Positron emission tomography (PET) with FDG monitors the enhanced glycolytic activity and increased expression of glucose transporters associated with tumor cells. FDG accumulates in tumors via the same transporters used by glucose. Similarly, once in the cell, the radiotracer is phosphorylated to FDG-6PO4- At this point the handling of FDG differs from glucose. Because of the lack of an oxygen at the 2-position in FDG, it cannot proceed further in glycolysis or glycogen synthesis and becomes a "trapped tracer." This characteristic of FDG has led to its widespread use for whole-body imaging of patients with cancer. Also, because of the relatively long physical half-life of fluorine-18, it can be distributed to imaging facilities without on-site cyclotrons.