x-rays

ray (ra)

1. a line emanating from a center.

2. a more or less distinct portion of radiant energy (light or heat), proceeding in a specific direction.

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α-rays , alpha rays high-speed helium nuclei ejected from radioactive substances; they have less penetrating power than beta rays.

β-rays , beta rays electrons ejected from radioactive substances with velocities as high as 0.98 of the velocity of light; they have more penetrating power than alpha rays, but less than gamma rays.

digital ray 

1. a digit of the hand or foot and the corresponding portion of the metacarpus or metatarsus, considered as a continuous structural unit.

2. in the embryo, a mesenchymal condensation of the hand or foot plate that outlines the pattern of a future digit.

γ-rays , gamma rays electromagnetic radiation of short wavelengths emitted by an atomic nucleus during a nuclear reaction, consisting of high-energy photons, having no mass and no electric charge, and traveling with the speed of light and with great penetrating power.

grenz rays  very soft x-rays having wavelengths about 20 nm, lying between x-rays and ultraviolet rays.

medullary rays  the intracortical prolongations of the renal pyramids.

roentgen rays  x-r's.

x-rays  electromagnetic vibrations of short wavelengths (approximately 0.01 to 10 nm) or corresponding quanta that are produced when electrons moving at high velocity impinge on various substances; they are commonly generated by passing high-voltage current (approximately 10,000 volts) through a Coolidge tube. They are able to penetrate most substances to some extent, to affect a photographic plate, to cause certain substances to fluoresce, and to strongly ionize tissue.

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x-rays [eks´rāz]

high-energy electromagnetic radiation produced by the collision of a beam of electrons with a metal target in an x-ray tube; the penetrability and hardness of the x-rays increase with the voltage applied to the tube, which controls the speed with which the electrons strike the target. Called also roentgen rays.



For diagnostic radiography, tube voltages in the range 80 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). Kilovoltage lower than 80 kVp is often used for the extremities, 25 to 30 kVp is used for mammography, and up to 150 kVp can be used for chest imaging.

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 (their radiolucency) or absorb x-rays (their 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.

X-ray Contrast Media. A contrast medium is a substance introduced into a structure in order to increase the radiographic contrast with surrounding tissues. The radiopaque contrast media include a variety of organic iodine compounds and the insoluble salt barium sulfate. Radiolucent contrast media are gases such as air, oxygen, or carbon dioxide.



Barium is used for gastrointestinal studies. Water-soluble, iodinated contrast media excreted by the kidneys are used for many procedures, including all types of angiography and for intravenous and retrograde urography; the most commonly used are diatrizoate and iothalamate. Those excreted by the liver are used for oral or intravenous cholangiography or cholecystography. Oily iodinated media are used for lymphangiography, bronchography, and myelography.

All iodinated contrast media can cause reactions, which may range from the common reactions of mild flushing and a feeling of warmth and nausea and vomiting to rare life-threatening reactions requiring immediate aggressive therapy. The cause of these reactions may be allergy; however, this is disputed.

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.

Standard stationary anode x-ray tube; diagram in longitudinal section. From Dorland's, 2000.

Simple radiograph. A, X-ray machine; B, patient; and C, x-ray film. From Malarkey and McMorrow, 1996.