Abstract
Electroencephalography (EEG) signals greatly suffer from gradient artefacts (GAs) due to the time-varying field gradients in the magnetic resonance (MR) scanner during the simultaneous acquisition of EEG and functional magnetic resonance imaging (fMRI) data. The GAs are the principal contributors of artefacts while recording EEG inside an MR scanner, and most of them come from the interaction of the EEG cap and the subject’s head. Many researchers have been using a spherical phantom to characterize the GA in EEG data in combined EEG–fMRI studies. In this study, we investigated how the phantom shape could affect the characterization of the GA. EEG data were recorded with a spherical phantom, a head-shaped phantom, and six human subjects, individually, during the execution of customized and standard echo-planar imaging (EPI) sequences. The spatial potential maps of the root-mean-square (RMS) voltage of the GA over EEG channels for the trials with a head-shaped phantom closely mimicked those related to the human head rather than those obtained for the spherical phantom. This was confirmed by measuring the average similarity index (0.85/0.68). Moreover, a paired t-test showed that the head-shaped phantom’s and the spherical phantom’s data were significantly different (p < 0.005) from the subjects’ data, whereas the difference between the head-shaped phantom’s and the spherical phantom’s data was not significant (p = 0.07). The results of this study strongly suggest that a head-shaped phantom should be used for GA characterization studies in concurrent EEG–fMRI.
Highlights
The simultaneous recording of electroencephalography (EEG) and functional magnetic resonance imaging has enabled many researchers to investigate and study new possibilities in functional neuroimaging to understand the human brain and help in diagnosing and treating brain-related diseases and disabilities [1]
The potential map of the RL gradient of the spherical phantom showed that the frontal, fronto-central, and temporal lobe electrodes contributed to the largest negative voltages, whereas the occipital and parietal lobe electrodes contributed to the largest positive voltages, respectively
We investigated experimental EEG data obtained from the simultaneous EEG–functional magnetic resonance imaging (fMRI)
Summary
The simultaneous recording of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) has enabled many researchers to investigate and study new possibilities in functional neuroimaging to understand the human brain and help in diagnosing and treating brain-related diseases and disabilities [1]. Because of the fusion of the excellent spatial resolution of fMRI [1,2] and the high temporal resolution of EEG in combined EEG–fMRI studies, this technique has enabled us to better understand the relationship between spontaneous or evoked electrical activity and hemodynamic response in the human brain [3,4,5]. In concurrent EEG–fMRI recordings, the magnetic fields of the MRI scanner generate artefacts in the EEG data [6,7]. These artefacts are larger than the neuronal activity of interest by an order of magnitude or more [8].
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