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

<i>Objective.</i>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.<i>Approach.</i>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<i>y</i>- and<i>z</i>-directed electric fields (<i>E</i>-field). Current excitations to the antenna arrays are tuned to control (i) the<i>E</i>-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.<i>Main results.</i>In the deep brain region, the cross-sectional focality sizes (75% threshold) are 0.73 cm², 1.18 cm²and 2.45 cm²in the<i>XOY, YOZ</i>and<i>XOZ</i>planes, respectively. The focality is much smaller than previously reported in the conventional method with kHz carrier waves. Further, the<i>E</i>-field direction at the focality can be steered along the<i>yz</i>-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 °C and the maximum SAR_1gis 2.70 W kg^-1. Both measures are within commonly accepted safe operation ranges.<i>Significance.</i>Compared to conventional TIS methods that utilize kHz carrier signals, our proposed approach achieves drastically improved spatial resolution and enables precise steering of the<i>E</i>-field. The proposed work holds significant potential for clinical applications, offering enhanced resolution and reduced input power for NDBS.