Radiology is a branch of medical science in which various forms of radiant energy are used to diagnose and treat disorders and diseases. For nearly 80 years, radiology was based primarily on the use of X rays. Since the 1970s, however, several new imaging techniques have been developed. Some, like computed tomography, makes use of X rays along with other technology, such as computer technology. Others, like ultrasound and magnetic resonance imaging, use forms of radiant energy other than X rays. Radiological techniques can also be used for therapeutic purposes, methods used to treat diseases and disorders. The use of radiology for therapy depends on the fact that X rays kill living cells. Under normal circumstances, this fact provides a good reason for people to avoid coming into contact with X rays. The destruction of healthy cells by X rays is, in fact, one of the ways in which cancers may develop.