Wilhelm Roentgen invented the use of cathode rays to take X-rays of the body in 1895, according to About.com. Roentgen was honored for his discovery by receiving the Nobel Prize in physics in 1901.
Roentgen documented his accidental discovery by using the rays to take a photograph of his wife's hand, the first X-ray ever performed, according to the Nobel Prize Foundation. He used the letter "X" when he named the discovery "X-ray" because so much about the concept was unknown. Roentgen declined to seek a patent for his invention, stating he wished for the world to have free access to this important technology.