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 ?
Mothers were also asked about their exposures to X-rays during pregnancy and the year prior to pregnancy.
The amounts of X-rays they found were small - they counted 33 photons in the first observation a year and a half after the supernova exploded, and 10 in another about 200 days later - but present," the (https://news.
Lisse's team used the Chandra X-ray telescope, once in 2014 and three times in 2015, to look for X-rays.
A spokesman for Worcestershire Acute Hospitals NHS Trust said: "We apologise for any distress this has caused and can confirm that the backlog concerns a secondary review of x-rays with the majority of these x-rays having been reviewed by the requesting clinician, mostly on the same day.
The current diagnosis of NEC mainly relies on abdominal plain X-rays and observation of patients' clinical manifestations.
Conclusion: Rejected/repeated X-rays are a common problem in every radiology department .
When X-rays encounter grains of dust in interstellar space they can be deflected, and if the dust clouds are dense they can scatter a noticeable fraction of the X-rays away from their original trajectory, putting them on a triangular path.
Most photons emitted by the vacuum tube originate from thermal energy but about i% is emitted as X-rays (Charles Sturt University, 2002; Garip, 1998).
Although the National Heart, Lung, and Blood Institute guidelines on the use of chest x-rays in children with asthma did not change during the study period, "we also believe that they do not provide the criteria that are sufficiently explicit to affect the discretionary use of x-rays," they said.
Dakota Dental's dentists in Apple Valley, had already proactively switched to using digital X-rays - a full six years before this study was published and the ADA issued its response.
Like light, X-rays are a kind of radiation, or invisible energy waves.
In one case, involving X-rays on children, a five-fold increase in risk was seen.