On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging

Purpose: To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS). Materials and 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. Conclusion: To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging.