A patient with suspected liver disease is given a radioactive tracer that is absorbed by liver tissue. A detector outside the body is used to build up an image of the liver. Explain why the radioactive tracer used in this procedure must be a gamma emitter with a short half-life, rather than an alpha emitter with a long half-life.

OCR A-Level Physics B: Advancing Physics (H557) — 6.4 Medical imaging · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Radioactive tracers are used in medical imaging to produce images of internal organs. The tracer is introduced into the body and a detector outside the body records the radiation emitted.

Model answer (5 marks)

Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue to reach the detector outside the body.
Alpha radiation has very low penetrating power and would be absorbed by body tissue before reaching the detector.
Alpha radiation is highly ionising, which would cause significant damage to body tissue / cells.
A short half‑life means the tracer decays quickly / becomes less radioactive quickly, so the patient is not exposed to radiation for a long time / reducing the radiation dose received.
A long half‑life would mean the patient remains radioactive / contaminated for a long period of time, presenting a greater long‑term hazard.

Examiner tips

  • Use the word ‘penetrating’ to link to gamma radiation’s ability to reach the detector. Mention the ionising nature of alpha particles to justify tissue damage. Explain the benefit of a short half‑life in terms of patient safety.
  • common_mistakes
  • :
  • Confusing alpha with beta radiation or not stating the low penetration of alpha. Forgetting to mention the short half‑life’s role in reducing patient dose. Using vague terms like ‘safe’ without linking to half‑life or radiation type.

Mark scheme (5 marks)

  1. Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue to reach the detector outside the body.
  2. Alpha radiation has very low penetrating power and would be absorbed by body tissue before reaching the detector.
  3. Alpha radiation is highly ionising, which would cause significant damage to body tissue / cells.
  4. A short half-life means the tracer decays quickly / becomes less radioactive quickly, so the patient is not exposed to radiation for a long time / reducing the radiation dose received.
  5. A long half-life would mean the patient remains radioactive / contaminated for a long period of time, presenting a greater long-term hazard.

Key terms in this question

half-life

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