n. a technique of image manipulation commonly used in *cross-sectional imaging to manipulate a *grey scale image. Typically there is too much data obtained in a scan to see on a single image: the radiologist therefore chooses the window level centred on the density of the tissue of interest and a window width wide enough to include the densities of all the tissues that need to be seen. Tissues denser than this window usually appear white, and tissues darker appear black. Sometimes several different images of the same scan are required at different window settings to assess adequately all the necessary detail (for example, window settings to observe the lung are different from those for the bones or the soft tissues in the chest on CT). See also Hounsfield unit.
(in radiology) a scale representing the possible gradient of densities from black to white for each *pixel in an image. In an *analogue image this gradient is smooth. A *digital image has many discrete steps. The more steps allowed, the closer to representing the true analogue image it comes, although more steps require more computer memory. Images can be manipulated by *windowing. This concept is particularly valuable in *computerized tomography. See Hounsfield unit; digitization.... grey scale
the numerical unit assigned electronically to each *pixel in a computerized tomography (CT) image, according to its X-ray density. The fixed points on the scale are arbitrarily assigned as ?1000 for air and 0 for water. The CT image is viewed in a ‘window’. The range of Hounsfield units displayed (window width) and the centre point of the range of interest (window level) can be varied by the radiologist in order to observe specific tissues (see windowing). The unit was named after Sir Godfrey Hounsfield (1919–2004), who developed CT scanning in the 1950s. Symbol: HU.... hounsfield unit
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