I've sprained my ankles, and knees, multiple times. I've heard a "pop" and had it swell up within minutes. The only time I had an MRI done was after the last time, when my PCP referred me to an orthopedic doctor, who felt that my history, and looseness of my ankle joint, suggested that surgery was necessary. He ordered the MRI as a pre-surgical test to determine what sort of damage he would be repairing. Sprain diagnosis, as far as I can tell, it done on a presumptive basis in most cases. If their isn't a broken bone, but there is swelling and pain from an injury that can result in a sprain, then they tell you to treat it as a sprain for at least a few weeks.