x-rays


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Related to x-rays: Gamma rays, MRI, ultraviolet

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.
References in periodicals archive ?
They are targeting to finish the installation of the x-ray before the Lenten Season.
Researchers assessed how AI models identified pneumonia in 158,000 chest X-rays across three medical institutions: the National Institutes of Health; The Mount Sinai Hospital; and Indiana University Hospital.
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That said, linking up all the chains together, the team came to the conclusion that these high-energy X-rays and gamma rays are produced due to the acceleration of electrons close to the speed of light.
Transmissive targets have a very thin layer of target material (< 10 [micro]m thick) directly bonded to the x-ray window (typically around 500[micro]m or 0.5mm thick).
Next, we randomly transferred 30 spiders from each of 10 out of the 20 colonies subjected to x-rays into separate well-ventilated plastic containers.
'I want people to make sure to get annual x-rays,' he added.
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The x-rays had been carried out at Alexandra Hospital in Redditch, Worcester Royal and Kidderminster Hospital.
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Those X-rays which were rejected/repeated were analyzed to know the frequency of various causes of rejection/repetition.
The application of X-rays for medical purposes made revolutionary advances in medicine, the approach of doctors to patients has changed since the doctors could visualize the inside of the patient's body for the first time without surgical opening.