One day last summer, Alison slipped off her jewelry, stepped into a hospital gown and lay down inside a full-body MRI scanner. As the machine issued calming instructions — breathe in, hold, breathe out — it captured thousands of images, from her head to her toes.A tech worker and mother of two in her 50s, Alison (whose full name can’t be shared under participant privacy rules) had joined a nationwide health study after spotting a flyer in her local library. Her mother died young of cancer, and w...