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GENERAL RADIOLOGY
PROCEDURES |
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Virtually everyone is familiar with diagnostic X-rays—the basic radiologic
technique—but many do not realize that the underlying principle is quite
simple. To make an X-ray image, the area to be examined is placed between
a metal cassette holding the film and the X-ray tube. The heat generated
by electrical current passing through this tube produces a beam of
radioactive particles.
X-rays pass most easily through air and soft tissue, but are stopped when they encounter bone, which is made up of calcium, phosphate, and other minerals. When the film, which is covered with millions of microscopic particles of silver, is developed, the particles that have interacted with the X-rays will be embedded on the film. If no interaction took place, the silver will wash away. Thus, if you look at a chest X-ray, the lungs appear black because the X-rays easily pass through their air spaces and interact with the silver on the film. In contrast, the ribs appear white because they are bone and do not let the X-rays pass through them as easily to react with the silver on the film, which is washed away during processing. X-rays pass through soft body tissues—muscles, tendons, nerves, blood vessels, intestines, and other organs—but with a more or less uniform density. To obtain X-ray images in which an internal organ stands out, a contrast material is introduced into the body. Depending on what is being examined, this may be swallowed or inserted (for example, the barium drink or enema that is given before X-raying the intestinal tract), or injected into a vein or artery. The major drawback to X-ray examinations is that they expose the body to ionizing radiation, which carries certain health risks. Although the amount of radiation exposure from a routine X-ray examination is not enough to be dangerous for most people, there are important exceptions. Even a low dose of X-rays can cause birth defects when a fetus is exposed to them during critical stages of development. Radiation exposure can also damage the male sperm and female eggs and result in genetic defects. X-rays and other radiation exposure have a cumulative effect, so receiving frequent examinations over a period of time increases the risk of cancer. For these reasons, X-rays are not recommended during pregnancy unless they are absolutely necessary, in which case the mother and fetus should be protected with a lead shield. |
