A Positron Emission Tomography (PET) scan is a diagnostic imaging test that allows physicians to observe the metabolic activity of cells within the body. This is achieved by injecting a small amount of a radioactive material, known as a radiotracer, into the patient’s bloodstream. The radiotracer temporarily makes the patient radioactive, which is necessary for the scanner to create detailed images of how tissues and organs are functioning. Patients often want to know how quickly this radioactivity dissipates after the procedure is complete.
The Source of Radioactivity and Decay
The temporary radioactivity comes from the injected radiotracer, typically Fluorine-18 Fluorodeoxyglucose (F-18 FDG). This compound is structurally similar to glucose, allowing it to be taken up by energy-consuming cells in the brain, heart, and certain tumors. The F-18 component is an unstable isotope that emits a positron, which is the source of the radiation detected by the PET scanner.
The rate at which the radioactive material naturally loses its activity is measured by its physical half-life. The half-life of F-18 is approximately 110 minutes, or just under two hours. This means that the amount of F-18 radioactivity remaining in the body is reduced by half every 110 minutes. After about 10 hours, the radioactivity from physical decay alone is reduced to less than 1% of the original dose.
Determining When Radioactivity is Gone
The actual clearance time from the body is determined by two combined processes: physical decay and biological clearance. Biological clearance refers to how quickly the body naturally processes and excretes the radiotracer. F-18 FDG not taken up by tissues is eliminated relatively quickly through the urinary system.
Roughly 20% of the F-18 FDG dose is rapidly excreted in the urine within the first few hours after injection. The combined effect of this biological excretion and the short physical half-life means the majority of the radiotracer is gone within 4 to 6 hours after the scan. After this initial period, overall radiation exposure levels drop significantly.
Most facilities recommend a conservative timeframe of 12 to 24 hours before resuming close, prolonged contact with sensitive individuals. This practical safety window ensures that the small amount of remaining radioactivity poses a negligible risk. The longer recommendation accounts for the need to protect vulnerable populations from even minimal residual exposure.
Practical Safety Guidelines After a Scan
Following specific safety measures for the first 12 to 24 hours after a PET scan helps minimize potential residual exposure to others. Increasing fluid intake is a primary action, as drinking plenty of water helps flush the radiotracer out of the system via urine. Patients are often advised to drink at least four to six glasses of water in the 24 hours following the procedure to promote elimination.
Special attention to hygiene is important because the radiotracer is excreted in bodily fluids. Patients should wash their hands thoroughly after using the bathroom. They are often instructed to flush the toilet twice, particularly in the first few hours following the scan, to clear trace amounts of the material from the plumbing.
Limiting close contact with sensitive individuals is a common recommendation during this period. Patients are typically asked to maintain distance from infants, small children, and pregnant women for at least 6 to 8 hours, and sometimes up to 24 hours. Distance is a highly effective way to reduce radiation exposure to these vulnerable populations.
Some facilities recommend using a separate bed from children or pregnant partners for the first 24 hours to ensure safe distance during prolonged close contact. If traveling shortly after the scan, especially by air, it is advisable to carry a document from the medical center confirming the procedure. This is because residual radioactivity could potentially trigger sensitive radiation detectors in airports.