An MRI scan of the lower limb is done to study its structure and anatomy so as to diagnose injuries, fractures, tumors or torn ligaments. In this scan, parts of the lower limb such as foot and ankle are examined thoroughly. Your doctor is likely to order this scan if he observes a mass in your limb during a physical scan, finds an abnormal x-ray report, identifies a birth defect in the lower limb, sees swelling, pain or rash in the leg or if you complain of pain in the bone along with fever.