Abstract
Recent progress in quantitative ultrasound (QUS) techniques (e.g. axial transmission) allows the in vivo evaluation of cortical bone. QUS for the characterization of this multiscale material is currently confined to wave velocity analysis. Additional parameters such as broadband ultrasonic attenuation (BUA) may help to gain deeper insight for cortical bone evaluation. However, the frequency dependence of attenuation has been sparsely investigated in cortical bone. The present study focuses on the feasibility of measurements of the slope of the frequency dependent attenuation coefficient in bovine cortical bone. BUA measurements are compared to other bone properties obtained following a multimodal approach. Samples are cut along the bone axis and circumference from three 36-month old bovine femurs. Ultrasonic measurements are performed in transmission using self-made PVDF transducers. BUA is evaluated for each sample between 3.5 and 4.5 MHz in three perpendicular directions. In addition, Bone Mineral Density (BMD) is assessed using a dual X-ray absorptiometry device. The microstructure of each sample is assessed with optical microscopy, allowing a classification of the samples into 4 groups: Haversian (H, osteons diameter 150~300 mum, pores size: 20~50 mum), plexiform (PI, lamellae thickness: 100~200 mum, pores size 8~12 mum), porotic (Po, largest pores size: 50~100 mum) and mixed microstructure (M). BUA is found to depend significantly on the anatomical location, propagation direction and microstructure. The distribution of BUA values according to the microstructure is similar to the pores size distribution, indicating that scattering regime is one of the important factors affecting attenuation. Axial BUA is significantly lower than radial and tangential BUA, which may also be explained by scattering effects, since osteons and lamellae are oriented in the axial direction. The Figure shows that axial BUA is significantly correlated to BMD. However, for plexiform samples, BUA is not correlated to BMD. In these samples with fewer pores, BUA seems to be affected by viscoelasticity, reflecting microscopic bone quality. Our results suggest that attenuation could be used as a complementary index of cortical bone status.
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