Abstract

Purpose: To characterize and eliminate a new type of image
artifact in concurrent transcranial magnetic stimulation
and functional MRI (TMS-fMRI) caused by small leakage currents originating from the high-voltage capacitors in the
TMS stimulator system.
Materials and Methods: The artifacts in echo-planar images
(EPI) caused by leakage currents were characterized
and quantified in numerical simulations and phantom
studies with different phantom-coil geometries. A relaydiode
combination was devised and inserted in the TMS
circuit that shorts the leakage current. Its effectiveness for artifact reduction was assessed in a phantom scan resembling a realistic TMS-fMRI experiment.
Results: The leakage-current-induced signal changes exhibited a multipolar spatial pattern and the maxima exceeded 1% at realistic coil-cortex distances. The relay-diode combination effectively reduced the artifact to a negligible level.
Conclusion: The leakage-current artifacts potentially obscure effects of interest or lead to false-positives. Since the artifact depends on the experimental setup and design (eg, amplitude of the leakage current, coil orientation, paradigm, EPI parameters), we recommend its assessment for each experiment. The relay-diode combination can eliminate the artifacts if necessary.