I often get questions regarding the difference between a standard X-ray scan and a dual X-ray absorptiometry (DXA) scan. I wrote a recent blog post on radiation and DXA that examined the amount of radiation an individual is exposed to via a DXA scan as well as other sources, both man-made as well as naturally occurring sources. So in this post, I want to discuss some of the differences between a standard X-ray scan and a DXA scan. First of all, a DXA scanner does emit X-ray energy as does a standard X-ray scanner, however it differs from the standard X-ray in several ways.  The first difference between the scanners is the goal of the two scanners.  The goal of a standard X-ray scanner is to produce a high-quality picture of the bone and other aspects of the body.  It cannot provide any information on the density of the tissue that it is imaging. The DXA scanner on the other hand is designed to measure the density of body tissue but does not produce a high-quality picture. These differences are evident below in the two images of a hip.  The image on the left is a standard X-ray image of a right hip.  You can see the high definition of the image that allows doctors to detect fractures, etc.  The image on the right is a DXA image of the right hip.  You can see the fuzziness of the image and most DXA scan reports even state that the image is not for injury diagnosis.  

What you cannot see in the DXA image above is the information that a DXA scanner provides regarding the density of the bone of the right hip that is being imaged. In the table below, you see the detailed information that DXA scanners provide regarding bone mineral density (BMD).  The table below provides the specific BMD for the various regions of the right hip (i.e., Ward’s Triangle, Neck, Shaft, and Total Right Hip).  Also, it provides information on these values compared to aged-matched individuals as well as young adult values for these measures.  This is not possible with a standard X-ray scan.

Given the different information that can be acquired by these two scanners, they have different ways of not only delivering X-ray energy, but also the length of time the individual is exposed to the X-ray energy.  Standard X-ray scanners typically use a focused delivery of the X-ray energy over a longer period of time than the DXA scanner. This is what allows for a very detailed image of the area of interest that is observed in a standard X-ray.  A DXA scanner on the other hand uses two X-ray energies, one of which is absorbed by soft tissues and the other by bones.  The two X-ray energy levels utilized by the DXA scanner are often delivered via a fan beam. The fan beam allows for the DXA scanner to scan an area or the entire body in a very short period, thus significantly reducing the exposure to radiation.  The amount of radiation exposure from a DXA scan is so low that it is difficult to directly estimate the degree of risk associated with this low level of radiation except by extrapolation from studies that involve distinctly higher levels of radiation exposure. The radiation exposure of a DXA is comparable to the natural background radiation one receives in a 3 hour period.  A standard chest X-ray, on the other hand, is equal to the natural background radiation one receives over a 10 day period. 

So what can you take away from this blog?  Although the standard X-ray scanner and the DXA scanner both use X-ray energy to acquire information regarding bones, they go about it in different ways.  Each of these scanners is useful in diagnosing abnormities of bone, but each plays a very different role. Finally, this blog hopefully will provide you with a little more insight regarding the differences between a standard X-ray scanner and a DXA scanner.

References
Radiation Risk in Perspective. Health Physics Society. Accessed May 24, 2006. http://hps.org/documents/radiationrisk.pdf

Radiation Dose in X-Ray and CT Exams. RadiologyInfo.org. Accessed June 4, 2020. https://www.radiologyinfo.org/en/info.cfm?pg=safety-xray

United States Nuclear Regulatory Commission. (January 31, 2020). Personal Annual Radiation Dose Calculator. Retrieved from https://www.nrc.gov/about-nrc/radiation/around-us/calculator.html 

National Council on Radiation Protection and Measurements. (May 29, 2015) NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States. Retrieved from https://ncrponline.org/publications/reports/ncrp-report-160

About the Author
Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.