Quantitative ultrasound assessment of bone.

In the last two decades, several non-invasive techniques have been developed to measure bone density at axial and peripheral skeletal sites. The dual-energy X-ray absorptiometry (DXA) technique allows accurate measurement of bone density, but does not provide information about the structural and qualitative features of bone, which play an important role in fracture risk determination. Increasing interest in quantitative ultrasonography (QUS) has recently developed; it may be considered a safe and quite inexpensive diagnostic technique. Ultrasound devices routinely measure two parameters: broadband ultrasound attenuation (BUA) and speed of sound (SOS). Two other parameters, stiffness and index of consistency (QUI), can be derived from BUA and SOS. SOS is influenced by the elasticity of bone as well as by its density. BUA is determined by mechanisms of diffraction, scattering and absorption in the bone, marrow and soft tissue. Absorption predominates in cortical bone and scattering in trabecular bone. BUA is a measure of the approximately linear frequency dependence of ultrasound attenuation. Several QUS devices are now available for clinical use for measuring various parameters at skeletal sites with different contents of trabecular and cortical bone. Standardization of instruments is one of the major limitations of this technique today. Many studies have demonstrated that BUA and SOS, measured at any level, can discriminate normal subjects from osteoporotic patients. Moreover, there is evidence documenting the ability of QUS to predict osteoporotic fracture risk and to give further BMD-independent information on bone. QUS at the heel can now be considered as an alternative technique to identify subjects with a high risk of bone fragility. Further studies are needed for better definition of the role of QUS in clinical practice.