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Plain X-Rays

by Jon A. Jacobson, MD

X-rays are high-energy radiation waves that can penetrate most substances (to varying degrees). In low doses, x-rays are used to produce images that help doctors diagnose disease. In high doses, x-rays (radiation therapy) is used to treat cancer. X-rays may be used alone as plain x-rays or combined with other techniques, such as computed tomography (CT).


For x-ray imaging, a person is positioned so that the body part to be evaluated is between the x-ray source and a device that records the image. The examiner goes behind a screen that blocks x-rays and runs the x-ray machine for only a few seconds. The person must remain still when the x-ray is taken. Several x-rays may be taken to obtain images from different angles.

Did You Know

  • Radiation exposure from x-rays is very small.

An x-ray beam is passed through the part body to be evaluated. Different tissues block different amounts of the x-rays, depending on the tissue’s density. The x-rays that pass through are recorded on a film or radiation detector plate, producing an image that shows the different levels of tissue density. The denser the tissue, the more x-rays it blocks and the whiter the image:

  • Metal appears completely white (radiopaque).

  • Bone appears almost white.

  • Fat, muscle, and fluids appear as shades of gray.

  • Air and gas appear black (radiolucent).


Plain x-rays are typically the first imaging test done to evaluate the arms, spine, legs, or chest. These body parts contain important structures with very different densities that are easily distinguished on x-rays. Thus, x-rays are used to detect the following:

  • Fractures: The almost white bone contrasts clearly with the gray muscles around it.

  • Pneumonia: The black air in the lungs contrasts clearly with the white infected tissues, which block the x-rays.

  • Blockages of the intestine: The black air in the blocked intestine contrasts clearly with the gray surrounding tissues.


X-Rays With a Radiopaque Dye

Plain x-rays can be done after a radiopaque dye (contrast agent) is given, usually by injection, by mouth, or into the rectum.

In conventional angiography, x-rays are taken after a dye is injected into blood vessels.

Before x-rays of the gastrointestinal tract, people may be asked to swallow barium or gastrografin (opaque substances) in a liquid or food. The x-rays then show the esophagus, stomach, and small intestine outlined by the barium or gastrografin. Or, an examiner may inject barium through a tube inserted into the anus (barium enema), then carefully pump air into the lower part of the intestine (colon) to expand it. Barium makes ulcers, tumors, blockages, polyps, and diverticulitis easier to see. A barium enema may cause mild to moderate crampy pain and an urge to defecate.


This technique produces images that show motion, similar to those of a video camera. Fluoroscopy can show organs or structures as they function: the heart beating, the intestines moving food along, or the lungs inflating and deflating. Fluoroscopy is commonly used to determine whether a catheter is correctly placed in the heart during electrophysiologic testing (for abnormal heart rhythms) and during coronary catheterization. When the gastrointestinal tract is evaluated, a radiopaque dye, such as barium, is usually given by mouth.


For plain x-rays, each image requires only a very small amount of radiation. For chest x-rays, the amount of radiation exposure with a single image is similar to the amount most people get from the environment in 2.4 days (background radiation exposure—see see Radiation Injury). However, some x-ray tests require several images, a high dose of radiation for each image, or both. As a result, the total radiation exposure is higher. For example, for x-rays of the lower back, done in a series, the amount of radiation equals about 3 months of background exposure. For mammography, the amount equals about 1 to 2 months of background exposure. Fluoroscopy usually requires high doses of radiation, so other imaging tests are done instead when possible.

Examiners take precautions to minimize a person’s exposure to radiation. Women who are or could be pregnant should tell their doctor. Then, the examiner can take all possible precautions to shield the fetus from exposure. To evaluate the abdomen or pelvis of a pregnant woman, the doctor can sometimes substitute an imaging test that does not use radiation, such as ultrasonography. However, plain x-rays that do not involve the abdomen or pelvis usually expose the uterus to only very small amounts of radiation.

Some particular tests have other risks. For example, barium swallowed or inserted by enema may cause constipation.

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