What Is Positron Emission Tomography?

Abstract

The scientific basis of nuclear medicine is the radiotracer, with its union of a pharmaceutical and a radionuclide. There are a number of existing tracers approved for use in the United States and elsewhere, but by far the most commonly used (and the only one used for melanoma and sarcoma) is fluorodeoxyglucose (FDG). FDG is a positron emitter, with the emitted positron being annihilated and producing two photons detected by the ring of detectors in the PET scanner. The PET is done together with a CT scan, which is used for both anatomic localization and attenuation correction. FDG is selectively taken up by cancer cells due to their heavy anaerobic metabolism, though other causes such as inflammation can also produce an increase in uptake. Common alterations in biodistribution include elevated glucose levels and/or insulin secretion and brown fat activation. The SUV (standardized uptake value) has been created as a semiquantitative measure of local uptake, correlates with prognosis, and is sometimes used for therapy assessment, but has technical problems resulting in significant variability.