

  • Cardiac function (eg, evaluating myocardial viability, detecting hibernating myocardium)

  • Neurologic function (eg, evaluation of dementia and seizures)

PET applications continue to be investigated. Not all applications are reimbursable in the US.

Variations

PET-CT

Functional information provided by PET is superimposed on anatomic information provided by CT.

Disadvantages of PET

The typical effective radiation dose during PET is about 7 mSv. The effective radiation dose with PET-CT is 5 to 18 mSv.

Production of FDG requires a cyclotron. FDG has a short half-life (110 min); thus, shipment from the manufacturer and completion of the scan must occur very rapidly. The resulting expense, inconvenience, and impracticality greatly limit the availability of PET.

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