Radioactivity Health Dictionary

The radiological examination of the KIDNEYS using a gamma camera. This is a device that can follow the course of an injected radioactive (see RADIOACTIVITY) compound which is concentrated and excreted by the kidneys. This provides information on kidney function.... renography

This is a form of a chemical element with the same chemical properties as other forms, but which has a di?erent atomic mass. It contains an identical number of positively charged particles called protons, in the nucleus, giving it the same atomic number, but the numbers of neutrons di?er. A radioactive isotope, or radionuclide, is one that decays into other isotopes, and in doing so emits alpha, beta or gamma radiation.

Applications of radionuclides to diagnosis The use of radionuclides in diagnosis 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, 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 body pool of the material is therefore not appreciably altered, and metabolism is not disturbed. Thus in studies of iodine metabolism the ratio of radioactive atoms administered to stable atoms in the body pool is of the order of 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. Therefore it is theoretically 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 necessitates tracing the path of the material through the body by means of external counting over the body surface, it is obviously essential to use an isotope that emits gamma radiation or positrons. If, however, only measurements on blood sample or excreta are required, it is possible to use pure beta emitters. Whole-body counters measure the total radioactivity in the body, and these are of great value in absorption studies.

Moving images can provide information on body functions such as the movements of the heart, blood ?ow, bile ?ow in the liver, and urine in the kidneys. The development of COMPUTED TOMOGRAPHY or CT scanning has replaced radionuclide scanning for some imaging procedures.

Five main groups of diagnostic uses may be de?ned:

(1) METABOLIC STUDIES 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 isotopes of calcium enable kinetic studies of bone formation and destruction to be performed.... isotope

A unit of radioactivity (see radiation units).... becquerel

Genes found in every cell that control growth, repair, and replacement.

Abnormalities of oncogenes are known to be a factor in the development of cancerous cells.

Mutations in oncogenes, resulting from damage by carcinogens, can cause a cell to grow unrestrainedly and infiltrate and destroy normal tissues (see cancer).

Factors known to cause cancer include ultraviolet light, radioactivity, tobacco, alcohol, asbestos, some chemicals, and certain viruses.... oncogenes

n. a device, used in diagnostic radiology or radiotherapy, to produce a narrow beam of radiation by means of metallic sheets, acting like a diaphragm in a camera, that control the size of the beam from a radiation source. Many newer *linear accelerators use multi-leaf collimators, a specialized form of collimator using individual ‘leaves’ (1 cm or smaller) to shape the radiation *treatment field around the tumour. Collimators are also used on radiation detectors, in particular in *gamma cameras, for which the exact source of radioactivity needs to be known to produce an accurate image.... collimator

an apparatus that detects gamma rays emitted by radioactive isotopes that have been introduced into the body as *tracers. It contains an activated sodium iodide crystal (see scintillator) and a large array of photomultiplier tubes. Using lead *collimators, the position of the source of the radioactivity can be plotted and displayed on a TV monitor or photographic film (see scintigram).... gamma camera

(thallium scan) a method to detect and quantify myocardial *ischaemia. An intravenously injected *radionuclide that is taken up by normal heart muscle can be imaged using a *gamma camera. Areas of scar due to *myocardial infarction emit little or no radioactivity and are seen as ‘cold spots’. Exercise is mimicked by infusing drugs to increase the heart rate in order to provoke cold spots in the diagnosis of ischaemic heart disease.... myocardial perfusion scan

n. a technique using radioactive antibodies as *tracers to estimate the levels of natural substances in a blood sample. The antibodies bind to antigens, and the amount of radioactivity trapped is a measure of the amount of the target antigen present. The technique is widely used in the estimation of hormone levels.... radioimmunoassay

n. a diagram showing the distribution of radioactive *tracer in a part of the body, produced by recording the flashes of light given off by a *scintillator as it is struck by radiation of different intensities. This technique is called scintigraphy. By scanning the body, section by section, a ‘map’ of the radioactivity in various regions is built up, aiding the diagnosis of cancer or other disorders. Such a record is known as a scintiscan. These images are now usually obtained using a *gamma camera.... scintigram