**2.2 Quantitative ultrasound (QUS)**

QUS is a non-ionizing technology for BMD detection using acoustic waves, which uses different parameters to reflect the situation of bone mass indirectly [14]. Since Longton et al. (2008) first used QUS to measure bone tissue in 1984.The theory, methods, and instruments for measuring BMD with QUS have been greatly developed [15]. There are four types of US transmissions: trabecular transverse transmission, cortical transverse transmission, cortical axial transmission, and pulse-echo measuring devices [16–18]. Among them, Trabecular Transverse Transmission is mainly used to measure cancellous bone and the detection site is calcaneus. Cortical Axial Transmission is used for cortical bone detection and detection site is Radius [19]. Other measurements sites of QUS devices are finger phalanges, tibia, less common femur, posterior processes of the spine and ulna. Through QUS, two parameters are mainly obtained: Broadband Ultrasound Attenuation (BUA) and Speed of Sound (SOS). In theory, the two QUS principal variables are both related not only to BMD but also to trabeculae orientation, the proportion of trabecular and cortical bone, the composition of organic and inorganic components, bone elasticity damage and fatigue [20]. But currently the extent of its impact on BUA and SOS is unknown. The correlation between QUS parameters and DXA-BMD is good. It can distinguish patients with osteoporosis from normal people, but the false negative rate is high. At present, there is no uniform standard for the diagnosis of osteoporosis
