Radioactive isotopes and radiation have two broad categories of medical application: diagnostic (using tracers and imaging) and therapeutic (using radiation to destroy diseased tissue).
A medical tracer (or radioisotope tracer) is a radioactive isotope introduced into the body — by injection, ingestion, or inhalation — that accumulates in a specific organ or tissue. External detectors measure the emitted radiation to map organ function or locate abnormalities.
| Property | Reason |
|---|---|
| Gamma emitter | Gamma rays penetrate tissue and reach external detectors; alpha/beta particles are absorbed internally, causing unnecessary dose |
| Short half-life | Long enough for the diagnostic procedure, short enough to minimise long-term radiation dose to the patient |
| Chemically bindable | Can be attached to biologically active molecules that target specific organs |
| Non-toxic | Must not cause chemical harm at the quantities used |
Technetium-99m () — the most widely used tracer. It emits low-energy gamma rays (140 keV), has a half-life of 6 hours, and can be chemically bound to many compounds to target specific organs (bone, brain, heart, kidneys). The 'm' stands for metastable — the nucleus exists in an excited state before emitting a gamma photon.
Iodine-131 () — used to assess thyroid function. The thyroid gland naturally concentrates iodine; by measuring the rate of uptake, doctors can diagnose hyperthyroidism or hypothyroidism.
Sodium-24 () — injected into the bloodstream to monitor blood flow and locate obstructions or leaks in the circulatory system.
A PET scanner is a medical imaging device that produces 3-D images of metabolic activity inside the body. It exploits electron-positron annihilation to locate the source of radiation with high precision.
Tracer injection — A positron-emitting radioisotope (e.g., Fluorine-18, , half-life ≈ 110 min) is attached to a biologically active molecule such as glucose (forming FDG — fluorodeoxyglucose) and injected into the patient.
Accumulation — The labelled glucose accumulates in metabolically active regions (e.g., tumours, active brain areas) because those cells consume more glucose.
Positron emission — The nucleus undergoes decay, emitting a positron:
Annihilation — The emitted positron travels a very short distance (< 1 mm) before meeting an electron in the surrounding tissue. They annihilate, producing two gamma-ray photons emitted in exactly opposite directions (180° apart):
Coincidence detection — A ring of detectors surrounds the patient. When two detectors register a gamma photon simultaneously (within a nanosecond window), the annihilation event is located along the line connecting those two detectors. This is called coincidence detection.
Image reconstruction — A computer analyses thousands of coincidence events to reconstruct a 3-D cross-sectional image (tomogram) showing the distribution of metabolic activity.
When the electron and positron are approximately at rest, their total momentum is zero. By the Law of Conservation of Momentum, the two photons must have equal and opposite momenta — they travel in exactly opposite directions.
The rest-mass energy of an electron (or positron) is:
When an electron and positron at rest annihilate, the total energy released is:
This energy is shared equally between the two photons, so each photon carries:
This specific energy is the signature used by PET scanner detectors to confirm that a detected photon came from an annihilation event.
Gamma rays are the preferred radiation for external detection because of their high penetrating power. Several medical applications rely on detecting gamma rays emitted from within or directed at the body:
Used with tracers like . A collimator (lead grid) allows only gamma rays travelling in a specific direction to reach a scintillation detector (sodium iodide crystal). The crystal converts gamma photons into visible light, which is then converted to an electrical signal by a photomultiplier tube, building up a 2-D image of the organ.