On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging.
OBJECTIVE
To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS).


METHODS
Distortions due to TMS pulses (0.25 msec, 2.0 T) were studied at 2.0 T before and during EPI.


RESULTS
Best results were obtained if both the EPI section orientation and the frequency-encoding gradient were parallel to the plane of the TMS coil. Under these conditions, a TMS pulse caused image distortions when preceding the EPI sequence by less than 100 msec. Recordings with a magnetic field gradient pick-up coil revealed transient magnetic fields after TMS, which are generated by eddy currents in the TMS coil. TMS during image acquisition completely spoiled all transverse magnetizations and induced disturbances ranging from image corruption to mild image blurring, depending on the affected low and high spatial frequencies. Simultaneous TMS and radio-frequency (RF) excitation gave rise to T1-dependent signal changes that lasted for several seconds and yielded pronounced false-positive activations during functional brain mapping.


CONCLUSIONS
To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging.
Bestmann
Sven
S
Biomedizinische NMR Forschungs GmbH, Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany. sbestma@gwdg.de
Baudewig
Jürgen
J
Frahm
Jens
J
D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D001931:Brain Mapping / Q000379:methods; D017352:Echo-Planar Imaging; D019054:Evoked Potentials, Motor / Q000502:physiology*; D006801:Humans; D019047:Phantoms, Imaging; D010812:Physical Stimulation; D012016:Reference Values; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation