1.5 GHz non-invasive directional deep brain stimulation with improved focus size and minimized input power.

Temporal Interference Stimulation (TIS) has recently been introduced for non-invasive deep brain stimulation (NDBS). While numerous studies have highlighted its advantages over conventional technologies, TIS still encounters challenges such as limited resolution and a lack of validation using human-like models. This article introduces an innovative method for NDBS which alleviates the resolution limit. We utilize as our excitation a 1.5 GHz microwave carrier modulated by a 10 Hz envelope. The microwave carrier enables strong electromagnetic focusing while the envelope triggers neural activity. To form this excitation, two dipole antenna arrays are placed around the head for the generation of y- and z-directed electric fields (E-field). Current excitations to the antenna arrays are tuned to control (i) the E-field to the desired focality position and (ii) its direction at the focality position. Full-wave simulations with a realistic head model are conducted to demonstrate the method. In the deep brain region, the cross-sectional focality sizes (75% threshold) are 0.73 cm2, 1.18 cm2 and 2.45 cm2 in the XOY, YOZ and XOZ planes, respectively. The focality is much smaller than previously reported in the conventional method with kHz carrier waves. Further, the E-field direction at the focality can be steered along the yz-plane by adjusting the excitation weights of the antenna arrays. Multiphysics simulations on temperature distribution and specific absorption rate (SAR) show that the maximum temperature increase within a 30-minute stimulation session is 0.76℃ and the maximum SAR1g is 2.70 W/kg. Both measures are within commonly accepted safe operation ranges. Compared to conventional TIS methods that utilize kHz carrier signals, our proposed approach achieves significantly improved spatial resolution and enables precise steering of the E-field. The proposed work holds significant potential for clinical applications, offering enhanced resolution and reduced input power for NDBS.