Spatial distribution of anisotropic acoustic impedance assessed by time-resolved 50-MHz scanning acoustic microscopy and its relation to porosity in human cortical bone

We used quantitative scanning acoustic microscopy (SAM) to assess tissue acoustic impedance and microstructure of cortical bone of human radii with the aim to provide data on regional distribution of acoustic impedance along the circumferential and across the radial directions in the entire cross-section of the radius diaphysis as well as to determine the range of impedance values in transverse (perpendicular to bone axis) and longitudinal (parallel to bone axis) cross-sections. Several microstructural features related to cortical porosity were analyzed in order to determine whether these features differ in different parts of the cortex and to assess the relationship between the microstructure and tissue acoustic impedance. Fifteen fresh bone specimens (human radius) were investigated using a SAM (center frequency of 50 MHz and − 6 dB lateral resolution of approximately 23 µm). The sample acoustic impedance was obtained by means of a calibration curve correlating the reflected signal amplitude of reference materials with their corresponding well-known acoustic impedance. Tissue acoustic impedance and microstructural features were derived from the morphometric analysis of the segmented impedance images. A higher porosity was found in the inner cortical layer (mean ± SD = 8.9 ± 2.3%) compared to the peripheral layer (2.7 ± 1.5%) (paired t-test, p < 10− 5). ANOVA showed that most of the variance can be explained by the regional effect across the radial direction with a minor contribution due to between-sample variability. Similar to porosity, the number and diameter of pores were greater in the inner layer. In contrast to porosity, ANOVA showed that impedance variability can mostly be explained by between-specimen variability. Two-way ANOVA revealed that after compensation for the between-sample variability the variation in acoustic impedance across the radial direction was much larger than that along the circumferential direction. In addition to the significant difference between the inner cortical layer (8.25 ± 0.4 Mrayl) and peripheral layer (8.0 ± 0.5 Mrayl) (unilateral paired t-test, p < 10− 4), the values in the anterior region (8.2 ± 0.5 Mrayl) were found to be significantly higher than those of the posterior region (7.9 ± 0.6 Mrayl). Impedance mean value of longitudinal sections was lower than mean value measured in transverse cross-sections, resulting in an impedance acoustic anisotropy ratio of 1.17 ± 0.03 in the inner cortical layer and 1.19 ± 0.02 in the peripheral layer. SAM is a valuable tool to provide data on the spatial distribution of microstructural and microelastic bone properties that is useful to improve our understanding of the impact of bone microstructure on tissue material properties.