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Radiology => Magnetic Resonance Imaging => X-Rays
X-Rays
INTRODUCTION
X Ray, penetrating electromagnetic radiation, having a shorter wavelength than light, and produced by bombarding a target, usually made of tungsten, with high-speed electrons (see Cathode Ray; Electromagnetic Radiation; Electron; Light; Radiation). X rays were discovered accidentally in 1895 by the German physicist Wilhelm Conrad Roentgen while he was studying cathode rays in a high-voltage, gaseous-discharge tube. Despite the fact that the tube was encased in a black cardboard box, Roentgen noticed that a barium-platinocyanide screen, inadvertently lying nearby, emitted fluorescent light whenever the tube was in operation. After conducting further experiments, he determined that the fluorescence was caused by invisible radiation of a more penetrating nature than ultraviolet rays (see Luminescence; Ultraviolet Radiation). He named the invisible radiation "X ray" because of its unknown nature. Subsequently, X rays were known also as Roentgen rays in his honor.
NATURE OF X RAYS
X rays are electromagnetic radiation ranging in wavelength from about 100 A to 0.01 A (1 A is equivalent to about 10-8 cm/about 4 billionths of an in.; see Wave Motion). The shorter the wavelength of the X ray, the greater is its energy and its penetrating power. Longer wavelengths, near the ultraviolet-ray band of the electromagnetic spectrum, are known as soft X rays (see Spectrum). The shorter wavelengths, closer to and overlapping the gamma-ray range, are called hard X rays (see Radioactivity). A mixture of many different wavelengths is known as "white" X rays, as opposed to "monochromatic" X rays, which represent only a single wavelength. Both light and X rays are produced by transitions of electrons that orbit atoms, light by the transitions of outer electrons and X rays by the transitions of inner electrons. In the case of bremsstrahlung radiation (see below), X rays are produced by the retardation or deflection of free electrons passing through a strong electrical field. Gamma rays, which are identical to X rays in their effect, are produced by energy transitions within excited nuclei. See Atom and Atomic Theory.
X rays are produced whenever high-velocity electrons strike a material object. Much of the energy of the electrons is lost in heat; the remainder produces X rays by causing changes in the target's atoms as a result of the impact. The X rays emitted can have no more energy than the kinetic energy of the electrons that produce them (see Energy). Moreover, the emitted radiation is not monochromatic but is composed of a wide range of wavelengths with a sharp, lower wavelength limit corresponding to the maximum energy of the bombarding electrons. This continuous spectrum is referred to by the German name bremsstrahlung, which means "braking," or slowing down, radiation, and is independent of the nature of the target. If the emitted X rays are passed through an X-ray spectrometer, certain distinct lines are found superimposed on the continuous spectrum; these lines, known as the characteristic X rays, represent wavelengths that depend only on the structure of the target atoms. In other words, a fast-moving electron striking the target can do two things: It can excite X rays of any energy up to its own energy; or it can excite X rays of particular energies, dependent on the nature of the target atom.
X-RAY PRODUCTION
The first X-ray tube was the Crookes tube, a partially evacuated glass bulb containing two electrodes, named after its designer, the British chemist and physicist Sir William Crookes. When an electric current passes through such a tube, the residual gas is ionized and positive ions, striking the cathode, eject electrons from it. These electrons, in the form of a beam of cathode rays, bombard the glass walls of the tube and produce X rays. Such tubes produce only soft X rays of low energy. See Ion; Ionization.
An early improvement in the X-ray tube was the introduction of a curved cathode to focus the beam of electrons on a heavy-metal target, called the anticathode, or anode. This type generates harder rays of shorter wavelengths and of greater energy than those produced by the original Crookes tube, but the operation of such tubes is erratic because the X-ray production depends on the gas pressure within the tube.
The next great improvement was made in 1913 by the American physicist William David Coolidge. The Coolidge tube is highly evacuated and contains a heated filament and a target. It is essentially a thermionic vacuum tube (see Vacuum Tubes) in which the cathode emits electrons because the cathode is heated by an auxiliary current and not because it is struck by ions as in the earlier types of tubes. The electrons emitted from the heated cathode are accelerated by the application of a high voltage across the tube. As the voltage is increased, the minimum wavelength of the radiation decreases.
