The pictures and stories are the stuff of slapstick: wheelchairs, gurneys and even floor polishers jammed deep inside M.R.I. scanners whose powerful magnets grabbed them from the hands of careless hospital workers. The police officer whose pistol flew out of his holster and shot a wall as it hit the magnet. The sprinkler repairman whose acetylene tank was yanked inside, breaking its valve and starting a fire that razed the building. But the bigger picture is anything but funny, medical safety experts say. As the number of magnetic resonance imaging scanners in the country has soared from a handful in...