DXA body composition: theory and practice.

Dual energy X-ray absorptiometry (DXA) was used to measure human body composition on a high speed fan beam scanner. High and low energy attenuation pairs, produced by the various combinations of fat mass and fat-free mass in the human body, were compared to attenuation values produced by standard materials (aluminum and acrylic). These standards were measured in various combinations to construct calibration curves for fat and fat-free mass. Primary calibration of the aluminum/acrylic combinations was achieved by direct comparison to the dual energy attenuation produced by stearic acid and pure water. Whole body examinations were accomplished using three 45 s longitudinal passes of the fan beam. These passes were acquired and assembled to create a giant, isocentric fan beam with a single center of focus. In vivo precision was 0.009 g/cm2 for BMD and 425 g for fat mass and fat-free mass (SD).