Gamma-Emitting Radioisotopes as Tracers Revision Notes | A Level AQA Physics

1. Why Use Radiation Inside the Body?

Picture a patient whose thyroid gland may be overactive. A surgeon cannot see inside without operating, and a standard X-ray shows structure but not function. The question is: how do you watch an organ working in real time, from outside the body?

The answer is a radioactive tracer — a radioisotope that travels to the organ of interest and emits radiation that passes out through the body to a detector. The detector builds up an image showing where the isotope has concentrated, revealing how the organ is functioning.

Not every type of radiation works here. Alpha particles are stopped within a few millimetres of tissue (or a few centimetres in air) and would never reach an external detector. Beta particles travel further but are still largely absorbed before leaving the body and deposit a significant dose in surrounding tissue. Gamma radiation, however, is highly penetrating and passes through the body to be detected externally — which is precisely why gamma emitters are chosen as tracers.

The tracer must also be chemically inert or, more usefully, attached to a molecule the body naturally directs to a specific organ. This combination of nuclear physics and biochemistry is what makes radionuclide imaging so powerful.

2. The Properties a Good Tracer Must Have

Before looking at specific isotopes, it helps to establish the criteria against which any tracer is judged. These criteria follow directly from the physics and biology of the situation.

• Emits gamma radiation only (or predominantly), so the radiation penetrates tissue and reaches the external detector without depositing excessive dose in the patient.
• Has a gamma energy in a range detectable by a gamma camera — typically between about 100 keV and 300 keV. Too low and the photons are absorbed before leaving the body; too high and the detector cannot stop and measure them efficiently.
• Has a half-life long enough to allow the tracer to reach the target organ and for imaging to be completed, but short enough that the patient's radiation dose is minimised after the procedure.
• Can be chemically labelled — attached to a compound that has a biological affinity for the target organ, so the tracer concentrates where imaging is needed.
• Is produced in a form that can be prepared and administered safely in a clinical setting.

These criteria create a set of trade-offs. A very short half-life minimises dose but may not allow enough time for imaging. A very long half-life gives plenty of imaging time but irradiates the patient long after the scan is complete. The ideal tracer sits in a carefully balanced middle ground.

3. Technetium-99m: The Benchmark Tracer

Technetium-99m (written $^{99m}\text{Tc}$) is the most widely used medical tracer in the world, and understanding why reveals exactly what the ideal tracer looks like. The 'm' stands for metastable — the nucleus is in an excited energy state and releases that energy by emitting a gamma photon, without emitting alpha or beta radiation in the process.

The 140 keV gamma energy sits almost perfectly in the detection window of a standard gamma camera. The photons are energetic enough to escape the body without significant absorption, yet not so energetic that the detector fails to stop and register them. This energy is often described as near-ideal for clinical imaging.

The six-hour half-life is equally well-suited. A typical scan takes one to four hours from administration to completion. After 24 hours (four half-lives), the activity has fallen to $\frac{1}{16}$ of its initial value, meaning the patient's dose drops rapidly once imaging is done. The isotope is also produced on-site from a molybdenum-99 generator, making it readily available in hospital nuclear medicine departments.

Crucially, technetium chemistry is versatile. Technetium-99m can be attached to a large number of different pharmaceutical compounds — each with an affinity for a different organ. Labelled with methylene diphosphonate (MDP), it concentrates in bone. Labelled with sestamibi, it targets heart muscle. This flexibility makes it the workhorse of nuclear medicine.

4. Iodine-131: Targeted Therapy and Imaging

Iodine-131 ($^{131}\text{I}$) takes a different approach. Unlike technetium-99m, which relies entirely on chemical labelling to reach its target, iodine-131 exploits a natural biological process: the thyroid gland actively absorbs iodine from the bloodstream to produce thyroid hormones. Administer iodine-131 and the thyroid concentrates it automatically.

The gamma emission at 364 keV allows external imaging of the thyroid, showing whether it is overactive, underactive, or contains abnormal nodules. However, iodine-131 also emits beta particles, which deposit energy locally in thyroid tissue. For diagnostic imaging alone, this beta emission is a disadvantage — it increases the radiation dose to the patient without contributing to the image.

This is where iodine-131 becomes a dual-purpose isotope. When used therapeutically — for example to destroy cancerous thyroid tissue — the beta emission is the active agent. The beta particles have a range of only a few millimetres in tissue, so they irradiate the thyroid selectively while sparing surrounding structures. The eight-day half-life is longer than technetium-99m, which means the patient carries a higher cumulative dose, but for therapy this is acceptable because the goal is to deliver a controlled, localised dose to destroy abnormal cells.

For pure diagnostic imaging of the thyroid, iodine-123 is now often preferred over iodine-131 because it emits gamma radiation only and has a shorter half-life, reducing patient dose. However, iodine-131 remains the standard for thyroid therapy and is still used diagnostically where iodine-123 is unavailable.

5. Indium-111: Imaging Infection and Tumours

Indium-111 ($^{111}\text{In}$) fills a clinical niche that neither technetium-99m nor iodine-131 can easily address: imaging sites of infection, inflammation, and certain tumours. Its properties reflect this specialised role.

The 2.8-day half-life is longer than technetium-99m but shorter than iodine-131. This is necessary because the clinical procedures using indium-111 take longer. When white blood cells are labelled with indium-111 and re-injected, they migrate to sites of infection over 24 to 48 hours. The isotope must remain active long enough for this migration to occur and for imaging to be completed — a six-hour half-life would be too short.

