A method for removing imaging artifact from continuous EEG recorded duringfunctional MRI

Combined EEG/fMRI recording has been used to localize the generators of EEG events and to identify subject state in cognitive studies and is of increa sing interest. However, the large EEG artifacts induced during fMRI have pr ecluded simultaneous EEG and fMRI recording, restricting study design. Remo ving this artifact is difficult, as it normally exceeds EEG significantly a nd contains components in the EEG frequency range. We have developed a reco rding system and an artifact reduction method that reduce this artifact eff ectively. The recording system has large dynamic range to capture both low amplitude EEG and large imaging artifact without distortion (resolution 2 m u V, range 33.3 mV), 5-kHz sampling, and low-pass filtering prior to the ma in gain stage. Imaging artifact is reduced by subtracting an averaged artif act waveform, followed by adaptive noise cancellation to reduce any residua l artifact. This method was validated in recordings from five subjects usin g periodic and continuous fMRI sequences. Spectral analysis revealed differ ences of only 10 to 18% between EEG recorded in the scanner without fMRI an d the corrected EEG. Ninety-nine percent of spike waves (median 74 mu V) ad ded to the recordings were identified in the corrected EEG compared to 12% in the uncorrected EEG. The median noise after artifact reduction was 8 mu V. All these measures indicate that most of the artifact was removed, with minimal EEG distortion. Using this recording system and artifact reduction method, we have demonstrated that simultaneous EEG/fMRI studies are for the first time possible, extending the scope of EEG/fMRI studies considerably. (C) 2000 Academic Press.