27.01.2015

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The roentgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air.
The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is numerically equal to the gray (Gy).
X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted.
A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators. The most commonly known methods are photographic plates, photographic film in cassettes, and rare earth screens. Before the advent of the digital computer and before invention of digital imaging, photographic plates were used to produce most radiographic images.
Since photographic plates are sensitive to X-rays, they provide a means of recording the image, but they also required much X-ray exposure (to the patient), hence intensifying screens were devised.
Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels.


An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. Photographic film largely replaced these plates, and it was used in X-ray laboratories to produce medical images. In the pursuit of a non-toxic contrast material, many types of high atomic number elements were evaluated. After the plate is X-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. In more recent years, computerized and digital radiography has been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g.


When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film. Photographic plates are mostly things of history, and their replacement, the "intensifying screen", is also fading into history. The metal silver (formerly necessary to the radiographic & photographic industries) is a non-renewable resource. Where photographic films required wet processing facilities, these new technologies do not.



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