Soft tissue
Soft tissue and bone have different attenuation coefficients to X-rays. A single X-ray beam passing through the body will be attenuated by both soft tissue and bone, and it is not possible to determine, from a single beam, how much attenuation was attributable to the bone. However, the attenuation coefficients vary with the energy of the X-rays, and, crucially, the ratio of the attenuation coefficients also varies. DXA uses two energies of X-ray. The difference in total absorption between the two can be used, by suitable weighting, to subtract out the absorption by soft tissue, leaving just the absorption by bone, which is related to bone density.