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Introduction: Simultaneous EEG/fMRI acquisition can identify the brain networks involved in generating specific EEG patterns. Yet, the combination of these methodologies is hampered by strong susceptibility artifacts and electromagnetic interference. Specifically, (a) the susceptibility of EEG electrodes / gels distorts the MR image, and (b) depending on the loop area of the electrode-ground circuitry, and the rate/strength of gradient switching, MR image acquisition induces interference in the EEG signal (gradient artifact). Here we measure the strength of these artifacts and demonstrate the effectiveness of gradient artifact compensation (Allen, 2000).
Methods: We experimented with saline phantoms and with an alert macaque. MRI was performed in a 4.7 T magnet (Bruker), with a gradient field strength of 50 mT m-1. The EEG was recorded from the skull of the monkey using the BrainAmp system (5 kHz sampling rate, input range: +/- 6.4 mV, 250 Hz bandwidth).
Results: Common EEG electrodes / gels produced a susceptibility artifact of < 4 mm, which is in the range of the monkey's muscle thickness, and thus does not affect the MR images of the brain. The amplitude of the unfiltered gradient artifact in our setup was 50 mV (loop area: 166 cm2, slew rate: 333.33 T m-1 s-1) which would ordinarily swamp the EEG signal (~50 μV). After analog low-pass filtering at 250 Hz (30 dB) followed by gradient artifact compensation (Allen, 2000) however, we were able to recover 90 of a 10 μV amplitude, 10 Hz control signal. The EEG can be recovered during image acquisition using this strategy.
Conclusion: Our results demonstrate the feasibility of simultaneous EEG and MRI experiments in the macaque monkey at high magnetic fields with strong gradients. Ongoing experiments examine the mechanism of generation of specific EEG patterns using event related fMRI in monkeys.
