(rād-ē-ā'shŏn )

[L. radiatio, a shining]

1. The process by which energy is propagated through space or matter.
2. The emission of rays in all directions from a common center.
3. Ionizing rays used for diagnostic or therapeutic purposes. Two types of radiation therapy are commonly used for patients with cancer: teletherapy and brachytherapy.

   SEE: brachytherapy.

4. Any form of radiant energy emission or divergence, as of energy in all directions from luminous bodies, radiographical tubes, particle accelerators, radioactive elements, and fluorescent substances.
5. In neurology, a group of fibers that diverge from a common origin.

acoustic r.A band of fibers that connect auditory areas of the cerebral cortex with the medial geniculate body of the thalamus. SYN: auditory radiation; sublenticular fasciculus.

actinic r.Ionizing electromagnetic radiation that can produce chemical changes, e.g., the damage done to skin by ultraviolet sunlight.

auditory r.Acoustic radiation

background r.Total radioactivity from cosmic rays, natural radioactive materials, and other radiation that is present in a specific area.

braking r.Bremsstrahlung radiation

bremsstrahlung r.Diagnostic radiation produced at the target of the anode in an x-ray tube. An electron is accelerated at high speed from the x-ray tube cathode filament. It interacts with the nuclear field of a target atom, changing direction and losing energy that is emitted in the form of an ionizing radiation photon. The result is a heterogeneous beam. SYN: braking radiation.

characteristic r.In radiology, the production of radiation in an anode caused by an interaction between an electron from the electron stream and an inner-shell electron of the target material. The result is an ejected electron, a positive atom, and an x-ray photon characteristic of the difference in binding energies between the atomic shells.

r. of corpus callosum All the fibers emanating from the corpus callosum into each cerebral hemisphere.

corpuscular r.Radiation composed of discrete elements or particles such as elements of atomic nuclei, i.e., alpha, beta, neutron, positron, or proton particles.

cosmic r.Ionizing radiation from the sun and other extraterrestrial sources. It has a short wavelength, high velocity, and an exceptional ability to penetrate tissue. It accounts for about one tenth of the yearly total of ionizing radiation exposure for each person. Colloquially, it is known as “cosmic rays.”

electromagnetic r.Photons that travel at the speed of light. They exhibit both magnetic and electrical properties.

SEE: electromagnetic spectrum for table.

heterogeneous r.Radiation containing waves of various wavelengths.

homogeneous r.Radiation containing photons of similar wavelength.

infrared r.Infrared ray.

interstitial r.Radiation treatment accomplished by inserting sealed sources of a particle emitter directly into tissues.

ionizing r.Electromagnetic waves capable of producing ions after interaction with matter. Examples include x-rays, gamma rays, and beta particles.

SEE: ionizing radiation injury.

irritative r.An overdose of ultraviolet irradiation resulting in erythema and , in exceptional cases, blister formation.

low-level r.Electromagnetic waves at intensity levels below that known to cause obvious damage to living things. Low-level radiation includes that emitted by power lines, nuclear power plants, and appliances such as electric blankets, television sets, and computer terminals.

nonionizing r.

ABBR: NIR

optic r.A system of fibers extending from the lateral geniculate body of the thalamus through the sublenticular portion of the internal capsule to the calcarine occipital cortex (striate area) on the banks of the calcarine fissure. SYN: geniculocalcarine tract.

photochemical r.Light rays that penetrate tissues only fractions of a millimeter, are absorbed by cells, and cause physical and biological changes. This type of radiation causes surface heating.

photothermal r.Radiation of heat by a source of light, as that from an electric bulb.

primary r.That radiation being emitted directly to the patient from an x-ray source.

pyramidal r.The radiation of fibers from the cerebral cortex to the pyramidal tract.

remnant r.Ionizing radiation that passes through the part being examined to make the radiographical image.

scattered r.X-rays that have changed direction because of a collision with matter.

secondary r.X-rays produced by the interaction between primary radiation and the substance being radiated.

solar r.Radiation from the sun. Sixty percent infrared and forty percent visible and ultraviolet.

spatially fractionated r.

ABBR: SFR

striatomesencephalic r.Fibers originating in the corpus striatum and terminating principally in the substantia nigra of the midbrain.

striatosubthalamic r.Ansa lenticularis.

striatothalamic r.Groups of fibers connecting the corpus striatum with the thalamus and subthalamus.

synchrotron r.Radiation released by charged particles accelerated by a synchrotron. It may be used to obtain noninvasive images of body structures, e.g., the coronary arteries, or to study the structure of proteins, tissue samples, or other objects of biological or medical interest.

thalamic r.Groups of fibers connecting the thalamus with the cerebral hemispheres. These include frontal, centroparietal, occipital, and optic radiations.

thermal r.Radiation in the form of heat.

ultraviolet r.Radiant energy extending from 3900 to 200 angstrom units (A.U.). Divided into near ultraviolet, which extends from 3900 to 2900 A.U., and far ultraviolet, which extends from 2900 to 200 A.U.

visible r.The radiation of the visible spectrum. It may be broken up into different wavelengths representing different colors: Red, 6220–7700 A.U.; Orange, 5970–6220 A.U.; Yellow, 5770–5970 A.U.; Green, 4920–5770 A.U.; Blue, 4550–4920 A.U.; Violet, 3900–4550 angstrom units (A.U.).

x r.1A form of electromagnetic radiation with wavelengths in the range of 0.01 to 10 nm, frequencies from 3 × 10¹⁶ Hz to 3 × 10¹⁹ Hz, and energies in the range 120 eV to 120 keV.2Treatment with or exposure to x-rays.