X-ray image intensifier

An X-ray image intensifier (XRII) is an that converts X-rays into visible light at higher intensity than the more traditional fluorescent screens can. Such intensifiers are used in X-ray imaging systems (such as fluoroscopes) to allow low-intensity X-rays to be converted to a conveniently bright visible light output. The device contains a low absorbency/scatter input window, typically aluminum, input fluorescent screen, photocathode, electron optics, output fluorescent screen and output window. These parts are all mounted in a high vacuum environment within glass or, more recently, metal/ceramic. By its intensifying effect, It allows the viewer to more easily see the structure of the object being imaged than fluorescent screens alone, whose images are dim. The XRII requires lower absorbed doses due to more efficient conversion of X-ray quanta to visible light. This device was originally introduced in 1948. The overall function of an image intensifier is to convert incident x-ray photons to light photons of sufficient intensity to provide a viewable image. This occurs in several stages. The input window is convex is shape, made up of aluminium to minimise the scattering of X-rays. The window is 1 mm in thickness. Once X-rays pass through the aluminium windows, it encounters input phosphor that converts X-rays into light photons. The thickness of input phosphor range from 300 to 450 micrometres reach a compromise between absorption efficiency of X-rays and spatial resolution. Thicker input phosphor has higher absorption efficiency but poor spatial resoution and vice versa. Sodium activated Caesium Iodide is typically used due to its higher conversion efficiency thanks to high atomic number and mass attenuation coefficient, when compared to zinc-cadmium sulfide. The input phosphor are arranged into small tubes, to allow photons to pass through the tube, without scattering, this improving the spatial resolution. The light photons are then converted to electrons by a photocathode.

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