On the relative contributions of absorption and scattering to ultrasound attenuation in trabecular bone: a simulation study

Ultrasound has been proposed as a means to non-invasively assess bone and particularly bone strength and fracture risk. Although there has been some success in this application, there is still much that is unknown regarding the propagation of ultrasound through bone. Because strength and fracture risk are a function of bone mineral density as well as architectural structure and tissue quality, this study was carried out to further elucidate the mechanisms of interaction between ultrasound and bone. Frequency-dependent attenuation of an ultrasound wave in trabecular bone has been shown to be strongly dependent on bone mass and architecture and is currently used in several clinical devices for bone assessment. Since attenuation is due to both absorption by the biological tissues per se and scattering, it is of interest to understand the relative contributions of each. A sample of calcaneal trabecular bone was scanned with micro-CT and subjected to morphological image processing (erosions and dilations) to obtain a total of 11 three-dimensional (3D) date sets. Eleven two-dimensional (2D) slices obtained from the 3D data sets were then analyzed to evaluate their bone volume fractions (VF). Computer simulations of ultrasonic propagation through each of thru 11 2D slices, which varied in VF from 0.088-0.181, were carried out in one of two modes. In the first instance, the component tissue (i.e., marrow and bone) were lossy, while in the second set of simulations the component tissues were lossless. In both cases the slope of the attenuation was computed over the frequency range 300 kHz - 900 kHz for the entire data set. Results obtained showed an average reduction in the attenuation slope of only 4.4% (SD=1.8%) in the lossless case as compared (pairwise) with the lossy case. This data indicates that scattering is the primary mechanism in trabecular bone with respect to overall attenuation measurements, and further suggests that models relating scattering to bone architecture and mass should be developed to further enhance the ability of ultrasound to non-invasively access bone.