A chest X-ray is often one of the first tests used to investigate lung and heart conditions because it is painless, quick, and safe. To create the image, X-rays are passed through your chest on to photographic film. Dense tissues, such as the bones, absorb X-rays and appear white; soft tissues appear grey; and air appears black. Damaged or abnormal lung tissue or excess fluid in the chest shows up as a white area because it does not contain air as it should. Chest X-rays are usually taken from behind, although side views may also be required.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.