The role of angular reflection in assessing elastic properties of bone by scanning acoustic microscopy

For an assessment of the mechanical performance of bone, a quantitative description of its mechanical heterogeneity is necessary. Previously, scanning acoustic microscopy (SAM) was used as a non-destructive method to estimate bone stiffness on the micrometer scale. While up to now only the normal incidence of acoustic waves is taken into account, we extend in our study the evaluation procedure by considering the full opening of the acoustic lens. The importance of this technical aspect is demonstrated by determining the contrast in Young's modulus between newly formed osteons and the surrounding higher mineralized interstitial bone. Several regions of human cortical bone of a femur in cross-section were imaged. For all the regions quantitative backscattered-electron imaging (qBEI) to estimate the local mass density was combined with SAM measurements. These measurements reveal a non-monotonic dependence between acoustic reflectivity and Young's modulus, which shows that it is actually necessary to consider the lens opening in a quantitative way. This problem was experimentally and theoretically approached by using lenses with two different opening angles operated at different frequencies (52° at 400MHz and 80° at 820MHz) to image the same specimen. The mass density of bone in osteons was found to be 1930kg/m(3) on average, while the higher mineral content in interstitial bone results in a 9% increase of the density. The contrast in the effective Young's modulus E, as determined through SAM, is more pronounced, with an average value of 14GPa in osteons and a more than 60% increase in interstitial bone. Additionally, SAM maps show oscillations in E with a periodicity of the typical bone lamella thickness of approximately 7µm in both osteons and interstitial bone. This mechanical heterogeneity can be explained by the varying orientation of the mineralized collagen fibers.