On the feasibility of temporal interference stimulation of human brains using two arrays of electrodes

BACKGROUND: Temporal interference (TI) stimulation leverages two pairs of electrodes to induce an interfering electrical field in the brain. Studies show that TI can stimulate the deep brain without affecting shallow areas. Optimal placement of these electrodes for maximal focal modulation remains computationally challenging. We are the first to propose two arrays instead of two or multiple pairs of electrodes to boost modulation focality. However, it outputs too many electrodes with overlaps across two frequencies, making it difficult to implement in practice. Objective: Based on recent progress in developing multi-channel TI devices and computational work, here we advocate two-array TI with feasibility data. Methods & Results: We review TI optimization algorithms, and compare them over 25 heads across six brain targets. We show that the latest optimization algorithm for two-pair TI innately works for two-array TI with the fastest speed (under 30s) and a similar amount of electrodes as in multi-pair TI. At four of the six targets, this fastest algorithm for two-array TI achieves similar or even better focality than TI using 16 pairs of electrodes that takes days to optimize. We show a hardware implementation of two-array TI on our 8-channel TI device. We argue that two-pair TI is only preferred for maximal modulation strength or when hardware is limited to two current sources. We restate the focality-intensity tradeoff in the context of TI and provide a first voxel-level map (at 4 mm resolution) of achievable TI focality and modulation strength in the MNI-152 head template. Conclusions: Compared to two-pair or multi-pair TI, we promote two-array TI for its similar performance in focality and lower cost in both optimization time and electrodes needed. We hope this work will pave the way for future adoptions of two-array TI for more focal non-invasive deep brain stimulation.