Using tracers inside the body
A radioactive tracer is a radioactive substance introduced into the body so that the distribution or activity of particular tissues and organs can be investigated.
The radioisotope can be attached to a molecule that is naturally taken up by a particular tissue. A molecule labelled in this way forms a radiopharmaceutical.
The radiopharmaceutical should travel through the body without significantly changing the process being investigated. Its chemical properties determine where it accumulates, while its radioactive decay makes the distribution detectable outside the patient.
Properties of a useful diagnostic tracer
- It should emit gamma radiation so photons can escape from the body and be detected externally.
- The gamma photons should have enough energy to leave the body while unnecessary ionisation is kept low.
- The physical half-life should be short enough to limit prolonged irradiation but long enough for the investigation to be completed.
- The isotope must be capable of labelling a compound with an affinity for the organ or process being investigated.
Technetium-99m
Technetium-99m is a widely used diagnostic tracer. It emits gamma radiation of about and has a physical half-life of about hours.
Its chemistry allows it to label many different radiopharmaceuticals, so it can be used to investigate a wide range of organs, blood flow and tumours.
Iodine-131
Iodine-131 has a physical half-life of about days and emits both gamma radiation and particles.
Iodine is absorbed by the thyroid, making this isotope useful for investigation and treatment of thyroid tissue.
Indium-111
Indium-111 is a gamma emitter with a physical half-life of about hours.
It can label antibodies and blood cells, making it useful for locating infections and investigating some blood disorders and uncommon tumours.
| Tracer | Radiation | Physical half-life | Gamma energy | Example use |
|---|---|---|---|---|
| Technetium-99m | h | Many organs, blood flow and tumours | ||
| Iodine-131 | and | d | about | Thyroid investigation and treatment |
| Indium-111 | h | about and | Labelling antibodies and blood cells |
The molybdenum–technetium generator
The short -hour half-life of technetium-99m is useful for reducing patient exposure, but it also means the isotope cannot conveniently be produced far from the hospital and stored for long periods.
Molybdenum-99 has a longer physical half-life of about hours and decays to produce technetium-99m. This makes it practical to transport molybdenum-99 to a hospital and generate technetium-99m there when required.
Transport
Molybdenum-99 is produced away from the hospital. Its longer half-life allows useful activity to remain during transport.
Decay
Inside the hospital generator, molybdenum-99 decays and continuously produces technetium-99m.
Extract
A saline solution is passed through the generator so technetium-99m can be collected and prepared for use in a radiopharmaceutical.
The generator combines the transport advantage of the longer-lived parent isotope with the lower patient exposure provided by the short-lived technetium-99m daughter.
What tracer concentration shows
Regions containing more tracer produce more detectable gamma photons, so the resulting image can provide information about function as well as structure.
- A glucose-labelled tracer can reveal regions with unusually high metabolic activity.
- Labelled white blood cells can accumulate around an infection.
- Labelled blood cells can be used to investigate blood distribution.
Exam Tip: When explaining why a tracer is suitable, separate its nuclear properties from its chemical properties. Radiation type, energy and half-life determine detectability and dose; chemical affinity determines where the tracer accumulates.