Assessing a stimulator modification for simultaneous non-invasive auricular vagus nerve stimulation and magnetic resonance imaging

The vagus nerve, a core component of the parasympathetic nervous system, mediates information transfer between the body’s internal milieu and the brain. This nerve can be stimulated non-invasively at the ear using transcutaneous auricular vagus nerve stimulation (taVNS). Concurrent functional magnetic resonance imaging (fMRI) permits study of taVNS-induced changes in brain dynamics, a key requisite for precision neurostimulation. To date, however, there is no standardized protocol for how to safely apply taVNS during MRI. One major risk is temperature increase exceeding innocuous thresholds due to coupling of the emitted radio frequency (RF) pulse during imaging. Thus, we developed and tested a stimulator cable configuration with floating ground cable traps and filter plate connectors. We measured temperature, resonance of the stimulation electrodes, and current interference using unmodified and modified stimulation cables. Measurements were conducted with phantoms and human participants at 3T field strength. Demonstrating the safety of the modifications in different contexts, the efficacy of the modification was independently tested in two other sites using different 3T MRI scanner models, stimulators, and stimulation strength. The modified compared to the unmodified cable considerably reduced RF heating as the relative temperature increase stayed well below the 2 K threshold specified by the ASTM F2182 standard. Additionally, in accordance with ASTM 2119, we can rule out potential distortion and signal loss around the electrodes due to current flow from the stimulator and even demonstrate that impaired image quality in brainstem and midbrain regions is recovered using the modified cable. Hence, we show that adding floating ground cable traps to the stimulator cable allows the safe use of taVNS with fMRI and may improve image quality in functional imaging. To enable other researchers to modify their hardware in the same way, we provide details of the modifications.