n. a unit of exposure dose of X- or gamma-radiation equal to the dose that will produce 2.58 × 10?4 coulomb on all the ions of one sign, when all the electrons released in a volume of air of mass 1 kilogram are completely stopped.
The gray is the unit of radiation that is actually absorbed by any tissue or substance as a result of exposure to radiation. 1 Gy is the absorption of 1 joule of energy (from gamma radiation or X-rays) per kilogram of irradiated matter. The gray supersedes an older unit called the rad (1 Gy = 100 rads).
Because some types of radiation affect biological organisms more than others, the sievert is used as a measure of the impact of an absorbed dose. It uses additional factors, such as the kind of radiation and its energy, to quantify the effects on the body of equivalent amounts of different types of absorbed energy. The sievert replaces an older unit, the rem (1 Sv=100 rems).... radiation unit
X-rays are produced artificially by bombarding a heavy metal tungsten target with electrons, in a device known as an X-ray tube. Low doses of the X-rays that are emitted are passed through body tissue and form images on film or a fluorescent screen. The X-ray image, also known as a radiograph or roentgenogram, shows the internal structure of the area that is being examined. Dense structures, such as bone, absorb X-rays well and appear white on an Xray image. Soft tissues, such as muscle, absorb less and appear grey.
Because X-rays can damage living cells, especially those that are dividing rapidly, high doses of radiation are used for treating cancer (see radiotherapy).
Hollow or fluid-filled parts of the body often do not show up well on X-ray film unless they first have a contrast medium (a substance that is opaque to X-rays) introduced into them. Contrastmedium X-ray techniques are used to image the gallbladder (see cholecystography), bile ducts (see cholangiography), the urinary tract (see urography), the gastrointestinal tract (see barium X-ray examinations), blood vessels (see angiography; venography), and the spinal cord (see myelography).
X-rays can be used to obtain an image of a “slice” through an organ or part of the body by using a technique known as tomography. More detailed images of a body slice are produced by combining tomography with the capabilities of a computer (see CT scanning).
Large doses of X-rays can be extremely hazardous, and even small doses carry some risk (see radiation hazards).
Modern X-ray film, equipment, and techniques produce high-quality images with the lowest possible radiation exposure to the patient.
The possibility of genetic damage can be minimized by using a lead shield to protect the patient’s reproductive organs from X-rays.
Radiographers and radiologists wear a film badge to monitor their exposure to radiation.
(See also imaging techniques; radiography; radiology.)... x-rays
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