Towards High-Resolution Ultrasound Neuromodulation With Crossed-Beam Phased Arrays

Transcranial focused ultrasound stimulation (tFUS) has emerged as a potential noninvasive therapeutic technology. Due to skull attenuations at high ultrasound (US) frequencies, successful tFUS with sufficient penetration depth requires sub-MHz US waves, leading to relatively poor stimulation specificity particularly in the axial direction (perpendicular to the US transducer). This shortcoming can potentially be overcome by utilizing two individual US beams properly crossed in time and space. For large-scale tFUS, a phased array is also required to dynamically steer focused US beams at desired neural targets. This article presents the theoretical foundation and optimization (through a wave-propagation simulator) of crossed-beam formation using two US phased arrays. It also experimentally validates crossed-beam formation with two custom-made 32-element phased arrays (operating at 555.5 kHz) positioned relatively at different angles. In measurements, the sub-MHz crossed-beam phased arrays achieved 0.8/3.4 mm lateral/axial resolution at a focal distance of ∼ 46 mm, compared to the lateral/axial resolution of 3.4/26.8 mm of the individual phased array at 50 mm focal distance (∼ 28.4-fold improvement in reducing the main focal zone area). The crossed-beam formation in the presence of a rat skull and a tissue layer was also validated in the measurements.