X-rays are high-energy radiation waves that can penetrate most substances (to varying degrees). In very 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 Computed Tomography (CT) In computed tomography (CT), which used to be called computed axial tomography (CAT), an x-ray source and x-ray detector rotate around a person. In modern scanners, the x-ray detector usually... read more 
(CT). (See also Overview of Imaging Tests Overview of Imaging Tests Imaging tests provide a picture of the body’s interior—of the whole body or part of it. Imaging helps doctors diagnose a disorder, determine how severe the disorder is, and monitor people after... read more and Background radiation Sources of Radiation Exposure Radiation injury is damage to tissues caused by exposure to ionizing radiation. Large doses of ionizing radiation can cause acute illness by reducing the production of blood cells and damaging... read more .)
Procedure for X-Rays
For conventional 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 fraction of a second. The person must remain still when the x-ray is taken. Several x-rays may be taken to obtain images from different angles.
An x-ray beam is aimed at the body part 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).
Uses of X-Rays
Plain x-rays are typically the first imaging test done to evaluate the arms, legs, or chest and sometimes the spine and abdomen. These body parts contain important structures with very different densities that are easily distinguished on x-rays. Thus, plain x-rays are used to detect the following:
Fractures Overview of Fractures A fracture is a crack or break in a bone. Most fractures result from force applied to a bone. Fractures usually result from injuries or overuse. The injured part hurts (especially when it is... read more
: The almost white bone contrasts clearly with the gray muscles around it.Pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more
: The black air in the lungs contrasts clearly with the white infected tissues, which block more of the x-rays.Blockages of the intestine Intestinal Obstruction An obstruction of the intestine is a blockage that completely stops or seriously impairs the passage of food, fluid, digestive secretions, and gas through the intestines. The most common causes... read more : The black air in the blocked intestine contrasts clearly with the gray surrounding tissues.
Mammography
In mammography Mammography Breast cancer occurs when cells in the breast become abnormal and divide uncontrollably. Breast cancer usually starts in the glands that produce milk (lobules) or the tubes (ducts) that carry... read more 
, x-rays are also used to screen and diagnose breast disorders, including breast cancer.
Radiation exposure is a concern because breast tissue is sensitive to radiation. Specialized mammography units and digital imaging techniques are used to minimize radiation exposure.
Variations of X-Rays
X-rays with a radiopaque contrast agent
Plain x-rays can be done after a radiopaque contrast agent Radiopaque Contrast Agents During imaging tests, contrast agents may be used to distinguish one tissue or structure from its surroundings or to provide greater detail. Contrast agents include Radiopaque contrast agents... read more (sometimes inaccurately called dye) is given, usually by injection into a vein, by mouth, or injected through a tube into the rectum. The radiopaque contrast agent makes the tissue or structure being imaged appear more radiopaque (whiter) than surrounding tissues, so that it can be better seen on an x-ray.
In conventional angiography Angiography In angiography, x-rays are used to produce detailed images of blood vessels. It is sometimes called conventional angiography to distinguish it from computed tomography (CT) angiography and magnetic... read more , x-rays are taken after a radiopaque contrast agent is injected into blood vessels.
Before x-rays of the gastrointestinal tract, people may be asked to swallow barium or gastrografin (which are radiopaque contrast agents) 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 detect. A barium enema may cause mild to moderate crampy pain and an urge to defecate.
For imaging of the esophagus, stomach, and upper intestinal tract, endoscopy Endoscopy Endoscopy is an examination of internal structures using a flexible viewing tube (endoscope). In addition to examinations, doctors can use endoscopy to do biopsies and give treatment. Endoscopes... read more has largely replaced x-rays taken after barium or gastrografin is used.
Fluoroscopy
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
During electrophysiologic testing Electrophysiologic Testing Electrophysiologic testing is used to evaluate serious abnormalities in heart rhythm or electrical conduction (see Overview of Abnormal Heart Rhythms). In people in whom an arrhythmia is already... read more (for abnormal heart rhythms) and during coronary catheterization Cardiac Catheterization and Coronary Angiography Cardiac catheterization and coronary angiography are minimally invasive methods of studying the heart and the blood vessels that supply the heart (coronary arteries) without doing surgery. These... read more
to determine whether a catheter is correctly placed in the heartWith a radiopaque contrast agent (such as barium), usually given by mouth, to evaluate the gastrointestinal tract
During evaluation of musculoskeletal injuries to observe movement of bones and joints
Disadvantages of X-Rays
Other imaging tests may provide better detail, be safer or faster, or help doctors diagnose a disorder more accurately than plain x-rays.
The main disadvantage is
Exposure to radiation
Radiation exposure
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 Background radiation Radiation injury is damage to tissues caused by exposure to ionizing radiation. Large doses of ionizing radiation can cause acute illness by reducing the production of blood cells and damaging... read more ).
However, some x-ray tests require several images, a higher dose of radiation for each image, or both. As a result, the total radiation exposure is higher, as in the following examples:
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 higher doses of radiation than routine plain x-rays, 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.
Other disadvantages
Some particular tests have other risks Risks of Radiation in Medical Imaging Imaging tests that use radiation, usually x-rays, are a valuable tool in diagnosis, but exposure to radiation has some risks (see also Radiation Injury). Different diagnostic tests require different... read more . For example, barium swallowed or inserted by enema may cause constipation.
