Elizabeth Rafferty, MD, is trying not to lapse into rhapsodic cliches. "I don't want to call it a magic bullet, because that would oversell," she says. "It's not a panacea." Then, five minutes later: "I don't want to say it's catching on like wildfire." After a few minutes more, though, Rafferty can't help herself. She lets her enthusiasm loose. "People have been waiting for it for a long time," she says. "It's a step, but it's a step by a person who has a stride of 7 feet." "It" is a 3-D mammography machine, the Selenia Dimensions system, one of which sits in the breast imaging clinic at Massachusetts General Hospital that Rafferty, a radiologist, runs. The machine, which is made by the Bedford company Hologic and developed partly at MGH under Rafferty's supervision, produces images that are so vivid and clear they seem to speak out loud: "Hey, right here! This is a tumor!"