The two gamma energies (171 keV and 245 keV) are both within a usable range for gamma camera detection, though neither is as close to the camera's optimum as technetium-99m's 140 keV. Indium-111 emits no alpha or beta particles; the primary imaging dose comes from its gamma photons, though electron capture also produces Auger electrons — an advantage for a diagnostic-only isotope compared with tracers that emit beta particles.

The labelling chemistry for indium-111 is more complex than for technetium-99m. Indium-111 is typically chelated — bound within a cage-like organic molecule — before being attached to white blood cells or targeting peptides. This additional preparation step means indium-111 scans are more resource-intensive, which is one reason technetium-99m is preferred when it can do the job.

6. Comparing the Three Tracers

Now that all three isotopes have been introduced, it is worth placing them side by side. Exam questions frequently ask you to compare tracers or justify a clinical choice — so being able to articulate the differences precisely is essential.

Notice the pattern: the choice of tracer is always a balance between the half-life needed for the clinical procedure, the radiation type and energy needed for safe and effective detection, and the chemical properties needed to direct the isotope to the right organ.

7. Radiolabelling: Directing the Tracer to Its Target

The physics of gamma emission gets the radiation out of the body. The chemistry of radiolabelling gets the isotope to the right place inside the body. These two aspects work together, and the AQA specification requires you to understand both.

Radiolabelling means attaching a radioisotope to a compound that has a biological affinity for a specific organ or tissue type. The compound acts as a carrier, transporting the isotope through the bloodstream to its destination. Once there, the compound is taken up by the target tissue, and the isotope concentrates in that region. A gamma camera positioned outside the body then detects the emitted photons and maps their origin, producing a functional image.

For technetium-99m, the range of available labelling compounds is exceptionally wide. Methylene diphosphonate (MDP) mimics phosphate and is incorporated into bone mineral, making it ideal for bone scans to detect fractures, metastases, or infection. Macroaggregated albumin (MAA) particles are too large to pass through lung capillaries and become temporarily trapped there, enabling lung perfusion scans. Sestamibi is taken up by metabolically active heart muscle, allowing myocardial perfusion imaging. The same isotope, different carrier, different organ — this versatility is a major clinical advantage.

For indium-111, the labelling process involves chelation. The indium ion is bound within an organic chelating agent (such as DTPA or oxine), which then binds to the surface of white blood cells without damaging their function. The labelled cells are re-injected and behave normally, migrating to sites of infection where the indium-111 activity can be imaged 24 to 48 hours later.

Worked example: Evaluating Tracer Suitability: An Exam-Style Worked Example

A patient requires imaging of a suspected bone metastasis. Evaluate whether technetium-99m labelled with MDP or indium-111 labelled with white blood cells is the more appropriate tracer for this procedure. [4 marks]

1. Target: bone tissue. Bone scans are typically completed within 2–4 hours of tracer administration. — Establishing the clinical context tells you what half-life and labelling chemistry are needed.
2. Half-life: 6 hours. Gamma energy: 140 keV (optimal for gamma camera). MDP has affinity for bone mineral. Imaging completed well within one half-life. — The short half-life is sufficient for the procedure and minimises patient dose. The labelling compound directly targets bone. [1 mark: correct identification of half-life suitability; 1 mark: reference to MDP affinity for bone]
3. Half-life: 2.8 days. White blood cells migrate to sites of infection/inflammation, not specifically to bone metastases. Imaging requires 24–48 hours. — Indium-111 labelled white blood cells are designed to locate infection, not tumour metastases. The longer half-life also means a higher cumulative dose for a diagnostic scan. [1 mark: correct identification that white blood cell labelling targets infection, not metastases]
4. Technetium-99m with MDP is the more appropriate tracer because: (i) MDP has a direct affinity for bone mineral, concentrating at sites of increased bone turnover such as metastases; (ii) the 6-hour half-life is well matched to the imaging timescale; (iii) gamma-only emission at 140 keV minimises patient dose and is optimal for the gamma camera. — [1 mark: justified conclusion referencing at least two relevant properties]

Answer: Technetium-99m labelled with MDP is the more appropriate tracer. MDP has a biological affinity for bone mineral and concentrates at sites of increased bone turnover. The 6-hour half-life matches the imaging timescale, and gamma-only emission at 140 keV minimises patient dose while providing optimal gamma camera detection.

8. Exam Technique and Common Mistakes

Questions on tracers appear regularly in the medical physics section of AQA A Level papers. They test recall of specific properties, but the higher-mark questions ask you to evaluate, compare, or justify — and that requires more than memorised numbers.

When a question asks you to 'explain why gamma radiation is used rather than alpha or beta', the mark scheme wants a chain of reasoning: gamma is highly penetrating, so it passes through body tissue to reach the external detector; alpha and beta are absorbed within the body and would not be detected externally. Stating 'gamma is more penetrating' alone typically earns one mark; adding 'so it can be detected outside the body' earns the second.

For 'compare' questions on two tracers, structure your answer around the same properties for each isotope: radiation type, gamma energy, half-life, and labelling method. Examiners award marks for explicit comparisons ('technetium-99m has a shorter half-life than iodine-131, so it delivers a lower cumulative dose to the patient'), not for describing each isotope in isolation.