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In radiography, film is exposed to X-rays, which have shorter wavelengths and different energy levels than the light rays used in photography. Radiography creates the images on photographic film, or on video or a digital file, by using a form of electromagnetic radiation.

In 1895, Wilhelm Conrad Roentgen discovered X-rays. Also called Roentgen rays, X-rays are generated in a glass vacuum tube (an X-ray tube) that contains two differently charged electrodes, one of which gives off electrons. When the electrons travel from one electrode to the other, some of the energy they emit is X-radiation. X-rays can pass through skin and muscle and other soft body tissue, while bones and denser objects show up as white images on the photographic emulsion when film is exposed to X-rays. A picture of the inside of the body can thus be developed.

Radiation in all forms has potential to be harmful. Exposure to ultraviolet radiation may tan the skin, but it can also result in burning and other damage to tissue, including the development of cancer cells. Low-level infrared radiation can warm tissues, but at higher levels it cooks them like microwaves do; the process can destroy cells. Radiologists and other professionals and radiology patients take protective measures to avoid all unnecessary exposure to radiation when X-rays are used.

There are other forms of diagnostic imaging that do not expose patients to any potentially harmful radiation. Sound waves are used in ultrasound technology, or sonography, to obtain a picture of internal organs. High-frequency sound waves beamed into the patient's body bounce back and create echoes that can be recorded on a paper strip, photograph, video, or digital file. Ultrasound is very frequently employed to determine the size and development of a human fetus. Magnetic resonance imaging (MRI) uses magnetic fields, radio waves, and computers to create images of the patient's body.

The growth of computers and computer technology in the 1980s and 1990s helped to increase the use of non-radiation imaging techniques. Computers can now handle a vast quantity of data much more rapidly, making it possible to enhance images to great clarity and sharpness. Radiologists today conduct diagnostic imaging procedures that include magnetic resonance imaging, computer tomography, and positron emission tomography, among other types of procedures.

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