The isotopes used in PET scanners have very short half-lives. Fluorine-18 has a half-life of about 110 minutes, carbon-11 about 20 minutes and oxygen-15 only about 2 minutes. This is what makes them safe for patients, because the activity soon falls and the dose stays small. It also creates a problem, because the activity starts falling the moment the isotope is made.
Each half-life that passes halves the activity. After one half-life 50 per cent is left, and every further half-life halves what remains again, so the activity keeps shrinking. If a delivery took several half-lives, the tracer would have decayed so much that too little activity remained to give a clear image.
For this reason PET isotopes are made in a particle accelerator called a cyclotron that is at, or very close to, the hospital. The tracer is prepared and injected quickly, so that it can be used before most of its activity has gone. It is produced near the scanner for exactly this reason.
