Dual-energy X-ray absorptiometry (DXA, or formerly DEXA) is a technique used to measure bone mineral density (BMD). The preferred regions for BMD measurement are lumbar spine, proximal femur and whole body. The technique relies on transmission measurements made at two photon energies to allow calcium, and thereby bone mineral, be assessed. There has been significant growth in the application of DXA over the last decade. Earlier reports found that the patient dose per examination is at the lower end of the diagnostic radiology range [UNSECAR, 2000]; with good practice it should be possible to maintain this position despite some increase in dose.With regard to nomenclature, the abbreviation “DXA” was proposed by the International Society for Clinical Densitometry (ISCD) in 2003, as an alternative to “DEXA”; this form is used here.
BMD techniques were originally based on single-photon absorptiometry, in which transmission of a scanning pencil beam, from a radionuclide source, through the patient was measured. The first-generation of modern DXA scanners used a pencil X-ray beam; later designs employ fan beams, cone beams and c-arm technology and, thereby, allow more rapid and convenient scanning. Bone mineral assessment has now become widely available, frequently outside the framework for conventional radiological services. This has given rise to concerns about referral patterns, justification and optimisation. There are relatively few studies of dose from or the accuracy of DXA by comparison with those available for other radiology modes. However, a recent widely based survey of equipment and its use has been undertaken for the EU within the SENTINEL Project [Larkin et al., 2008, Sheahan et al., 2005].