The Half-Life of F-18: Understanding Radioactive Decay and Why It Makes PET/CT Safe
One of the most reassuring facts about the radioactive tracer used in your PET/CT scan is something called its half-life. Once you understand what this means, the idea of being injected with a radioactive substance becomes far less daunting and you will understand why fluorine-18 (18F) is specifically chosen for its safety profile as much as its diagnostic power.
What Is Radioactive Decay? All radioactive materials are unstable — their atoms are trying to reach a more stable state by releasing energy. This process of releasing energy is called radioactive decay. Over time, as the atoms stabilise, the material becomes less and less radioactive, until it effectively disappears. The half-life is simply the time it takes for half of the radioactive material to decay. After one half-life, 50% of the radioactivity remains. After two half-lives, 25% remains. After three, 12.5%. After ten half-lives, less than 0.1% remains.
Fluorine-18: A 110-Minute Half-Life The radioactive component of the radiopharmaceutical 18F-FDG — fluorine-18 — has a half-life of 110 minutes. Let us trace what that means from the moment of your injection: • At injection (time 0): 100% of the radioactivity is present • After 110 minutes (~2 hours): 50% remains. The scan is typically performed during this window, when the tracer is most active • After 220 minutes (~4 hours): 25% remains • After 6 hours: Less than 7% remains • After 12 hours: Less than 0.5% remains — effectively gone
By the time you wake up the morning after your scan, the radioactivity has decayed to negligible levels — without any medication, filtering, or special treatment needed. The body eliminates 18F-FDG naturally. The glucose molecule is metabolised normally. The fluorine-18 decays into stable oxygen-18 — a completely non-radioactive element — and leaves the body through urine. Drinking water after your scan simply speeds this up.
Why 110 Minutes Is the Sweet Spot The half-life of 110 minutes is not an accident , it is one of the reasons fluorine-18 is so well suited to PET/CT imaging. Here is why this specific duration matters: • Long enough to be useful: There is sufficient time to produce the tracer at Positron Diagnostics, quality test it, administer it, allow the 45–60 minute uptake period, and complete the scan — all while meaningful radioactivity is still present • Short enough to be safe: Unlike some isotopes with half-lives of days or weeks, fluorine-18 is essentially eliminated from the body within hours, minimising cumulative radiation exposure • Predictable decay: Because the half-life is so well characterised, our medical physicists can calculate the exact dose to administer to each patient, accounting for decay between production and injection How Does This Compare to Other Isotopes? Not all radioactive tracers decay this quickly. For perspective: • Technetium-99m (used in bone scans and some SPECT imaging): 6-hour half-life which is still short, but 3x longer than F-18 • Iodine-131 (used in thyroid therapy): 8-day half-life, that requires specific post-treatment precautions for several days
The short half-life of fluorine-18 is one of the key safety features that makes 18F-FDG-based PET/CT such a well-tolerated and widely used diagnostic tool worldwide.
What This Means for You Practically After your PET/CT scan at Positron Diagnostics: • Drink plenty of water to help flush the tracer through your kidneys • Avoid prolonged close contact with pregnant women and young children for the rest of that day as a precaution • You may return to normal activities the same day • No special disposal of clothing, bedding, or personal items is required
Questions about your scan or the tracer used? Our team is happy to speak with you: www.positron.health +357 22 053 555