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Description
Transcranial focused ultrasound (tFUS) is advancing as a non-surgical technique for neuromodulation and tissue ablation by transmitting ultrasound waves through the skull and focusing them inside the brain. Most studies characterize ultrasound waves passing through the skull at normal incidence and for focusing at deep brain targets, while focusing on shallow brain targets and application of the guided waves remains largely unexplored. The skull bone causes major distortion to the ultrasound waves because of the high acoustic impedance, PS mode conversions, and heavy scattering in the diploë layer. Our goal in this study is to explore how we may stimulate parts of the brain with guided waves that might be otherwise hard to target, and to explore the application of guided waves for skull characterization. We developed a numerical simulation framework for tFUS to simulate the acoustic-viscoelastic wave equation using the spectral-element method. We employ an ex-vivo human skull model in the numerical simulation framework and place a focused transducer bowl source at some incidence angle to the skull. We analyze the ultrasound waves transmitted through the skull into the shallow areas close to the skull layer. Incident P waves convert to S waves at the outer skull, form guided waves via internal reflections, and reconvert to P waves upon exiting the inner skull layer. Thus, when we place incident waves at certain angles to the skull, the setup introduces shear wave effects in the transmitted wavefield. We investigate these complex ultrasound wavefields as they pass through the skull bone to observe their behavior in shallow regions.