What is the role of dual-photon absorptiometry (DPA) 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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Answer

Dual-photon absorptiometry (DPA) is an extension of the SPA principle that was developed to compensate for errors in SPA bone-mass measurements due to the varying composition and thickness of surrounding soft tissues. This deficiency of SPA was overcome by using 2 distinct photon energies, usually gadolinium-153. Photons of different energy are differentially attenuated by bone and soft tissues. Therefore, their absorption by bone, and hence bone density, can be calculated by measuring the percentage of each transmitted beam and then by applying simple simultaneous equations. The source of photons is 153Ga, which emits photons of 2 discrete energies (44 and 100 keV). The scanning approach is similar to that of SPA.

DPA represents an improvement over SPA in that it allows the direct measurement of vertebral or femoral bone density. DPA eliminates the requirement that soft tissue thickness be constant across the scanning path (allowing its use in areas such as the spine and femur). DPA can be used to quantify changes in patients with metabolic bone disease or in those undergoing treatment with drugs that alter bone mineral content.

The desirable characteristics of DPA include its capability in assessing vertebral, proximal femoral, or total body bone content; its independence from effects of marrow fat and other soft tissue; and its relatively low radiation dose. However, it is more expensive than other techniques, it has a longer scanning time, and it is not as widely available as SPA.


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