X-Rays - How They Work

Learn how an X-ray image is made.

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X-ray of scoliosis. University of Utah Hospital - Radiology Department (X-Ray Image) [Public domain], via Wikimedia Commons

X-rays are the oldest form of diagnostic imaging tests.  Although advances in technology have brought us MRIs, CT scans and more, X-rays continue to be used widely for diagnosing spine and other disorders.

While X-rays are generally not recommended to doctors as the initial step in diagnosing neck or back pain, they can be key for finding sources of pain and/or movement limitations.

X-rays are particularly useful for seeing fractures and other things that may be going on in your bones.

 Doctors generally integrate the information gleaned from X-rays and/or other diagnostic imaging tests with the information gathered during your medical history intake and physical exam. 

Often digitized, X-ray films are easy and inexpensive to take, says Robert Boyles, P.T., and clinical assistant professor of physical therapy at the University of Puget Sound.

The downside of X-ray tests is they emit ionizing radiation; this may be harmful to the tissue that is exposed to the radiation, Boyle continues.

How X-Ray Images Are Made

X-rays are generated when an electron beam hits a metal target. The beam and the metal are enclosed in a tube, called the X-ray tube. Once generated, the X-ray leaves the tube through a small opening. This opening functions a bit like an iris in a camera because it regulates the size of the beam.

Outside the tube, the X-ray beam is directed at a plate of film. The body part being imaged is placed in between the tube and the plate of film.

 It absorbs the x-ray to varying degrees.

The density, thickness and the atomic weight of the body tissue being X-rayed determine how much beam reaches the film, and how much is blocked. The denser the body part, the less transparent its image is on the film, and vice versa.

When an X-ray beam reaches the film, the film turns black and the effect is called radio-lucent.

White areas on an X-ray are made when something blocks the beam.  This effect is called radio-opaque.

Either way, this process is called attenuation, or the degree to which X-rays are absorbed or deflected by the body part being imaged. Attenuation is related to radiation dose.

Related:  How much radiation exposure will you get from a spine MRI?

Let's compare a few body substances for their opaqueness and translucency on X-rays. Relatively speaking, bone is the most dense tissue and air or gases are the least. In between are water and fat - water is more dense on an X-ray than fat.

Because bones are of high density, they show up as white on the resulting film. Skin, on the other hand, is much less thick and therefore becomes translucent, or even transparent on the film. This is how the X-ray so readily shows the bone underneath the skin.

If contrast dye is injected in a CT scan (which is basically an X-ray that yields 3D images), that dye is more dense than your bone, and will therefore show up as white.

Sometimes metal markers are added to X-rays.  Metal markers are thicker than contrast dye and also show up as white.

As you might imagine, the thickness of the body makes a difference in the quality of an X-ray. For this reason, larger people may need increased exposure time or a more intense beam in order to get the same film clarity as a smaller person.

When it Comes to Reading an X-Ray

Regardless of how clear an X-ray is, it's important that the radiologist who reads it does so carefully and thoroughly, and accurately correlates the findings with what they may indicate, Boyle comments. The doctor who prescribes the X-ray should communicate ahead of time with the reading radiologist about why the films are being taken. This may better equip the radiologist for focusing on the most important areas of concern, which in turn may help you get an accurate diagnosis for your symptoms, he concludes.

Related:  Communicate your symptoms with your doctor.

Sources:

U.S. Food and Drug Administration. What is Computed Tomography? FDA Website. Last Updated April 23 2014. http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115318.htm

U.S. National Library of Medicine, National Institutes of Health (NIH). Lumbar spine CT scan. Medline Plus Website. Last Updated. January 17, 2013. http://www.nlm.nih.gov/medlineplus/ency/article/007350.htm

Boyles, Robert, PT. Dsc, OCS, FAAOMPT. Basic Musculoskeletal Radiology and Imaging. MedBridge Education Website. Accessed August 28, 2014. http://www.medbridgeeducation.com

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