Technetium-99m is a radioactive isotope used as a medical tracer. Explain why Technetium-99m is suitable for use as a medical tracer, including reference to its half-life, the type of radiation it emits, and the difference between irradiation and contamination in this context.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Technetium-99m is injected into a patient's bloodstream and travels through the body, where it can be detected by external equipment to produce images of internal organs. It has a half-life of 6 hours and is a gamma emitter.
Model answer (5 marks)
Technetium‑99m has a half‑life of 6 h, which is short enough that the activity falls to negligible levels within a day, minimising the patient’s radiation dose.
The 6 h half‑life is long enough for the isotope to circulate through the bloodstream, localise in the target organ and be detected by a gamma camera before it decays.
It emits only gamma radiation (≈140 keV). Gamma rays can penetrate body tissues and be recorded externally, giving a clear image without the need for invasive sampling.
The patient is irradiated: the isotope delivers a dose of radiation to the body, but the patient does not become radioactive themselves because the isotope decays rapidly.
In contrast, contamination would leave radioactive atoms on or in the patient for a long time, continuing to emit radiation. Irradiation from a short‑lived tracer is therefore far less hazardous.
The 6 h half‑life is long enough for the isotope to circulate through the bloodstream, localise in the target organ and be detected by a gamma camera before it decays.
It emits only gamma radiation (≈140 keV). Gamma rays can penetrate body tissues and be recorded externally, giving a clear image without the need for invasive sampling.
The patient is irradiated: the isotope delivers a dose of radiation to the body, but the patient does not become radioactive themselves because the isotope decays rapidly.
In contrast, contamination would leave radioactive atoms on or in the patient for a long time, continuing to emit radiation. Irradiation from a short‑lived tracer is therefore far less hazardous.
Examiner tips
- Use the exact half‑life value (6 h) and explain both its shortness and sufficiency. Mention the gamma energy and its external detectability. Clarify the difference between irradiation (dose to patient) and contamination (persistent radioactivity).
Common mistakes
- Saying the half‑life is too long or too short. Confusing gamma emission with beta or alpha. Mixing up irradiation with contamination or forgetting to mention the patient is not made radioactive.
Mark scheme (5 marks)
- Technetium-99m has a half-life of 6 hours, which is short enough that it decays away relatively quickly after the procedure.
- The half-life is long enough for the isotope to flow through the body and be detected before it decays away.
- Technetium-99m is a gamma emitter, meaning gamma radiation can pass through body tissue and be detected externally.
- The patient is irradiated by the Technetium-99m, meaning they are exposed to radiation from the source but do not become permanently radioactive themselves.
- Contamination would be hazardous because radioactive atoms would remain on or in the patient for a long time, continuing to emit radiation; irradiation from a short half-life tracer is less hazardous because activity quickly decreases.
Key terms in this question
half-life · irradiation · medical tracer
Related
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