Technetium-99m is widely used as a radioactive tracer in hospitals. Explain why technetium-99m is well suited for use as a medical tracer, referring to its half-life, the type of radiation it emits, and the safety implications for the patient.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A patient at a hospital is injected with a small amount of technetium-99m so that doctors can monitor blood flow through an organ. Technetium-99m is a gamma emitter with a half-life of 6 hours, and it decays into a stable isotope that can be excreted by the body.
Model answer (5 marks)
Technetium‑99m is a gamma emitter; gamma rays are highly penetrating and can exit the body, allowing external detectors to image the tracer distribution.
Alpha or beta emitters would be absorbed within the tissues and would not reach the detector, so they are unsuitable.
Its half‑life is 6 h – long enough for the tracer to circulate and be imaged, yet short enough that the activity falls to negligible levels before the patient leaves the hospital.
The short half‑life therefore limits the patient’s total radiation dose and reduces long‑term risk.
After decay the isotope becomes stable technetium‑99, which is non‑radioactive and can be excreted, preventing any long‑term build‑up of radioactivity in the body.
Alpha or beta emitters would be absorbed within the tissues and would not reach the detector, so they are unsuitable.
Its half‑life is 6 h – long enough for the tracer to circulate and be imaged, yet short enough that the activity falls to negligible levels before the patient leaves the hospital.
The short half‑life therefore limits the patient’s total radiation dose and reduces long‑term risk.
After decay the isotope becomes stable technetium‑99, which is non‑radioactive and can be excreted, preventing any long‑term build‑up of radioactivity in the body.
Examiner tips
- Use the word ‘gamma emitter’ and explain penetration; mention alpha/beta unsuitable. Show the 6‑h half‑life as ‘long enough for imaging but short enough to minimise dose’. State that the decay product is stable and excreted. Keep each point concise, matching the 5 marks.
Common mistakes
- Confusing gamma with beta radiation; writing ‘gamma rays are absorbed’. Forgetting to mention the stable decay product. Giving an incorrect half‑life or not linking it to dose reduction.
Mark scheme (5 marks)
- Technetium-99m is a gamma emitter, and gamma radiation is the most penetrating type, so it can pass out through body tissue and be detected externally.
- Alpha or beta radiation would be absorbed by the body/tissue before it could be detected, making them unsuitable as tracers.
- A half-life of 6 hours is long enough for the tracer to flow through the body and be detected / for the medical procedure to be completed.
- The short half-life means the technetium-99m decays away quickly, so the patient is not exposed to radiation for a long time / reducing long-term risk to the patient.
- Technetium-99m decays into a stable, safe isotope that can be excreted by the body, so there is no long-term build-up of radioactive material inside the patient.
Key terms in this question
radioactive tracer · half-life
Related
- All OCR A-Level Physics A (H556) revision notes →
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