X rays are electromagnetic radiation that differentially penetrates structures within the body and creates images of these structures on photographic film or a fluorescent screen. These images are called diagnostic x rays.
Diagnostic x rays are useful in detecting abnormalities within the body. They are a painless, non-invasive way to help diagnose problems such as broken bones, tumors, dental decay, and the presence of foreign bodies.
X rays are a form of radiation similar to light rays, except that they are more energetic than light rays and are invisible to the human eye. They are created when an electric current is passed through a vacuum tube. X rays were accidentally discovered in 1895 by German physicist Wilhem Roentgen (1845-1923), who was later awarded the first Nobel Prize in physics for his discovery. Roentgen was also a photographer and almost immediately realized that the shadows created when x rays passed through the body could be permanently recorded on photographic plates. His first x-ray picture was of his wife's hand. Within a few years, x rays became a valued diagnostic tool of physicians world-wide.
How x rays work
X rays pass easily through air and soft tissue of the body. When they encounter more dense material, such as a tumor, bone, or a metal fragment, they are stopped. Diagnostic x rays are performed by positioning the part of the body to be examined between a focused beam of x rays and a plate containing film. This process is painless. The greater the density of the material that the x rays pass through, the more rays are absorbed. Thus bone absorbs more x rays than muscle or fat, and tumors may absorb more x rays than surrounding tissue. The x rays that pass through the body strike the photographic plate and interact with silver molecules on the surface of the film.
Once the film plates have been processed, dense material such as bone shows up as white, while softer tissue shows up as shades of gray, and airspaces look black. A radiologist, who is a physician trained to interpret diagnostic x rays, examines the pictures and reports to the doctor who ordered the tests. Plain film x rays normally take only a few minutes to perform and can be done in a hospital, radiological center, clinic, doctor's or dentist's office, or at bedside with a portable x-ray machine.
Special types of x-ray procedures
Mammograms are fixed plate x rays that are designed to locate tumors within the breasts. Dental x rays are designed to locate decay within the tooth. Sometimes a liquid called contrast material (for example, barium) is used to help outline internal organs such as the intestines. The contrast material absorbs x rays, helping to make soft tissue more easily visible on the x-ray films. Contrast material is commonly used in making x rays of the digestive system. The contrast liquid can be swallowed or injected, depending on the part of the body being x rayed. This may cause some minor discomfort.
Fluoroscopy is a special x-ray technique that produces real-time images on a television monitor. With fluoroscopy, contrast material is injected into a blood vessel. The physician can then watch the real-time movement of the contrast material to determine if there are blockages in circulation. Fluoroscopy is also used to help guide catheters into place in the heart during cardiac catheterization or to guide an endoscope during endoscopic surgery.
Computed tomography or CT scan works on the same principles as fixed plate x rays, only with a CT scan, an x ray tube rotates around the individual, taking hundreds of images that are then compiled by a computer to produce a two-dimensional cross section of the body. Although many images are taken to produce a CT scan, the total dose of radiation the individual is exposed to is low. Other common imaging techniques such as magnetic resonance imaging (MRI) and ultrasound do not use x rays.
How x rays are performed
Fixed plate x rays are extremely common diagnostic tests. A trained x-ray technologist takes the x ray. The individual is first asked to remove clothing and jewelry and to wear a hospital gown. The x ray technologist positions the patient appropriately, so that the part of the body to be x rayed will be between the x-ray beam and the film plate. Usually the individual either lies on an adjustable table or stands. Parts of the body that are especially sensitive to damage by x rays (for example, the reproductive organs, the thyroid) are shielded with a lead apron. Lead is very dense and effectively protects the body by stopping all x rays.
It is essential to remain motionless during the x ray, since movement causes the resulting picture to be blurry. Sometimes patients are asked to hold their breath briefly during the procedure. Children who are not old enough follow directions or who cannot stay still may need to be restrained or given medication to sedate them in order to keep them still enough to obtain useful results. Sometimes parents can stay with children during an x ray, unless the mother is pregnant, in which case she must protect the fetus from x-ray exposure.
If a contrast material is to be used, the individual will be given special instructions to prepare for the procedure and may be asked to remain afterwards until recovery is complete. (See Preparation and Aftercare below.)
Although unnecessary exposure to radiation should be avoided, the low levels of radiation one is exposed to during an x ray does not cause harm with a few exceptions. Pregnant women should not have x rays unless in emergencies the benefits highly outweigh the risks. Exposure of the fetus to x rays, especially during early pregnancy can increase the risk of the child later developing leukemia. Body parts not being x rayed should be shielded with a lead apron, especially the testes, ovaries, and thyroid.
No special preparation is needed for fixed plate x rays unless contrast material is used. When x rays are scheduled that involve the use of contrast material, the physician will give specific instructions for preparation. For example, in a lower GI series, the individual may have to fast and use special laxatives to cleanse the bowel before swallowing the contrast material. Parents can prepare children for x rays be explaining what will happen and that these tests are short and painless.
Little aftercare is needed following an x ray. In complicated x rays where contrast material is injected into a blood vessel, the individual may need to remain under medical care for a short while to assure that there is no allergic reaction to the contrast material and recovery is complete.
Contrast agent —Also called a contrast medium, this is usually a barium or iodine dye that is injected into the area under investigation. The dye makes the interior body parts more visible on an x-ray film.
Electromagnetic radiation —Packets of energy that develop when an electric current passes through a vacuum tube.
Endoscope —A medical instrument that can be passed into an area of the body (the bladder or intestine, for example) to allow visual examination of that area. The endoscope usually has a fiberoptic camera that allows a greatly magnified image to be shown on a television screen viewed by the operator. Many endoscopes also allow the operator to retrieve a small sample (biopsy) of the area being examined, to more closely view the tissue under a microscope.
Low dose exposure to x rays creates minimal cell damage and minimal risk when x rays are performed in an accredited facility. There is an increased risk that a developing fetus will develop leukemia during childhood if exposed to x-ray radiation; pregnant or potentially pregnant women should avoid x rays. There is also a slight risk of an allergic reaction to the contrast material or dye used in certain x rays.
Some parents are concerned about health consequences of their child's exposure to x-ray radiation. However, doses of radiation received in most x rays are quite similar to the environmental (background) radiation one is exposed to simply by living on Earth. Although unnecessary x rays should be avoided, in most cases, the benefits greatly outweigh the potentially small increased risk of exposure.
See also Computed tomography .
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American College of radiology 1891 Preston White Drive, Reston, Virginia 20191-4397. Telephone (800) 227-5463. http://www/acr.org/flash.html
Radiological Society of North America. 820 Jorie Boulevard, Oak Brook, Illinois 60523-2251. Telephone: (800) 381-6660. http://www.rsna.org
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Tish Davidson, A.M.