What is the role of single x-ray absorptiometry (SXA) in the workup of osteoporosis?

Updated: Jan 19, 2021
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Single x-ray absorptiometry (SXA) is the x-ray–based equivalent of SPA and uses a filtered x-ray spectrum (55 KeV, 300 µA) with k-edge filtration and solid-state detectors. As with SPA, the arm to be measured must be placed in a water bath to allow correction for the overlying soft tissues. The source and counter move together over the body part being examined, creating an image.

SXA has been used only with the radius and calcaneus. The area of interest is positioned in tissue-equivalent material to produce uniform soft-tissue uptake that can then be subtracted from the image for the calculation of bone density. The distal radius is the most sensitive region for measuring bone density in most disease processes because this site reflects the high turnover of trabecular bone.

The difference in photon absorption between bone and soft tissue allows the calculation of the total bone mineral content in the scanning path. Bone mineral content is expressed as grams of bone mineral per square centimeter imaged.

The equipment is relatively compact and mobile, and scanning usually takes about 5 minutes with the forearm in a standard position. The accuracy is 3%, and the precision is better than 1% in the distal forearm. The radiation dose is less than 0.1 µSv. The Osteoanalyser measures BMD in the heel; scanning is completed in 2 minutes with a precision better than 1%. [44]

Rectilinear scanning is performed in the distal (87% cortical bone) and ultradistal forearm (65% trabecular bone). Results are expressed as BMD or as bone mineral content in grams per square centimeter.

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