BRAIN SCANNING

any of a number of diagnostic methods for detecting intracranial abnormalities. The oldest of the brain-scanning procedures still in use is a simple, relatively noninvasive procedure called isotope scanning. It is based on the tendency of certain radioactive isotopes to concentrate selectively in tumours and blood vessel lesions. The procedure involves the injection of a radioactive isotope (such as technetium-99m or iodine-131) into a blood vessel that supplies the cranial region. As the substance becomes localized within the brain, it decays, therewith emitting gamma rays. The concentration of rays at a given site, as measured by a movable radiation detection device, can reveal the presence, the shape, and often the size of the intracranial abnormality. In many cases, isotope scanning has been replaced by computerized axial tomography (CAT), or computed tomography (CT). The CAT scan is a procedure in which the brain is X-rayed from many different angles. An X-ray source delivers a series of short pulses of radiation as it and an electronic detector are rotated around the head of the individual being tested. The responses of the detector are fed to a computer that analyzes and integrates the X-ray data from the numerous scans to construct a detailed cross-sectional image of the brain. A series of such images enables physicians to locate brain tumours, cerebral abscesses, blood clots, and other disorders that would be difficult to detect with conventional X-ray techniques. With the development in the mid-1970s of the CAT scan, computer-based technologies have revolutionized the field of medical diagnosis. One of the more significant new tomographic techniques is nuclear magnetic resonance (NMR) imaging. Like CAT, NMR generates images of thin slices of the brain (or other organ under study), but it does so without the hazard of X rays or other ionizing radiation. In addition NMR can reveal physiological and biochemical, as well as structural, abnormalities. (Although the benefits of NMR are myriad, the technique is not advised for individuals with pacemakers, aneurysm clips, large metallic prostheses, or dependence on iron-containing instruments.) Positron emission tomography (PET) is a computer-based procedure in which a radioactive tracer-labeled compound is introduced into the brain (or other organ under study), and its behaviour is tracked. This information, with computer modeling, eventually yields a cross-sectional image of the physiological process under study.