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All of these techniques exploit differences in density or activity of tissues, which alter the way in which electromagnetic or sound waves are transmitted, absorbed, or reflected. The data are converted to an image, resulting in a picture of the internal organs, in which areas differ in appearance based on differences in density or activity. These techniques can be used to visualize tumors, blood flow, metabolic activity, damaged organs, gall or kidney stones, fetuses, and so on. X-rays are electromagnetic radiations, which penetrate tissues to varying degrees based on their density. A photographic film is placed behind the body, producing an image. CT (cat) scans are a series of X-rays that are analyzed by a computer instead of exposing film; CT scans provide images in thin planes or slices, for more precise localization of dense areas. PET scans utilize positrons emitted from radioisotopes, which computers can analyze to provide information about metabolic activity and blood flow. In an MRI (NMR or nuclear magnetic resonance) scan, a magnetic field is used to alter the energy level of the nuclei of hydrogen atoms, which emit radiation when they return to their normal state; a computer analyzes the data, which are of relatively high contrast. Ultrasound utilizes ultrasonic (high frequency sound) waves, which produce echoes that vary with tissue density, and are analyzed by computer; this technique is especially useful for observing fetal development because it does not risk harming the fetus with radiation. None of these techniques is as informative as actually visualizing the area of interest by performing surgery, but they are noninvasive.
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