1. The act or condition of diverging in all directions from a center. 2. The sending forth of light, short radio waves, ultraviolet or x-rays, or any other rays for treatment or diagnosis or for other purpose. Cf.:irradiation (2). 3. SYN: radiatio. 4. A ray. 5. Radiant energy or a radiant beam. [L. radiatio, fr. radius, ray, beam]
- acoustic r. [TA] the fibers that pass from the medial geniculate body to the transverse temporal gyri of the cerebral cortex by way of the sublentiform part of the internal capsule. SYN: radiatio acustica [TA].
- afterloading r. method of administering r. that involves initial placement of local catheters with later installation of the r. source.
- alpha r. an emission of a nucleus of high kinetic energy from the nucleus of an atom undergoing radioactive decay or fission.
- annihilation r. the r. resulting when a positron from beta positive decay comes to rest. It encounters an electron, and they annihilate each other and convert their rest mass into two 0.51-MeV gamma rays emitted in exactly opposite directions. See pair production.
- anterior thalamic r. [TA] r. formed by fibers interconnecting, via the anterior limb of the internal capsule, the anterior and medial thalamic nuclei and the cerebral cortex of the frontal lobe (excluding the precentral gyrus bordering on the central sulcus). SYN: radiatio thalami anterior [TA].
- background r. irradiation from environmental sources, including the earth's crust, the atmosphere, cosmic rays, and ingested radionuclides.Natural sources account for the largest amount of r. received by most persons each year (average annual dose, 3.00 mSv), with medical and occupational sources providing only a fraction (average less than 0.60 mSv). It is currently believed that radon, a gas produced by radium decay within crystal rock, constitutes the major source of background r. throughout many parts of the U.S. Radon buildup in inadequately ventilated homes may pose a long-term health hazard. The deleterious effects of background r., estimated as causing 1–6% of spontaneous genetic mutations, rise with dose.
- beta r. radiant energy from a source of beta rays.
- central thalamic r. [TA] r. formed by fibers interconnecting, through the posterior limb of the internal capsule, the ventral lateral, ventral posterolateral and posteromedial, lateral dorsal, and lateral posterior nuclei and the precentral gyrus and parietal lobe of the cerebral cortex. SYN: radiatio thalami centralis [TA].
- Cerenkov r. light given off by a transparent medium when a high-energy particle speeds through it at a velocity greater than that of light in that medium.
- characteristic r. monochromatic r. that is produced when an electron is ejected from an atom and another takes its place by jumping from another shell; the energy of the emitted photon is the difference between that of the two shell positions. See photoelectric effect. SYN: characteristic emission.
- r. of corpus callosum [TA] the spreading out of the fibers of the corpus callosum in the centrum semiovale of each cerebral hemisphere. SYN: radiatio corporis callosi [TA].
- corpuscular r. r. consisting of streams of subatomic particles such as protons, electrons, neutrons, etc.
- electromagnetic r. r. originating in a varying electromagnetic field; e.g., long and short radio waves; light, visible and invisible; x-r. and gamma rays.
- gamma r. ionizing electromagnetic r. resulting from nuclear processes, such as radioactive decay or fission.
- geniculocalcarine r. SYN: optic r..
- Gratiolet r. SYN: optic r..
- hemibody r. a palliative cancer therapy involving r. to one-half of the body. [hemi- + body]
- heterogeneous r. r. consisting of different frequencies, various energies, or a variety of particles. SEE ALSO: polychromatic r..
- homogeneous r. r. consisting of a narrow band of frequencies, the same energy, or a single type of particle.
- hyperfractionated r. smaller fractions of a dose of r. given more frequently than daily.
- hypofractionated r. larger fractions of a dose of r. given less frequently than daily.
- inferior thalamic r. [TA] SYN: inferior thalamic peduncle.
- ionizing r. corpuscular ( e.g., neutrons, electrons) or electromagnetic ( e.g., gamma) r. of sufficient energy to ionize the irradiated material.
- K-r. usually a very penetrating form of x-r. excited by cathode rays (high-speed electrons) impinging upon a metal anode such as tungsten; the energy of the r. is a function of the binding energy of the K-shell electrons of the metal anode.
- L-r. an x-r. of slight penetrating power excited by cathode rays (high-speed electrons) impinging on a metal anode; the energy of the r. is a function of the binding energy of the L-shell electrons of the metal anode.
- monochromatic r. light rays or ionizing r. of a very narrow band of wavelengths (ideally, of a single wavelength). Cf.:photopeak, characteristic r..
- neutron r. an emission of neutrons from the nucleus of an atom by decay or fission.
- occipitothalamic r. SYN: optic r..
- optic r. [TA] the massive, fanlike fiber system passing from the lateral geniculate body of the thalamus to the visual cortex (striate or calcarine cortex, area 17 of Brodmann); the fibers follow the retrolenticular and sublenticular limbs of the internal capsule into the corona radiata but they curve back along the lateral wall of the temporal and occipital horns of the lateral ventricle to the striate cortex on the medial surface and pole of the occipital lobe. SYN: radiatio optica [TA], geniculocalcarine r., geniculocalcarine tract, Gratiolet fibers, Gratiolet r., occipitothalamic r., Wernicke r..
- polychromatic r. r. containing gamma rays, under ray of many different energies; in diagnostic radiology, typically bremsstrahlung.
- posterior thalamic r. [TA] r. formed by fibers interconnecting through the retrolenticular part of the posterior limb of the internal capsule, the pulvinar complex and lateral geniculate nucleus, and the posterior parietal and occipital lobes of the cerebral cortex. SYN: radiatio thalamica posterior [TA].
- pyramidal r. corticospinal fibers passing from the cortex into the pyramid. SYN: radiatio pyramidalis.
- scattered r. secondary r. emitted from the interaction of x-rays with matter; generally lower in energy, with a directional distribution that depends on the energy of the incident r.. SYN: secondary r..
- secondary r. SYN: scattered r..
- Wernicke r. SYN: optic r..

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ra·di·a·tion .rād-ē-'ā-shən n
1) energy radiated in the form of waves or particles
2 a) the action or process of radiating <with \radiation of the pain there may be tenderness over the sciatic nerve (J. A. Key)>
b ) (1) the process of emitting radiant energy in the form of waves or particles (2) the combined processes of emission, transmission, and absorption of radiant energy
3) a tract of nerve fibers within the brain esp one concerned with the distribution of impulses arising from sensory stimuli to the relevant coordinating centers and nuclei <the optic \radiations>

* * *

energy in the form of waves or particles, especially electromagnetic radiation, which includes (in order of increasing wavelength), gamma rays, X-rays, ultraviolet rays, visible light, and infrared rays (radiant heat), and the particles.

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ra·di·a·tion (ra″de-aґshən) [L. radiatio, q.v.] 1. divergence from a common center. 2. a structure made up of divergent elements, as one of the fiber tracts in the brain; for official names of specific structures, see under radiatio. 3. energy transmitted by waves through space or through some medium; usually referring to electromagnetic radiation when used without a modifier. By extension, a stream of particles, such as electrons, neutrons, protons, or alpha particles.

Medical dictionary. 2011.

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