The discovery of X-rays represents one of the most significant accomplishments in medical history. Though the shades of white, grey and black on an X-ray film may not indicate much to the untrained eye, to trained and experienced professionals, they reveal a multitude of information about internal anatomical structures.
Mode of Action
X-rays work because the body's tissues have different densities. Dense structures, such as bones, absorb radiation rather than allow it to pass through. They appear white on the film. Since soft tissues are less dense, fewer X-rays are absorbed, more penetrate the tissues and structures appear grey. X-rays readily penetrate air, which appears black on radiographs.
Lucency vs. Opacity
A lucency on an X-ray image represents an area absorbing less radioactive energy than the surrounding tissue. Lucent areas appear dark compared to the surrounding area. An opacity indicates an area absorbing more radioactive energy than the surrounding tissue and will appear as lighter spots.
X-rays reveal conditions that cause a reduction in density of body structures. Osteoporosis, osteomyelitis and cancer represent bone disorders in which the diseased and degraded areas show up as dark spots in the white bone. Conditions caused by the leakage or entrapment of air in regions of the body where it is not normally present, such that which accompanies a collapsed lung or some bowel conditions, also cause black lucent areas on X-rays.