Hounsfield scale
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The Hounsfield scale is a quantitative scale for describing radiodensity. It was established by Sir Godfrey Newbold Hounsfield, one of the principal engineers and developers of computed axial tomography (CAT, or CT scans).
CT machines were the first imaging devices for detailed visualization of the internal three-dimensional anatomy of living creatures, initially only as tomographic reconstructions of slice views or sections. Since the early 1990s, with advances in computer technology and scanners using spiral CT technology, internal three-dimensional anatomy is viewable by three-dimensional software reconstructions, from multiple perspectives, on computer monitors. By comparison, conventional X-Ray images show only compressed two-dimensional images of complex anatomy, i.e. radiodensity shadows on acetate film..
Specifically, Hounsfield defined 0 Hounsfield units (HU) as the radiodensity of distilled water at standard pressure and temperature, and -1000 HU as the radiodensity of air. These were chosen as universally available references and were oriented to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. humans.de:Hounsfield-Skala