Assessment of anisotropic tissue elasticity of cortical bone from high-resolution, angular acoustic measurements

Assessment of anisotropic elastic properties at the tissue level is still one of the major challenges in bone research. In previous studies, bone sections were cut in different directions relative to a principle axis of symmetry. This causes a high preparation and measurement effort. We have developed a new acoustic scanning procedure that allows one to measure the angular dependence of the acoustic impedance of cylindrically shaped samples (diameter: 4.4 mm) with a single measurement. Our scanning acoustic microscope was equipped with a rotational stage, and a scanning procedure was developed that measures the surface reflection of the rotating cylinder. It was shown in a previous study that the acoustic impedance derived from the reflection coefficient is highly correlated with the elastic coefficient in the probing direction. From the angular reflection, the independent elastic coefficients were derived using assumptions of transverse isotropy and continuum micromechanical model constraints. This method was applied to the inspection of human femoral bone samples. Four cylinders were prepared from the anterior, posterior, medial, and lateral regions. The measurements were performed with a 50 MHz transducer, providing a lateral resolution of 23 microm. Remarkable structural and elastic variations were observed between the four samples. The means and standard deviations of the derived elastic coefficients were: c33 = 29.9 +/- 5.0 GPa, c11 = 21.9 +/- 2.1 GPa, c12 = 9.2 +/- 1.5 GPa, c13 = 9.7 +/- 1.6 GPa, and c44 = 6.7 +/- 1.2 GPa. The results demonstrate that microstructural and anisotropic elastic tissue parameters can be assessed by ultrasound in very small bone samples.