Recent developments in trabecular bone characterization using ultrasound

Currently available quantitative ultrasound technologies to assess cancellous bone are based on the measurements in transmission of speed of sound or slope of frequencydependent attenuation (so called broadband ultrasonic attenuation). These two parameters are now considered as surrogate markers of site-matched bone mineral density. The ability of ultrasound techniques to provide non-bone mineral density-related bone properties (eg, microstructure) has not been clearly demonstrated yet. This is mainly because of two factors: a lack of understanding of ultrasound propagation with clear identification of the different underlying physical interactions; and the difficulty of performing experiments because of the limited sample size, the large number of statistical relationships to be tested with multiple variables, and the usual strong covariance observed between bone quantity and microarchitecture. The aim of this paper is to review the most recent development in the field of ultrasound characterization of trabecular bone. We present research work on ultrasound backscatter and how it could be used to estimate microarchitectural properties independently of bone quantity, and the first promising results obtained for the estimation of trabecular thickness. We then introduce numeric simulations of wave propagation through trabecular microarchitecture and show how it could contribute to elucidate and better characterize the physical underlying physics and result in more predictive models. These innovative acquisition schemes and the possibility of virtual experiments should altogether contribute to rapid advancement of ultrasonic bone characterization.