Radiologists are medical doctors who specialize in diagnosing and treating injuries and diseases using medical imaging procedures such as X-rays, computed tomography (CT), magnetic resonance imaging (MRI), nuclear medicine, positron emission tomography (PET), and ultrasound. They work as consultants to their clinical colleagues and are sometimes referred to as a "doctors doctor". Radiologists complete at least 13 years of training, including medical school, a four-year residency, and most often, an additional one- or two-year fellowship of very specialized training, such as radiation oncology, pediatric radiology, or interventional radiology. They are certified by the American Board of Radiology, and they have exacting requirements for continuing medical education throughout their practicing years. Radiologists are experts in interpreting a multitude of imaging exams and are highly skilled in understanding how to obtain the best quality imaging exam and what is the best test to obtain. They are at the forefront of imaging technology, pioneering the use of CT, MRI, PET, and fusion imaging as well as minimally invasive procedures such as endovascular treatment of aneurysms and tumors, percutaneous biopsies, and radiation therapy. Radiologists are also trained in radiation safety and protection, and they are highly trained in keeping people safe from the harmful effects of radiation.