Functional MRI with simultaneous EEG recording: Feasibility and application to motor and visual activation

Abstract

The possibility of combining the high spatial resolution of functional magnetic resonance imaging (fMRI) with the high temporal resolution of electroencephalography (EEG) may provide a new tool in cognitive neurophysiology, as well as in clinical applications such as epilepsy. However, the simultaneous recording of EEG and fMRI raises important practical problems: 1) the patients' safety, in particular the risk of skin burns due to electrodes heating; 2) the impairment of the EEG recording by the static magnetic field, as well as by RF and magnetic field gradients used during MRI; and 3) the quality of MR images, which may be affected by the presence of conductors and electronic devices in the MRI bore. Index terms: functional MRI; EEG; artifact; temperature; safety THE POSSIBILITY TO RECORD multichannel electroencephalogram (EEG) in the magnet (1) may offer new insights in the investigation of brain functions. The combination of two techniques, EEG and functional magnetic resonance imaging (fMRI), may yield a new neuroimaging tool that combines the excellent temporal resolution of the EEG with the good spatial resolution of fMRI (2). Such a tool could be used in both cognitive neuroscience and clinical neurology, such as epilepsy (3). Several studies have successfully applied EEG-triggered fMRI to visualize brain regions related to the generation of epileptogenic spikes (4 -6), but little has been done in cognitive neurosciences (7).Potential problems may appear when concurrent EEG and fMRI acquisition is performed. On the one hand, the presence of electrodes in the image field of view (FOV) may produce MRI artifacts due to susceptibility effect or radiofrequency (RF) interaction with the conductive wires. On the other hand, the EEG recording may be impaired by the presence of the main magnetic field (B 0 ), as well as by RF and switched gradients used during MRI acquisitions. Although methods have been proposed to restore electrophysiological signals during MR acquisition (8), no attempt has been made to implement this solution in practice-the principal reason being the difficulty to avoid preamplifier saturation of EEG systems due to induced current by switched gradients. Moreover, if fMRI is triggered by EEG-measured signals, good quality EEG recordings between MR acquisitions, rather than during MR scanning, are required. In the latter situation, only B 0 affects the quality of the EEG recording, producing electrocardiogram artifacts (1,9,10). However, it has been shown by different research groups that a good quality EEG recording can be obtained inside a 1.5 T magnet, allowing the detection of epileptic discharges (4 -6,11).Another important concern is the safety of the patient associated with the use of the EEG device during MRI acquisition. This risk is due to induced currents in EEG conductors, which can be caused by the static magnetic field, switching magnetic gradient fields, or RF interaction. A recent theoretical article reviewed the different mechanisms that induce currents in electrical wires us...

Simultaneous EEG–fMRI at ultra-high field: Artifact prevention and safety assessment

“…Importantly, although significant, the losses observed in temporal SNR were considerably less severe than in spatial SNR, in good agreement with previous theoretical and experimental work (Luo and Glover, 2012), showing that functional information is in fact less affected by the introduction of EEG equipment than anatomical MR signals per se. Also in agreement with previous studies (Bonmassar et al, 2001;Lazeyras et al, 2001;Mullinger et al, 2008b), the B 0 maps exhibited local susceptibility artifacts along the skin, likely coinciding with EEG electrodes, but focal enough not to extend into actual brain regions, which remained largely unaffected in terms of B 0 homogeneity. B 1 + maps, on the other hand, displayed clear alterations both globally and in specific regions, which were largely coincident with the more accentuated local SNR drops observed in functional and anatomical images.…”

“…B 1 + maps, on the other hand, displayed clear alterations both globally and in specific regions, which were largely coincident with the more accentuated local SNR drops observed in functional and anatomical images. Overall, the results obtained in this study strengthen the growing view that the properties of modern EEG caps have managed to limit susceptibility artifacts to a satisfactory level, even at ultra-high field (Krakow et al, 2000;Lazeyras et al, 2001). RF pulse disruption, in contrast, stands as an important degradation effect that can significantly reduce the available SNR, as well as compromise the performance of brain segmentation and other image processing steps (Mullinger et al, 2008b).…”

“…While safety guidelines do exist, the pursuit of increasingly higher field strengths, higher EEG channel densities, and various custom modifications (coil designs, MR sequences), has continuously demanded site-specific safety assessments for setup validation. Temperature measurements in phantoms and humans have been extensively adopted for this purpose (Lemieux et al, 1997;Lazeyras et al, 2001;Mullinger et al, 2008a). While useful and practical, these tests are limited in spatial coverage, and cannot assess local SAR variations occurring in vivo within the brain.…”

“…The inclusion of current-limiting resistors in the electrodes (Lemieux et al, 1997) and a careful selection of low-SAR MRI sequences (Noth et al, 2012) have been central to minimizing risks of injury. Temperature measurements in phantoms and humans help assessing the magnitude of heating effects (Lazeyras et al, 2001), and electromagnetic (EM) simulations provide high-resolution estimates of the SAR distribution across the head . In addition to safety concerns, simultaneous recordings from both modalities can be affected by severe artifacts, many of which are field strength-dependent.…”

“…Both mechanisms are field strength-dependent (Mullinger et al, 2008b). With modern EEG configurations, however, even the more conventional silver-or copper-based systems have been found to have an acceptable impact on fMRI data quality at fields up to 3 T (Bonmassar et al, 2001;Lazeyras et al, 2001). It has further been proposed that temporal SNR in fMRI is relatively well-preserved because physiological noise is also reduced with the overall signal loss (Luo and Glover, 2012).…”