Assessing a Stimulator Modification for Simultaneous Noninvasive Auricular Vagus Nerve Stimulation and MRI

The vagus nerve can be stimulated noninvasively at the ear using transcutaneous auricular vagus nerve stimulation (taVNS). Concurrent functional MRI (fMRI) permits study of taVNS-induced changes in brain dynamics, a key requisite for precision neurostimulation. 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 across three sites using different 3T MRI scanner models, stimulators, and stimulation strengths with phantoms and human participants.

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 demonstrate that impaired image quality in brainstem and midbrain regions is recovered using the modified cable.

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.