Skull microstructure and mode conversion in transcranial ultrasound imaging

Longitudinal-to-shear wave mode conversion that occurs at the interface between elastic bone and tissues can be used for aberration reduction by intentionally increasing the incidence angle between the ultrasound beam and the skull. However, in some cases, mode conversion produces artifacts in transcranial images. In this study, we seek to elucidate the fundamental mechanisms underlying the variation in imaging performance in the presence of wave mode conversion. 2D microCT scans of a skull sample were used to simulate transcranial ultrasound imaging of a point target. Simulation studies show that imaging at a large incidence angle (40°) reduced aberration for skulls having low porosity. However, as skull porosity increased, large angles of incidence resulted in degradation of resolution and increased target localization errors. Experimental results indicate that imaging at normal incidence introduced a localization error of 1.85±0.10 mm, while imaging at a large incidence angle (40°) resulted in an increased localization error of 6.54±1.33 mm and caused a single microtube target to no longer appear as a single, coherent target in the resulting image, which is consistent with simulation results. This study indicates that imaging performance is highly dependent on the porosity of the skull, particularly at incidence angles above the critical angle (e.g., 40°).