This is a form of a chemical element with the same chemical properties as other forms, but which has a different atomic mass. It contains an identical number of positively charged particles called protons in the nucleus, giving it the same atomic number and thus identity as an element, but the numbers of neutrons differ. A radioactive isotope, or radionuclide, is one that has an unstable nucleus and undergoes decay with emission of alpha, beta or gamma radiation.
The use of radionuclides in diagnosis, known as nuclear medicine or more recently molecular imaging, is based on the fact that it is possible to tag many of the substances normally present in the body with a radioactive label. Certain synthetic radioactive elements, such as technetium–99m, can also be used. Because it is possible to detect minute quantities of radioactive material, only very small doses are needed, making the procedure a safe one. Furthermore, the total amount of the material in the body is altered very little so metabolism is not disturbed. For example, in studies of iodine metabolism the ratio of radioactive atoms administered to stable atoms already in the body is about 1:1,000 million.
By measuring radioactivity in the body, in blood samples, or in the excreta it is possible to gain information about the fate of the labelled substance, and hence of the chemically identical inactive material already in the body. Therefore it may be possible to trace the absorption, distribution and excretion of any substance normally present in the body, provided that it can be tagged with a suitable radioactive label.
If the investigation means having to trace the path of the material through the body by external counting of radioactivity over the body surface, the isotope must emit gamma radiation or positrons. This generally involves planar imaging with a gamma camera, three-dimensional imaging with single photon emission computed tomography (SPECT), or positron emission tomography (PET).
Moving images can provide information on body functions such as the movements of the heart, blood flow, bile flow in the liver, and urine in the kidneys. COMPUTED TOMOGRAPHY (CT) has replaced radionuclide scanning for many imaging procedures. Hybrid devices combine the anatomical detail from CT with functional information from SPECT or PET.
Five main groups of diagnostic uses may be defined:
The use of radioactive materials in metabolic studies is based on the fundamental property that all isotopes of an element are chemically identical. The radioactive isotope is used as a true isotope tracer – that is, when introduced into the body (in whatever form) it behaves in the same way as the inactive element. For example, isotopes of iodine are used to measure thyroid function (see THYROID GLAND), and fluorine–18 labelled fluorodeoxyglucose to detect the increased transport and metabolism of glucose in tumours.
The fate of labelled substances given by mouth can be followed to assess their absorption, utilisation and excretion. In most of these studies the isotope is a true isotope tracer. For example, iron absorption can be measured with radioactive iron; vitamin B12 absorption may be investigated with vitamin B12 tagged with radioactive cobalt.
By introducing an isotope into a ‘compartment’, such as the blood or extracellular space, it is possible to measure the volume of that compartment by determining the dilution of radioactivity when equilibrium has been reached, such as measurement of total blood volume by use of chromium–51 labelled ERYTHROCYTES.
In this type of study the isotope is not necessarily used as a true isotopic tracer. In other words, it does not trace the path of the corresponding inactive isotope. For example, xenon–133 is used in measurements of blood flow in muscles, and in lung-function studies; krypton–85 is used to detect intracardiac shunts (abnormal blood flows in the heart). Neither of these elements is normally present in the body. The survival of ERYTHROCYTES may be followed and any organ where they are being stored (sequestration) revealed by labelling them with chromium–51.
Scanning is a technique which is used to determine the distribution of radioactive isotopes within the body or within one particular organ. The gamma camera maps the distribution of radioactivity in the body within its field of view, and a composite planar image is created as the camera moves over the body or a three-dimensional image computed from images taken at various angles. A bone scan obtained following injection of a technetium–99m complex of phosphate can be used to screen for metastatic spread of breast or prostate cancer to the skeleton. A toxic nodule in the thyroid may be identified by its selective concentration of radioiodine. Myocardial perfusion imaging is used to stratify patients at risk of sudden death due to coronary artery disease.
Radioactive isotopes are also used in medical treatment. The overactivity of the thyroid gland in thyrotoxicosis can be treated by swallowing iodine–131. The ingested iodine is taken up by the thyroid gland where it irradiates the gland, so reducing its activity. Phosphorus–32 is largely taken up in bone as this is the main source of body phosphate, and irradiation of the bone marrow results, controlling the overactivity that is characteristic of polycythaemia rubra vera (see under POLYCYTHAEMIA). In the related but separate field of cobalt teletherapy, cobalt–60 is used to deliver high energy gamma radiation to treat tumours. (See RADIOTHERAPY.)