Even in the vast world of apps, Dr. Patrick J. Gagnon has one with an unusual distinction: it had to be cleared for use by the Food and Drug Administration. Dr. Gagnon, a radiation oncologist, uses the app when he sees patients in his Fairhaven, MA, office. He pulls his iPhone out of his pocket, and then he and a patient, side by side, can view images on it and discuss treatment. "It's a nice way to go through a scan with a patient," he said. The app he uses, called Mobile MIM, made by MIM Software, can turn an iPhone or an iPad into a diagnostic medical instrument. It allows physicians to examine scans and to make diagnoses based on magnetic resonance imaging, computed tomography and other technologies if they are away from their workstations. Dr. Gagnon says the app will also prove useful when he wants to give physicians at other hospitals rapid access to images for immediate decisions.