X-ray Applications

The principal uses of X rays are in the field of scientific research, industry, and medicine.

Research.

The study of X rays played a vital role in theoretical physics, especially in the development of quantum mechanics. As a research tool, X rays enabled physicists to confirm experimentally the theories of crystallography. By using X-ray diffraction methods, crystalline substances may be identified and their structure determined. Virtually all present-day knowledge in this field was either discovered or verified by X-ray analysis. X-ray diffraction methods can also be applied to powdered substances that are not crystalline but that display some regularity of molecular structure. By means of such methods, chemical compounds can be identified and the size of ultramicroscopic particles can be established. Chemical elements and their isotopes may be identified by X-ray spectroscopy, which determines the wavelengths of their characteristic line spectra. Several elements were discovered by analysis of X-ray spectra.

A number of recent applications of X rays in research are assuming increasing importance. Microradiography, for instance, produces fine-grain images that can be enlarged considerably. Two radiographs can be combined in a projector to produce a three-dimensional image called a stereoradiogram. Color-radiography is also used to enhance the detail of X-ray photographs; in this process, differences in the absorption of X rays by a specimen are shown as different colors (see COLOR,). Extremely detailed and analytical information is provided by the electron microprobe, which uses a sharply defined beam of electrons to generate X rays in an area of specimen as small as 1 micron (about 1/25,000 in) square.

Industry.

In addition to the research applications of X rays in physics, chemistry, mineralogy, metallurgy, and biology, X rays are used in industry as a research tool and for many testing processes. They are valuable in industry as a means of testing objects such as metallic castings without destroying them. X-ray images on photographic plates reveal the presence of flaws, but a disadvantage of such inspection is that the necessary high-powered X-ray equipment is bulky and expensive. In some instances, therefore, radioisotopes, which emit highly penetrating gamma rays, are used instead of X-ray equipment. These ISOTOPE, (q.v.) sources can be housed in relatively light, compact, and shielded containers. Cobalt-60 and cesium-137 have been used widely for industrial radiography. Thulium-70 has been used in small, convenient, isotope projectors for some medical and industrial applications.

Many industrial products are inspected routinely by means of X rays so that defective products may be eliminated at the point of production. Other applications include the detection of fake gems and the detection of smuggled goods in customs examinations. Ultrasoft X rays are used to determine the authenticity of works of art and for art restoration.

Medicine.

X-ray photographs, called radiographs, and fluoroscopy are used extensively in medicine as diagnostic tools. In radiotherapy, X rays are used to treat certain diseases, notably cancer, by exposing tumors to X radiation. See CANCER,; RADIATION EFFECTS, BIOLOGICAL,; RADIOLOGY,.

The use of radiographs for diagnostic purposes was inherent in the penetrating properties of X rays. Within a few years of their discovery, X rays were being used to locate foreign bodies, such as bullets, within the human body. With the development of improved X-ray techniques, minute differences in tissues were revealed by radiographs, and many pathological conditions could be diagnosed by means of X rays. X rays provided the most important single method of diagnosing TUBERCULOSIS, (q.v.) when that disease was prevalent. Pictures of the lungs were easy to interpret because the air spaces are more transparent to X rays than the lung tissues. Various other cavities in the body can be filled artificially with contrasting media, either more transparent or more opaque to X rays than the surrounding tissue, so that a particular organ is brought more sharply into view. Barium sulfate, which is highly opaque to X rays, is used for the X-ray examination of the gastrointestinal tract. Certain opaque compounds are administered either by mouth or by injection into the bloodstream in order to examine the kidneys or the gallbladder. Such dyes can have serious side effects, however, and should be used only after careful consultation. The routine use of X-ray diagnosis has in fact been discouraged in recent years--by the American College of Radiology in 1982, for example--as of questionable usefulness.

A recent X-ray device, used without dyes, offers clear views of any part of the anatomy, including soft organ tissues. Called the body scanner, or computerized axial tomography (CAT or CT) scanner, it rotates 180° around a patient's body, sending out a pencil-thin X-ray beam at 160 different points. Crystals positioned at the opposite points of the beam pick up and record the absorption rates of the varying thicknesses of tissue and bone. These data are then relayed to a computer that turns the information into a picture on a screen. Using the same dosage of radiation as that of the conventional X-ray machine, an entire "slice" of the body is made visible with about 100 times more clarity. The scanner was invented in 1972 by the British scientist Godfrey N. Hounsfield, and was in general use by 1979. E.C.W., EARNEST C. WATSON, Ph.B., D.Sc. & J.T.S., JOHN T. SUCHY, M.A.

For applications of radioisotopes that emit gamma rays, see ISOTOPIC TRACER,. See also NUCLEAR MAGNETIC RESONANCE.