Don't do radiology because there is no expectation for baseline radiology knowledge. Do a rotation that gives you exposure to a field that 1) uses imaging heavily, 2) is not the specialty of your intern year, 3) you were not exposed to 3rd year of medical school. I did cardiothoracic surgery ICU, EM, GI. In an ideal world, there would be a lighter version of a rotation in a surgical subspecialty (orthopedics, neurosurgery, otolaryngology, thoracic surgery, urology, vascular surgery) designed for students not going into that specialty. These are rare, but ask around your school. Similarly, consider radiation oncology.