x-ray (eks´ra) see under ray.
x-ray.
x-ray,
X-ray
| x-ray or X-ray n. 1. A relatively high-energy photon with wavelength in the approximate range from 0.01 to 10 nanometers. Also called roentgen ray. 2. A stream of such photons used for their penetrating power in radiography, radiology, radiotherapy, and scientific research. Often used in the plural. Also called roentgen ray. 3. A photograph taken with x-rays. v. 1. To irradiate with x-rays. 2. To photograph with x-rays. |
x-ray.
1 Also called roentgen ray, x radiation. electromagnetic radiation with wavelengths between about 0.005 and 10 nm. X-rays are produced when electrons traveling at high speed strike certain materials, particularly heavy metals such as tungsten. They can penetrate most substances and are used to investigate the integrity of certain structures, to therapeutically destroy diseased tissue, and to make radiographic images for diagnostic purposes, as in radiography and fluoroscopy. Discrete x-rays are those with precisely fixed energies that are characteristic of differences between electron binding energies of a particular element. Tungsten, for example, has 15 different effective energies and no more, representing emissions from 5 different electron shells.
2 a radiograph made by projecting x-rays through organs or structures of the body onto a photographic film. Structures that are relatively radiopaque (allow few x-rays to pass through), such as bones and cavities filled with a radiopaque contrast medium, cast a shadow on the film. Also called x-ray film.
x-ray,
n a type of electromagnetic radiation characterized by wavelengths between approximately 103 Å and 10−4 Å, corresponding to photon energies of about 20 eV to 125 MeV. X-rays are invisible; penetrative, especially at higher photon energies; and travel with the same speed as visible light. Typical production involves bombarding a target of high atomic number with fast electrons in a high vacuum; they are also emitted as a product of some radioactive disintegrations (specifically originating from the extranuclear part of the atom). X-rays were first discovered by Wilhelm C. Roentgen in 1895; hence the term
roentgen rays, often applied to mechanically generated radiographs. Roentgen called them
x-rays after the mathematic symbol
x for an unknown. Also the colloquial term for radiograph. See also radiograph.
roentgen rays, often applied to mechanically generated radiographs. Roentgen called them
x-rays after the mathematic symbol
x for an unknown. Also the colloquial term for radiograph. See also radiograph.
)
Xerostomia.
radiograph, low-voltage filament circuit,
n the lower-energy circuit in a radiography machine that uses a step-down transformer to lower the line voltage to around 3 volts, just enough to heat the filament and produce an electron cloud.
radiograph, monochromatic,
n a radiograph that has a single wavelength or an extremely narrow band of wavelengths.
x-ray
electromagnetic radiation of wavelengths ranging between 5.0 × 10−6 and 5.0 × 10−4 μm (including grenz rays).
X-rays are produced by the collision of a beam of electrons with a metal target in an x-ray tube. Called also roentgen rays. The penetrability and hardness of the x-rays increases with the voltage applied to the x-ray tube, which controls the speed with which the electrons strike the target. For diagnostic radiography, tube voltages in the range 50 to 120 kilovolts peak (kVp) are normally used. For radiation therapy, voltages in the 1 to 2 megavolt range are used for most treatment. Accelerating electrons to speeds high enough to produce megavoltage x-rays requires a linear accelerator (lineac).
The x-ray exposure is proportional to the tube current (milliamperage) and also to the exposure time. In diagnostic radiography, the tube voltage and current and exposure time are selected to produce a high-quality radiograph with the correct contrast and film density. In radiation therapy, these exposure factors are selected to deliver a precisely calculated radiation dose to the tumor. The total dose is usually fractionated so that tumor cells can be oxygenated as surrounding cells die; this increases the sensitivity of the cells to radiation.
Body tissues and other substances are classified according to the degree to which they allow the passage of x-rays (radiolucency) or absorb x-rays (radiopacity). Gases are very radiolucent; fatty tissue is moderately radiolucent. Compounds containing high-atomic-weight elements, such as barium and iodine, are very radiopaque; bone and deposits of calcium salts are moderately radiopaque. Water; muscle, skin, blood and cartilage and other connective tissue; and cholesterol and uric acid stones have intermediate density. See also radiation and radiation therapy.
A double contrast study uses both a radiopaque and a radiolucent contrast medium; for example, the walls of the stomach or intestine are coated with barium and the lumen is filled with air. The resulting radiographs clearly show the pattern of mucosal ridges.
x-ray tube
a glass vessel with a high vacuum and two electrodes. A very high voltage electrical current is passed across the tube and drives a stream of electrons produced by a tungsten filament set in the face of the cathode to collide with the anode and generate x-rays.
X-ray
High-energy radiation A range of the electromagnetic spectrum used in low doses to diagnose disease and in high doses to treat CA. See Soft X-rays.
X-ray exposure
Diagnostic x-rays Impart 30-150 keV of energy; rare reports vaguely suggest a relationship between exposure to low- level X-rays and a slight ↑ in myeloproliferative disorders and a minimal ↑ risk for developing myeloma
Therapeutic x-rays
• Low level radiation, eg 5-10 keV or 'grenz' radiation–may be used to treat recalcitrant skin conditions–eg, psoriasis
• High level radiation, eg megaelectron-volt–MeV) radiation–may be used to treat internal malignancy
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