Body Composition by DXA
Body composition measurements from DXA have been available since DXA technology was developed 30 years ago, but are historically underutilized. Recently, there have been rapid developments in body composition assessment including the analysis and publication of representative data for the US, official usage guidance from the International Society for Clinical Densitometry, and development of regional body composition measures with clinical utility. DXA body composition is much more than whole body percent fat. In this paper celebrating 30 years of DXA for body composition, we will review the principles of DXA soft tissue analysis, practical clinical and research applications, and what to look for in the future.
Why use DXA to measure body composition?
DXA is a special imaging modality that is not typically available on general use x-ray systems because of the need for special beam filtering and near-perfect spatial registration of the two attenuations. The whole body can be scanned to measure whole body bone mass and soft tissue composition. DXA is the preferred method for bone and body composition for several reasons. First, there are few assumptions required for DXA composition measurements. The two X-ray attenuations passing through the body can be used to accurately calculate the mass of two different materials given simple algebra and the physical properties of those materials. There were details to work out, such as how to quantify the soft tissue mass in a divergent fan-beam geometry, but the fundamental nature of DXA gives it the promise of accuracy over a wide range of body sizes and body types. Second, DXA can measure regional body composition by subdividing the body using specific well-defined cut lines. Third, DXA is precise and stable for years. Using phantoms, it is straightforward to verify measurement stability of better than 0.5% change in body composition accuracy over decades of operation for a single DXA system.
