Endodontists are dentists who have specialized in this field; qualification as an endodontist typically requires an additional 2-3 years of training following dental school. Many endodontic residents do original research and earn a Master's degree as well as a specialty certificate. They specialize and limit their practice to root canal therapy and root canal surgery, and use their special training and experience in treating difficult cases that are referred to them by general dentists who opt not to perform these cases, such as teeth with narrow or blocked canals, or unusual anatomy. Endodontists may use advanced technology, such as operating microscopes, ultrasonics and digital imaging, to perform these special services, and often have great experience in successfully treating patients who present in pain.