Radiologists are medical doctors who specialize in diagnosing and treating diseases and injuries using medical imaging techniques, such as x-rays, computed tomography (CT), magnetic resonance imaging (MRI), nuclear medicine, positron emission tomography (PET) and ultrasound. Radiologists graduate from accredited medical schools, pass a licensing examination, and then go on to complete at least five years of unique post-graduate medical education in, among other topics:Radiation safety and protectionRadiation effects on the human bodyAppropriate performance and interpretation of quality radiological and medical imaging examinationsThese physicians often complete a fellowship — one to two additional years of specialized training in a particular subspecialty of radiology, such as breast imaging, cardiovascular radiology or nuclear medicine. Radiologists physicians are usually board certified by the American Board of Radiology (for a doctor of medicine) or the American Osteopathic Board of Radiology (for an osteopathic doctor); an indication of a high level of training, and demonstrated excellence in the field. Radiological procedures are medically prescribed and should only be conducted by appropriately trained and certified physicians under medically necessary circumstances. Radiologist physicians have four to six years of unique, specific, post–medical school training that includes radiation safety and ensure the optimal performance of radiological procedures and interpretation of medical images.