ALAIN POL, ISM / SCIENCE PHOTO LIBRARY ALAIN POL, ISM / SCIENCE PHOTO LIBRARY
False-colour computed tomography (CT) X-ray scan through the chest of a normal subject, showing the heart (orange) and lungs (blue). The red circle at bottom is the aorta, the largest artery in the body. A CT scan is an image of a two-dimensional axial slice through the body made by a series of fine X-ray beams emitted from a source that moves in a full circle around the subject. A system of detectors diametrically opposite the source detects up transmitted x-rays from numerous positions around the subject - up to 1,000 points may be used. Data relating to the attenuation of each beam by body tissues is assembled on computer to obtain a black & white image of the section.
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