DEXA (Dual Energy X-Ray Absorptiometry)

Recent advancement in the investigation of body composition and its relation with the onset of diseases has fostered the importance of using the dual energy X-ray absorptiometry (DEXA). The use of DEXA is an established method to measure bone mineral density (BMD) and mass, but is also used to assess lean body mass and whole body fat. DEXA measurements of body composition have previously been validated against a number of criterion methods including carcass analysis in animal studies. In general, the concept of the DEXA is to quantify the attenuated radiation subsequent to its passage through bone and tissue. This occurs by emitting two low-dosage X-ray beams at a patient's bones, [Page 182]which are calibrated at two energy levels. One beam absorbs mainly soft tissue, while the other concentrates on bone. The BMD can then be determined by subtracting the soft tissue from the total. Currently, DEXA systems are based on either pencil beam or fan beam systems, where the fan beam is the newer generation with faster scan times. Also, DEXA measurements are associated with low-dose radiation (2 microSieverts, :Sv, at the spine or total body measurement and 4 :Sv at the hip) compared to a chest X-ray (~60 :Sv). DEXA is one of the most precise measurements for bone densitometry (i.e., spine, hip, radius) and body composition (whole-body scan), with coefficient of variations between 1 to 2 percent. However, its accuracy is still questionable in the severely obese and very lean. For example, estimates of bone density decrease in accuracy with increases in tissue thickness. Changes in individuals' hydration levels may influence calculations for soft tissue estimates. Correction algorithms are included in operating software to account for varying effects of tissue thickness, but estimates may differ with different manufacturers. Measurements with DEXA have weight limitations from 114–160 kg, which also depend on the instrument manufacturer and model. Despite certain limitations, DEXA methodology is currently the best research tool to estimate body composition in the clinical setting in adults and the pediatric population to better understand the physiology and pathophysiology in obese individuals, and is important in the diagnosis of osteoporosis and low bone density.