X–ray tests work by passing small amounts of electromagnetic radiation through the body to produce images of bones, teeth and internal organs. The type of x–ray test administered depends on which part of the body needs to be examined.

The x–ray is the oldest imaging technology still in use today. It was discovered almost by accident in the 1890s by a German scientist named Wilhelm Conrad Roentgen. Because no one understood the rays that were at work in forming these images, the rays were initially called x–rays. Much more is now known about this form of radiation. 

Bones and cartilage tend to be dense and absorb x–rays better than do soft tissues (e.g., skin, fat, internal organs). As a result, bones appear white on x–rays whereas tissues often are gray. Organs or structures containing mostly air, such as the lungs, appear black. Generally, the denser the bone or organ, the more clearly it will show up with an x–ray. The results may first be viewed by a physician with special training in interpreting x–ray results (radiologist) before they are explained to the patient.

An x–ray machine produces a beam of radiation that is pointed by a tube at the part of the body being examined. The area to be examined is placed between the metal film cassette and the x–ray tube, which is surrounded in lead casing except for a small opening through which the rays are beamed. Within the x–ray tube, a beam of electrons is fired into a target made of a metallic element known as tungsten, emitting photons that travel straight and outward from a point on the target to the area being x–rayed. The resulting images are recorded on either film or a fluorescent screen.

The energy contained in the photons may cause some cell damage. Most of this cell damage is repaired quickly, but some may be permanent. Considering the low doses of radiation in modern x–ray tests, most physicians believe the benefits of x–rays outweigh the very small risk associated with them. However, young children and developing fetuses are more sensitive to radiation. Women should advise their physician if they are pregnant before getting an x–ray.

Additional techniques may be necessary to get a clear view of soft tissues, hollow areas or fluid–filled body parts. For example, a physician may administer a special dye known as a contrast medium to produce sharper contrasts in light and dark areas on the x–ray. These contrasts give the physician a clearer image of areas such as the urinary tract, the spinal cord, blood vessels and the gallbladder.