Abstracts

Rationale: To collect simultaneous high quality EEG and functional MRI for localizing interictal activity and studying generators of normal EEG. Methods: EEG signal is detected with Ag/AgCl electrodes and connected to local differential amplifiers via high impedance carbon fiber leads that reduce the risk of RF burns. The digitally multiplexed signal leaves the scan room via optical fiber. Equipment was built by Telefactor Corp. (W. Conshohocken, PA) and modified specially for our application. We record EEG in a hard-wired bipolar montage using twisted dual lead electrodes. The configuration is self-canceling for current induced by motion and gradients. Imaging was performed on our General Electric 3 Tesla scanner with EPI modifications by Advanced NMR Systems. An EPI sequence with a 4000 ms TR was used to acquire 6 slices spaced evenly over the TR period to yield windows of readable EEG between gradient bursts. Simultaneous EEG/fMRI was performed on a biological phantom using twisted and untwisted leads, on a 31 y.o. epileptic female and a 28 y.o. normal female. After blanking the EEG signal during EPI gradient activity with software, ballistocardiographic artifact was removed by averaging the EEG following an EKG trigger and subtracting the average from the raw EEG. To retain spectral information, we set fMRI scan timing to fall outside of EEG frequency bands of interest. We determined spectral power in user-defined bands with each TR to calculate fMRI signal maps. Results In the phantom study, twisted vs. untwisted leads reduced random noise by an average of 5.4 dB and gradient noise by an average of 6.3 dB. The EEG power spectrum contained expected alpha band signal increases during eyes closed. Discussion: Hardware modifications and post-processing minimize scanner artifact in the EEG and allow continuing development of fMRI for epilepsy and EEG study. Acknowledgments: We thank Telefactor Corp. for their generous continuing support. RIG is supported by a Cota-Robles fellowship from UCLA (1999-2000) and an Epilepsy Foundation fellowship (2000-2001). Partial support for the project is from NIDA (1R01-DA13054).