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.
(4) PHYSICAL TRACING STUDIES 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 the organ of sequestration revealed by labelling them with radioactive chromium.
(5) SCANNING OF ORGANS AND TISSUES Scanning is a technique which is used to determine the distribution of radioactive isotopes within the body or within one particular organ. In the conventional scanner, the radiation detector – which is a scintillation counter – ‘sees’ only a small cross-sectional area of the body at a time. The activity ‘seen’ at each point is registered, and a ‘map’ of the activity seen over the scanned area is recorded. Various methods of presentation have been used, and the recently improved display systems present the information gathered by the scanner more eﬀectively. More recent developments are stationary detectors such as the gamma camera, auto-ﬂuoroscope, and other devices which can view the whole of the area simultaneously. Thus when selective concentration of an isotope in a tissue occurs, it is possible to examine the distribution of that isotope by means of scanning. A toxic nodule in the thyroid may be identiﬁed by its selective concentration of iodine-131. Areas of absent function on the radioactive scan (‘cold’ areas) suggest the presence of tumours, abscesses, and similar lesions. Iodine-131 may be used to localise tumours of the thyroid, and chlormerodrin labelled with mercury-197 to delineate tumours of the kidneys. Of even greater practical application is the localising of brain tumours with human serum albumin labelled with iodine-131 or with radioactive technetium.
Treatment Radioactive isotopes are also used in medical treatment. The overactivity of the thyroid gland in thyrotoxicosis can be treated by the ingestion of radioactive iodine. The ingested iodine is taken up by the thyroid gland where local irradiation of the gland takes place, reducing its activity. Radioactive phosphorus is used in the treatment of polycythemia rubra vera. It 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 cobalt teletherapy the isotope cobalt 60 is used to deliver 1·2–1·3 million volt radiation which is equivalent to X-rays generated at a peak voltage of 3–4 million volts. (See RADIOTHERAPY.)