On the need of individually optimizing temporal interference stimulation of human brains due to inter-individual variability

Transcranial temporal interference stimulation (TI, TIS, or tTIS), also known as interferential stimulation (IFS), is able to focally stimulate deep brain regions, provided it is properly optimized. We previously presented an algorithm for optimizing TI using two arrays of electrodes and showed that it can achieve more focal stimulation compared to optimized high-definition transcranial electrical stimulation (HD-TES) and conventional optimized TI using two pairs of electrodes, especially in the deep brain areas such as the hippocampus. However, those modeling studies were only performed on an averaged head (MNI152 template) along with three individual heads. Existing TI work in the literature mostly does not optimize the stimulation but only utilizes a common (possibly optimized) montage of two pairs of electrodes on different individual heads. Here we aim to study the inter-individual variability of optimized TI by applying the same optimization algorithms on N = 10 heads using their individualized head models. Specifically, we compared the focality achieved by different stimulation techniques at three different regions of interest (ROI; right hippocampus, left dorsolateral prefrontal cortex, and left motor cortex). As expected, there is a variability in focality achieved by TI of up to 0.95 cm at the same ROI across subjects due to inter-individual differences in the head anatomy and tissue conductivity. Again we show that optimized TI using two arrays of electrodes achieve higher focality compared to optimized HD-TES at the same level of modulation intensity, especially at the deep brain locations. Compared to using a common montage obtained from a standard head (MNI152 template), individually optimized HD-TES does not significantly improve the stimulation focality at the ROIs. However, individually optimized TI does significantly improve the focality by up to ~4 cm for TI using two pairs of electrodes, and by up to ~1 cm for TI using two arrays of electrodes. This work demonstrates the need of individually optimizing TI to target deep brain areas, and advocates against using a common head model and montage for TI modeling and experimental studies.