A New York Times reporter says she found out how unreliable MRIs can be after a scan missed a serious stress fracture in her foot. MRIs use powerful magnets and radio waves to manipulate protons in the body's hydrogen atoms. The idea is that protons in different types of tissue respond in distinctive ways to this pushing and prodding, and the differing responses reveal the characteristics of the tissue. Magnetic resonance machines, though, vary enormously. Even more important, radiologists say, is the quality of the imaging coils they put around the body part being scanned and the computer programs they use to control the imaging and to analyze the images. And there is a huge variability in skill among the technicians doing the scans, the reporter writes.