Most of the X-ray tubes in present-day use are modified Coolidge tubes. The larger and more powerful tubes have water-cooled anticathodes to prevent melting under the impact of the electron bombardment. The widely used shockproof tube is a modification of the Coolidge tube with improved insulation of the envelope (by oil) and grounded power cables. Such devices as the betatron (see Particle Accelerators) are used to produce extremely hard X rays, of shorter wavelength than the gamma rays emitted by naturally radioactive elements.
PROPERTIES OF X RAYS
X rays affect a photographic emulsion in the same way light does (see Photography). Absorption of X radiation by any substance depends upon its density and atomic weight. The lower the atomic weight of the material, the more transparent it is to X rays of given wavelengths. When the human body is X-rayed, the bones, which are composed of elements of higher atomic weight than the surrounding flesh, absorb the radiation more effectively and therefore cast darker shadows on a photographic plate. Another type of radiation, which is known as neutron radiation and is now used in some types of radiography, produces almost opposite results. Objects that cast dark shadows in an X-ray picture are almost always light in a neutron radiograph.
° Fluorescence
X rays also cause fluorescence in certain materials, such as barium platinocyanide and zinc sulfide. If a screen coated with such fluorescent material is substituted for the photographic films, the structure of opaque objects may be observed directly. This technique is known as fluoroscopy. See Fluoroscope.
° Ionization
Another important characteristic of X rays is their ionizing power, which depends upon their wavelength. The capacity of monochromatic X rays to ionize is directly proportional to their energy. This property provides a method for measuring the energy of X rays. When X rays are passed through an ionization chamber (see Particle Detectors), an electric current is produced that is proportional to the energy of the incident beam. In addition to ionization chambers, more sensitive devices, such as the Geiger-Müller counter and the scintillation counter, can measure the energy of X rays on the basis of ionization. In addition, the path of X rays, by virtue of their capacity to ionize, can be made visible in a cloud chamber.
° X-Ray Diffraction
X rays may be diffracted by passage through a crystal or by reflection (scattering) from a crystal, which consists of regular lattices of atoms that serve as fine diffraction gratings (see Diffraction; Diffraction Grating). The resulting interference patterns may be photographed and analyzed to determine the wavelength of the incident X rays or the spacings between the crystal atoms, whichever is the unknown factor (see Interference). X rays may also be diffracted by ruled gratings if the spacings are approximately equal to the wavelengths of the incident X rays.
INTERACTION WITH MATTER
In the interaction between matter and X rays, three mechanisms exist by which X rays are absorbed; all three mechanisms demonstrate the quantum nature of X radiation. See Quantum Theory.
° Photoelectric Effect
When a quantum of radiation, or a photon, in the X-ray portion of the electromagnetic spectrum strikes an atom, it may impinge on an electron within an inner shell and eject it from the atom. If the photon carries more energy than is necessary to eject the electron, it will transfer its residual energy to the ejected electron in the form of kinetic energy. This phenomenon, called the photoelectric effect, occurs primarily in the absorption of low-energy X rays. See Photoelectric Cell; Photoelectric Effect.
° Compton Effect
The Compton effect, discovered in 1923 by the American physicist and educator Arthur Holly Compton, is an important manifestation of the absorption of X rays of shorter wavelengths. When a high-energy photon collides with an electron, both particles may be deflected at an angle to the direction of the path of the incident X ray. The incident photon, having delivered some of its energy to the electron, emerges with a longer wavelength. These deflections, accompanied by a change of wavelength, are known as Compton scattering.
° Pair Production
In the third type of absorption, especially evident when elements of high atomic weight are irradiated with extremely high-energy X rays, the phenomenon of pair production occurs. When a high-energy photon penetrates the electron shell close to the nucleus, it may create a pair of electrons, one of negative charge and the other positive; a positively charged electron is also known as a positron. This pair production is an example of the conversion of energy into mass. The photon requires at least 1.2 MeV of energy to yield the mass of the pair. If the incident photon possesses more energy than is required for pair production, the excess energy is imparted to the electron pair as kinetic energy. The paths of the two particles are divergent.
APPLICATIONS OF X RAYS
The principal uses of X radiation 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 micrometer (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 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 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-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 electronics engineer Godfrey N. Hounsfield, and was in general use by 1979.
For applications of radioisotopes that emit gamma rays, see Isotopic Tracer.
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