The DXA method evolved from earlier single and dual photon absorptiometry methods for evaluating bone mineral. DXA systems share in common an x-ray source that, after appropriate filtration, emits two photon energy peaks. The attenuation of the two energy peaks relative to each other depends on the elemental content of tissues through which the photons pass. Bone, fat, and lean soft tissues are relatively rich in calcium/phosphorus, carbon, and oxygen, respectively. DXA systems are designed to separate pixels, based on appropriate models and relative attenuation, into these three components. There are no known factors, including hydration effects that significantly influence the validity of DXA fat and bone mineral estimates. Excessive or reduced fluid volume would be interpreted as changes in lean soft tissue. The radiation exposure is minimal and can be used in children and adults of all ages. DXA measures in persons who fit within the DXA field-of-view have good reproducibility for total body and regional components.
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