How to Do Concurrent tES-fMRI to Optimize Target Circuit Engagement

Functional MRI (fMRI) is frequently used to assess modulation of brain activity by transcranial electrical stimulation (tES). However, concurrent tES-fMRI is technically challenging: first, it requires MRI-compatible tES equipment; second, the use of electric currents inside the MR scanner can lead to artifacts on the fMRI data; third, the observed brain activity can be due to direct effects of the currents on neural activity, but also be influenced by side effects. In this course, we will give an overview of methodological and experimental aspects of concurrent tES-fMRI with a focus on the requirements for an unambiguous interpretation of the observed BOLD activity patterns. We set specific emphasis on the impact of the electric current flow on echo-planar imaging (EPI) that is the standard sequence for fMRI due to its high temporal resolution, despite the well-known susceptibility to field inhomogeneities which cause geometric distortions mainly in the phase-encoding direction. These are well-known to cause severe distortions in regions of air-tissue interfaces. However, we show that subtle effects caused by the tES current-induced magnetic fields can also be seen. This issue has been demonstrated in subjects post-mortem, but is otherwise largely neglected. We show that the small in-vivo brain currents indeed induce negligible magnetic fields, but that the magnetic fields induced by the currents in the electrode cables can in fact cause artifacts comparable to the blood-oxygen-level-dependent (BOLD) contrast. We will explain how these artifacts arise during acquisition and processing of the fMRI data and discuss how to design more robust experiments. In particular, examples of artifact-prone cable configurations and susceptible fMRI paradigms will be presented, followed by an open discussion on best-practices for mitigating their impact on the fMRI analysis.