Plain films, or "x-rays," were historically the first radiologic technique used and are still the most commonly ordered in medical care. X-ray images are created by ionizing radiation passing through the body onto a special photographic plate (as described in the History of Radiology). Different body tissues absorb this radiation to varying degrees. This property is referred to as the radiodensity of a tissue. The more radiodense a tissue, the brighter it appears on the film. For example, bone is very radiodense, absorbing more radiation than most other body tissue. Bone, therefore, appears white on plain film. On the other hand, air is not very radiodense, and air-filled structures (such as the lungs) appear much darker on plain film.

Plain films make use of these properties in different ways. Fractures in bone can be visualized by finding dark lines, indicating an interruption of the normally very radiodense bone. Conversely, diseases such as pneumonia (in which fluid accumulates in the lung), appear as unusually bright, radiodense areas of the normally dark-appearing lung.

Plain films can be obtained quickly and are inexpensive. Therefore, x-rays are often used as the initial imaging modality. If read properly by a skilled radiologist or other well-trained physician, they are often the only imaging required for diagnosis for many diseases. In other cases, an abnormality may be noted on plain film which requires further study with a more sophisticated imaging modality, such as CT or MRI.

Possible risks associated with x-rays are due to exposure of the body to radiation. Studies suggest that too much exposure to ionizing radiation can increase the risk of cancer. However, unlike the high doses from radiation therapy for cancer, the amount of radiation used in diagnostic radiographic procedures is actually much lower than exposure from the natural environment (from sources such as the sun and microwaves).

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