A radiologist is a physician who specializes in the use of medical imaging to diagnose and treat illnesses or injuries in patients.
The different types of medical imaging are X-ray, computed tomography (CT) scans, magnetic resonance imaging (MRI), ultrasound, and nuclear medicine.
Radiologists are experts in these different types of tests and can advise a primary care doctor on which test is most appropriate in a specific case. These doctors also assist primary care doctors in analyzing the images produced by these tests in order to determine next steps necessary for treatment.

Radiologists help doctors get a closer look at what’s happening inside your body. If your primary care doctor wants to investigate your symptoms further, they may refer you to a radiologist to get an ultrasound or x-ray. Some radiologists specialize in mammography and breast imaging, which is who you see when you need a mammogram. A Radiologist can also determine if bones are broken or fractured after any kind of accident.
Radiologists are trained to perform MRIs and CT scans, both of which are used to determine the presence of diseases or disorders and help your doctor properly diagnose you. They can detect anything from tumors, bleeding and infections to bone and muscle disorders.

Nuclear medicine specialists are doctors who have extensive training in the use of nuclear substances, like radiation, to diagnose and treat certain illnesses.
These specialists perform diagnostic tests in which a small amount of a radioactive substance is injected into a vein, inhaled or taken orally by the patient. The radioactive substance then enables the nuclear medicine specialist to use a scanner to detect the radiation level in the part of the body being examined in order to create an image of it with a computer. The images tell the physician whether there are abnormal changes in metabolism which indicate whether a disease or injury is present. These tests are often used to detect cancer.
In addition to detecting cancer, substances like radioactive iodine can be either injected into the blood stream or taken orally to fight cancer and other diseases. After being ingested, the substance is absorbed by the thyroid gland and abnormal thyroid tissues are destroyed.