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. radiodensityshadows 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.
His scale is a quantitative measure of radiodensity and is used to evaluate CAT scans.
Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity.
The pixel value is displayed according to the mean attenuation of the tissue that it corresponds to on a scale from -1024 to +3071 on the Hounsfieldscale.
In conventional CT machines, a small X-Ray tube is physically rotated behind a circular shroud; in electron beam tomography the tube is far larger, note the internal funnel shape in the photo, with a hollow cross-section and only the electron current is rotated.
The pixel itself is displayed according to the mean attenuation of the tissue that it corresponds to on a scale from -1024 to +3071 on the Hounsfieldscale.
Water has an attenuation of 0 Hounsfield units (HU) while air is -1000HU, bone is typically +400HU or greater and metallic implants are usually +1000HU.
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