Although MRI is used for medical diagnosis, it utilizes a physics phenomenon discovered in the 1930s called nuclear magnetic resonance in which magnetic fields and radio waves, both harmless, cause atoms to give off tiny radio signals. In the 1940s, research physicists found that the length of time these response signals are emitted after an atom is stimulated by radio waves varies widely depending upon the substance being examined. This amazing phenomenon also holds true for biological tissue. It wasn't until 1970, however, that the basis for using magnetic resonance as a tool for medical diagnosis when it was found that different kinds of animal tissue emit response signals that vary in length and, furthermore, that cancerous tissue emit response signals that last much longer than non-cancerous tissue. Subsequent findings revealed that the response times of other kinds of diseased tissue, normally called "relaxation times," also vary dramatically.