Induced current effects on functional magnetic resonance imaging interpretability during monopolar deep brain stimulation

OBJECTIVE The objective was to evaluate the stability of stimulation current delivered by deep brain stimulation (DBS) systems during MRI scanning and to assess whether configuration-dependent variability in induced current may undermine the interpretability of functional MRI (fMRI) acquired during active stimulation. METHODS The authors measured the electrical output of 2 current-controlled DBS systems in a standardized phantom during 3-T MRI acquisition. Stimulation was delivered in both monopolar and bipolar configurations, with the DBS systems on and off. Induced current was recorded using a custom MRI-conditional setup, and peak amplitudes were quantified across multiple sequences, including gradient-intensive fMRI protocols. All data were normalized to baseline output and analyzed using Cohen’s d to assess the magnitude of MRI-induced current deviation. RESULTS Monopolar stimulation during MRI exhibited significant current fluctuations, with induced amplitudes ranging from −3.2 to +3.9 mA and frequent polarity inversion. These distortions were sequence dependent and most pronounced during fMRI acquisition. In contrast, bipolar stimulation demonstrated stable output with minimal deviation from programmed parameters. The variability observed in monopolar output was not attributable to impedance shifts and was consistent across both DBS systems tested. CONCLUSIONS MRI-induced current substantially alters the effective output of monopolar DBS, introducing uncertainty into any concurrent fMRI acquisition. Although functional imaging was not directly performed in human subjects, these findings imply that the observed blood oxygen level–dependent (BOLD) response during monopolar stimulation likely reflects the distorted, not programmed, stimulation. Bipolar configurations avoid this confounder and should be preferred when interpreting fMRI data acquired during DBS.