A radiologist is a medical doctor specializing in diagnosing and treating medical problems using imaging technologies. These technologies include but are not limited to X-rays, CT scans, MRIs, and ultrasounds. In order to become a radiologist after completing medical school, one must also pass an examination and complete at least four years of a residency program. Often, radiologists seek an additional one to two year fellowship to train in a specialty, such as nuclear medicine.