Sound-tissue interaction - The physical basis of bone ultrasonometry and limitations of existing methods

Ultrasound measurements of bone are generally obtained using transmission r ather than pulse-echo techniques because of its highly attenuating nature. Ultrasound velocity and attenuation measurements are utilized. For velocity , there are well-defined fundamental relationships describing the dependenc e on the elasticity and density of bone. However, the practical implementat ion and signal processing of velocity measurements has led to a significant variability in results from different commercial systems. We may measure e ither phase of group velocity, for the latter, adopting a range of pulse ar rival definitions. We are offered bone velocity, heel velocity, time of fli ght, and amplitude-dependent velocity. For attenuation measurements, howeve r, the reverse is true. We generally record the increase in attenuation wit h frequency (0.2-0.6 MHz), termed broadband ultrasound attenuation (BUA). A lthough first described in 1984, because of the complex interplay of attenu ation mechanisms, there still lacks a fundamental understanding of the depe ndence of BUA on the material and structural properties of cancellous bone. With the increasing number of commercial systems available, there is an urg ent need to understand the intrinsic (artefact free) and system estimation of ultrasound velocity and attenuation parameters that may be implemented t o characterise bone and provide clinical information.