X-rays (radiography), discovered in 1895, were the first imaging technique used for diagnosing medical problems.
X-rays are high-energy electromagnetic waves that have a shorter wavelength than visible light or radio waves. When a beam of X-rays is passed through the body, some parts of the body absorb more radiation than other parts, producing a darker, shadowy image on the X-ray film, or radiograph. X-ray images can also be viewed on a fluorescent screen, or monitor. Dense structures (such as bone) allow few X-rays to pass through, so these structures appear lighter, or white, on the film. Fat and other soft tissues absorb less radiation, and so they look somewhat gray. Hollow structures (such as the lungs) allow even more radiation to pass through, making them appear darker, or black, on film. Structures that are hollow (such as the intestines and blood vessels) can show up more clearly on X-rays if they are filled with a dye such as barium sulfate, which blocks the X-rays.