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We examine the potential for low-intensity focused ultrasound to non-invasively produce small (< 1mm3) focal acoustic fields for precise brain stimulation near the skull. Our goal is to utilize transcranial ultrasonic neuromodulation to transform communications and immersive gaming experiences and to optimize neuromodulation applications in medicine. To begin evaluating possible hardware design strategies for engineering ultrasonic brain interfaces, in the present study we evaluated the skull transmission properties of longitudinal and shear waves as a function of incidence angle for 0–2 MHz. We also employed K-wave and time-reversal numerical simulations to further inspect waveform interactions with modeled layers. Timereversal focusing for single-layer and three-layer skull cases were simulated for three different bandwidth ranges (MHz): Broadband(0–2), 1 MHz(0.4–1.4), and 0.2 MHz(0.4–0.6). Broadband and 1 MHz bandwidths emulate the performance of micromachined or piezo membrane ultrasonic arrays, while 0.2 MHz bandwidth is representative of the performance of conventional piezoelectric ultrasonic transducer. We found the 3dB focal volume was ~0.6 mm for broadband and 1 MHz, with the latter showing a slightly larger sidelobe. In contrast, 0.2 MHz nearly doubled the size of the 3dB focal volume while producing prominent sidelobes. Our results provide initial confirmation that a broadband, ultrasonic, linear array can access the first 15 mm of the human brain, which contains circuitry essential to sensory processing including pre-motor and motor planning, somatosensory feedback, and visual attention. These areas are critical targets for providing haptic feedback via non-invasive neural stimulation.
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