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Identification of Critical Angles in Shear Mode Conversion-Based Transcranial Ultrasound via Leaky Guided Wave Analysis

In this work, we study the role of leaky guided waves in transcranial ultrasound transmission via numerical models and experiments conducted at varying incidence angles using a temporal and a parietal bone segment. Our results obtained with a 500 kHz excitation indicate that transmission angles higher than the longitudinal critical angle have efficient shear mode conversion, which, compared to the normal incidence case, yields improved spatial resolution for both the temporal and parietal segments, as well as improved transmission amplitude for the temporal segment. We also show that the critical angles can be predicted from leaky guided wave radiation angle dispersion spectra. The guided wave radiation angles can be obtained with signal processing of experimental pressure time histories without knowledge about the bone geometry or material properties, making the leaky guided wave dispersion a powerful tool for identifying angles for which efficient shear mode conversion is expected to occur.