title: Image artifacts in concurrent transcranial magnetic stimulation (TMS) and fMRI caused by leakage currents: modeling and compensation.
abstract: OBJECTIVE
  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.
  
  
  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 relay-diode 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, London, United Kingdom. n.weiskopf@fil.ion.ucl.ac.uk
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
authors.forename: Oliver
authors.initials: O
authors.lastname: Josephs
authors.forename: Christian C
authors.initials: CC
authors.lastname: Ruff
authors.forename: Felix
authors.initials: F
authors.lastname: Blankenburg
authors.forename: Eric
authors.initials: E
authors.lastname: Featherstone
authors.forename: Anthony
authors.initials: A
authors.lastname: Thomas
authors.forename: Sven
authors.initials: S
authors.lastname: Bestmann
authors.forename: Jon
authors.initials: J
authors.lastname: Driver
authors.forename: Ralf
authors.initials: R
authors.lastname: Deichmann
mesh_terms: D016477:Artifacts; D001921:Brain / Q000502:physiology*; D003198:Computer Simulation; D017352:Echo-Planar Imaging / Q000379:methods*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008959:Models, Neurological; D019047:Phantoms, Imaging; D050781:Transcranial Magnetic Stimulation / Q000379:methods*