6.35P Explain why isotopes used in PET scanners have to be produced nearby

The idea

PET scanners use positron-emitting radioisotopes, typically fluorine-18, which has a half-life of about 110 minutes. Activity falls by half every half-life, so after a few hours most of the tracer has decayed and the sample is too weak to give a useful scan. A short half-life also means the patient receives a lower dose over the investigation. Because the isotope cannot be stored, it must be made in a cyclotron and delivered quickly, so production must be close to the hospital scanner. A long journey wastes decay time and reduces the activity reaching the patient, so nearby production keeps the tracer usable.

What earns the marks

• States that the isotope has a short half-life so its activity falls quickly.
• Explains that a long journey would leave too little activity for a useful scan.
• Links nearby production to minimising decay between manufacture and use.
• Identifies that the isotope cannot be stored for long periods.

Common mistakes and how to put them right

• Confuses half-life with the time a patient stays in the scanner.

  Put it right: Keep the focus on decay of the tracer.

• Assumes the isotope is dangerous because it lasts a long time.

  Put it right by noting the short half-life limits the dose.

• Ignores transport time and claims distance does not matter.

  Put it right: Explain that decay during travel reduces activity.

Exam tips

• Name the short half-life as the key reason before adding transport detail.
• Use the idea of activity halving with each half-life to justify urgency.
• Keep the answer focused on decay and delivery, not on scanner cost.