[F]FDG, as a glucose analog, is taken up by high-glucose-using cells such as brain, brown adipocytes, kidney, and cancer cells, where phosphorylation prevents the glucose from being released again from the cell, once it has been absorbed. The 2-hydroxyl group (–OH) in normal glucose is needed for further glycolysis (metabolism of glucose by splitting it), but [F]FDG is missing this 2-hydroxyl. Thus, in common with its sister molecule 2-deoxy--glucose, FDG cannot be further metabolized in cells. The [F]FDG-6-phosphate formed when [F]FDG enters the cell cannot exit the cell before radioactive decay. As a result, the distribution of [F]FDG is a good reflection of the distribution of glucose uptake and phosphorylation by cells in the body.
The fluorine in [F]FDG decays radioactively via beta-decay to O. After picking up a proton H from a hydronium ion in its aqueous environment, the molecule becomes glucose-6-phosphate labeled with harmless nonradioactive "heavy oxygen" in the hydroxyl at the C-2 position. The new presence of a 2-hydroxyl now allows it to be metabolized normally in the same way as ordinary glucose, producing non-radioactive end-products.
Although in theory all [F]FDG is metabolized as above with a radioactivity elimination half-life of 110 minutes (the same as that of fluorine-18), clinical studies have shown that the radioactivity of [F]FDG partitions into two major fractions. About 75% of the fluorine-18 activity remains in tissues and is eliminated with a half-life of 110 minutes, presumably by decaying in place to O-18 to form [O]O-glucose-6-phosphate, which is non-radioactive (this molecule can soon be metabolized to carbon dioxide and water, after nuclear transmutation of the fluorine to oxygen ceases to prevent metabolism). Another fraction of [F]FDG, representing about 20% of the total fluorine-18 activity of an injection, is excreted renally by two hours after a dose of [F]FDG, with a rapid half-life of about 16 minutes (this portion makes the renal-collecting system and bladder prominent in a normal PET scan). This short biological half-life indicates that this 20% portion of the total fluorine-18 tracer activity is eliminated renally much more quickly than the isotope itself can decay. Unlike normal glucose, FDG is not fully reabsorbed by the kidney. Because of this rapidly excreted urine F, the urine of a patient undergoing a PET scan may therefore be especially radioactive for several hours after administration of the isotope.
All radioactivity of [F]FDG, both the 20% which is rapidly excreted in the first several hours of urine which is made after the exam, and the 80% which remains in the patient, decays with a half-life of 110 minutes (just under two hours). Thus, within 24 hours (13 half-lives after the injection), the radioactivity in the patient and in any initially voided urine which may have contaminated bedding or objects after the PET exam will have decayed to 2 =  of the initial radioactivity of the dose. In practice, patients who have been injected with [F]FDG are told to avoid the close vicinity of especially radiation-sensitive persons, such as infants, children and pregnant women, for at least 12 hours (7 half-lives, or decay to  the initial radioactive dose).