In the past osteoporosis was often diagnosed on the basis of a low trauma fracture, for example a fall from standing, and X-ray changes. Because the inverse relationship between bone mineral density (BMD) and fracture risk is well established, the diagnosis is usually made using BMD measurements. The World Health Organization defines osteoporosis as BMD 2.5 standard deviations or more below the mean value for young adults, and severe osteoporosis as a BMD below this cut-off and one or more fragility fractures. The main role of radiography is in the diagnosis of fractures secondary to osteoporosis. Single-photon absorptiometry involves passing a collimated beam of monoenergetic photons from a radioiodine source through a limb and measuring the transmitted radiation, using a sodium iodide scintillation detector. Dual-energy X-ray absorptiometry measures bone mineral density by determining the absorption of two beams of photons at two different energies.