Radio isotope imaging is a variant of this where the contrast medium is replaced by a radio labelled substance avidly taken up by the target tissue - for instance radio iodine can be used to target the thyroid gland. Tomography , is a newer technique in which source and photographic plate are moved in relation to the subject. Computer Assisted Tomography (CAT) is an even more sophisticated technique in which both x- ray source and detector (replacing film) are moved around the body, producing a data stream which is interpreted by a computer as a series of images equivalent to cross sections through the body at fixed intervals. The drawback is of course that X rays are now known to be harmful, inducing mutation and tissue damage. Alternatives include: • ultrasound which reflects from boundary layers between tissues of different densities • magnetic resonance imaging (MRI) produces images according the atomic structure of their major components, by subjecting them to a strong collapsing magnetic force. Again
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