When the Food and Drug Administration approved magnetic resonance imaging (MRI) scanners in 1984, the machines seemed incredible. They offered an inside view of the human body, making it easier to diagnose disease, injuries and physical abnormalities. Today, they’re part of a multibillion-dollar industry: In 2016, 118 out of every 1,000 Americans got an MRI. The use of CT scans was even higher: 245 per 1,000 people in 2016.