Ultrasonic evaluation of anisotropic structure of swine skull

For the ultrasound treatment in brain, such as high intensity focused ultrasound (HIFU), understanding of the complicated ultrasonic propagation in the skull bone is important, because of the anisotropic and heterogeneous character. In this study, we experimentally investigated the ultrasonic wave propagation in the skull sample, considering the anisotropic structure. A cylindrical bone sample was fabricated along the thickness direction of the right occipital bone of a swine (diameter: 11.5 mm, height: 3.0 mm). The ultrasound velocity in the MHz range was measured in the radial direction by rotating the sample (0 degree: sagittal-axis) and changing the measurement position. The structure of the sample was also evaluated by the X-ray micro CT. There are three orthogonal trabecular alignments in bone sample: main, intermediate-shafts (IS) and minor. In the outer layer of the skull, IS were mostly found along the frontal-axis. Ultrasound velocity became higher as the wave propagation direction approached to IS and values were in the range from 2110 to 2790 m/s. In the dipole, ultrasound velocity became higher as the wave propagation direction approached to main-axis and values were in the range from 2080 to 2600 m/s.