Radiology is the branch of medicine which deals with the use of electro-magnetic radiation to show features inside the human body, or as a treatment tool. It includes the use of X-rays (including CT scans), magnetic resonance imaging, and radiation therapy.
Radiology started to be developed in the early part of the 20th century after the discovery of X-rays by Wilhelm Roentgen in 1895 (for which he received the first Nobel Prize in Physics in 1901). Roentgen often demonstrated X-rays by using them to show the bones in his hand, and immediately hit upon the use of the rays for medical purposes.
Also see
Magnetic Resonance Imaging, X-rays, CT scan, Radiation therapy, PET scan