X-ray
The oldest and best known of the medical imaging techniques. X-rays, discovered by Röntgen more than 100 years ago, are high energy radiation produced in a special type of lamp called an X-ray tube. X-rays are also a form of electromagnetic radiation, but with a much shorter wavelength and a higher frequency than visible light. The amount of energy in x-rays is so great that the rays pass through the body/target to strike a photographic film positioned on the other side. However, some of the radiation is stopped, depending on the density of the tissues it passes through.
Tissues in the human body vary as to type, thickness and hardness, and some kinds of tissues allow more X-rays to pass through than others. Bones, for example, permit few X-rays to penetrate because of their high density and calcium content. Therefore, bones appear white on X-ray film. The more atoms a structure contains, and the heavier the atoms, the more X-rays the structure will absorb and the whiter the structure will appear on the developed film.
Blood vessels and soft tissues, such as the intestines or kidneys, allow X-rays to pass through fairly easily. Moreover, these tissues are not well differentiated from the surrounding tissues, i.e. they permit just approx. about as many X-rays to pass through as do the surrounding tissues. To obtain a clear picture of the kidneys, then, one needs to alter the way in which X-rays are transmitted in these organs. One way of achieving this is to inject a X-ray contrast agent into the blood stream, which goes to the kidneys.
The photographic film on the far side of the target will be blackened in a variying degree, depending on the intensity of the radiation that reaches it . The film will remain white where the radiation has been blocked by bone (high content of calcium), and dark where it has passed through air, as in the lungs, and different hues of grey where it has passed through muscle, fat and other soft tissues.
If a contrast medium has been injected, blood vessels and specific organs will let through less radiation, resulting in less film exposure, i.e. whiter areas on the film. CT is a further development of X-ray.
GE Healthcare Glossary