Frequency dependence of the anisotropic impedance estimation in cortical bone using time-resolved scanning acoustic microscopy

The influences of frequency and spatial resolution on the anisotropic impedance estimation of cortical bone was investigated in the frequency range from 25 to 100 MHz. A set of spherically focused transducers provided a spatial resolution in the range from 150 down to about 20 microns. Four embedded cortical bone samples (2 male, 2 female, 2 donors below 30, 2 donors above 70) were cut with different orientations relative to the long axis of the femur (0 to 90 degrees). From each section impedance maps were acquired in the C-scan mode. Histogram evaluations showed a similar angular dependence with a characteristic off-axis maximum of the estimated impedance for all samples and frequencies. The impedance values obtained with the 25 MHz transducer were significantly lower than those obtained with the 50 and 100 MHz transducers. Morphological parameters of the macrostructure, e.g. size and distribution of the haversian channels, appeared to have a significant influence on the measured properties, if the spatial resolution is not sufficient.