Slightly different from X-ray imaging is radionuclide scanning, which uses radioactive isotopes. These are unstable substances that emit gamma rays as they decay or change into a more stable state. The radioisotopes used in medicine decay quickly and give off only weak amounts of radiation, so that the risk of radiation is low. The isotope being used is injected into the body in suspension or solution. The type of isotope is chosen for the body’s ability to concentrate it in the part of the body under investigation. For example, the thyroid gland normally uses iodine to produce hormones, and so the radioactive isotope iodine-131 will, if introduced into the body, be concentrated in the thyroid gland. Technetium-99, collects in the liver, from where the isotope emits its gamma rays which are in turn picked up by a scintillation counter; pictures are generated from this by a rectilinear scanner or gamma-camera. Normal body metabolism removes the isotope eventually.
A radionuclide scan of, for instance, the thyroid gland will give information about tissue within this organ. A benign lump of functioning thyroid tissue will collect a high concentration of the radioactive iodine and show as an area of high activity – a ‘hot nodule’. But a malignant tumour may fail to take up radioiodine and so show as a dull spot – a ‘cold nodule’. In the latter case, early removal of the lump would usually be advised by the doctor.