Transmission efficiency comparison between dual-mode conversion incidence and normal incidence

Transcranial ultrasound stimulation (TUS) techniques have been studied as a non-invasive method for neuromodulation and neurostimulation. Owing to the nature of a highly attenuative skull, transmission efficiency is one of the most important factors in transcranial ultrasound. Conventional TUS techniques are generally conducted based on normal incidence. However, normal incidence is associated with absorption and scattering inside the skull. These problems due to normal incidence caused high transmission loss and heating of the skull. In this study, we propose and verify a dual-mode conversion technique by using an oblique incidence calculated based on a specific Lamb wave mode. To imitate the high attenuation of the skull, plexiglass and high-density polyethylene (HDPE) were used as high-attenuating materials. In addition, the reflectance function of the Lamb waves was used to demonstrate the relationship between the transmission efficiency and mode conversion to Lamb waves. The results of the simulation and experiments show that the dual-mode conversion technique has better transmission efficiency than normal incidence.