3.10.6.1
Radioactive tracers
Medical Physics | AQA A-Level Physics
- A radioactive tracer is a substance containing a radioactive isotope that is introduced into the body. Its movement and concentration can be tracked from outside using a detector.
- Common medical tracers:
- Technetium-99m (Tc-99m): emits gamma rays at 140 keV, half-life of 6 hours. The most widely used tracer because of its short half-life and gamma-only emission (no alpha or beta particles to damage tissue).
- Iodine-131 (I-131): emits beta and gamma radiation, half-life of 8 days, energy 360 keV. Used for thyroid imaging and treatment.
- Indium-111 (In-111): emits gamma rays at 170 and 250 keV, half-life of 68 hours.
- Tc-99m is produced from a molybdenum-technetium generatorA portable device containing Mo-99 (half-life 66 hours) which decays into Tc-99m. The Tc-99m is periodically washed out (eluted) with saline for use. This allows hospitals to have a continuous supply without a nuclear reactor on site.. Mo-99 (half-life 66 hours) decays to Tc-99m, which is eluted (washed out) with saline when needed.
- An ideal tracer should: emit gamma radiation (penetrates tissue to reach the detector), have a short half-life (to minimise dose), be non-toxic, and be metabolised by the target organ.