Systematic Review of Interleaved TMS-fMRI – Overview of Methodological Differences and Sources of Bias

s Brain Stimulation 14 (2021) 1589e1707 minimum amount of energy (i.e. voltage gradient). Thus, depending on the interface voltage, metal implants may act as insulators or as conductors. In this study, we built a FEM model of a metal implant exposed to a homogeneous electric field. For the sake of comparison, the same geometry was solved using two different models: a purely electric one and an electrochemical one accounting for the electrode-electrolyte interface processes. We also modeled a tES treatment in a realistic head model with a skull plate to compare the field distribution in the brain when the plate is modeled with high or low conductivity. Our results show that metal implants act as electric insulators for electric fields up to 200 V/m and they only resemble a perfect conductor for fields above 1000 V/m. Thus, given the low magnitude of the fields generated in tES (~ 1 V/m), skull plates are better modeled as insulators. In addition, our results show that the fields predicted by the two models differ by more than 25% over large regions of the brain, highlighting the importance of the model chosen for the skull plate. Our analysis applies to any (tES) modality that uses low frequency currents (<1 kHz) and also to TMS.