A comparison of source localisation techniques in concurrent EEG/fMRI

Introduction: Simultaneous fMRI and EEG is an attractive means by which to investigate non-invasively human brain function. The combination of the two techniques offers high spatial and temporal resolution. Furthermore, the two metrics derive from different physical mechanisms (electrical activity and haemodynamics) meaning that their combination may shed light on the process of neurovascular coupling. Recent work has shown that it is possible to localise sources of electrical activity using EEG data recorded during simultaneous fMRI. Further it was shown that source localisation can reduce the level of interference in EEG timecourses caused by the MR scanner. Previous work was however conducted in vivo, meaning that the accuracy of source localisation and time course estimation could not be tested. In this study we employ an EEG dipole phantom to achieve this. A priori knowledge of the dipole location and timecourse allows assessment of the accuracy of two source localisation techniques (dipole fitting and beamformer) which are tested in the presence and absence of MR gradient fields. The interference reduction properties of beamformers and dipole fitting are compared, and the advantages of high EEG channel density discussed. Methods: A 1 cm long current dipole was placed in a spherical homogeneous conducting (1±0.3S/m) phantom and orientated in the x-y plane. An 8.5Hz sinusoidally oscillating current was applied to the dipole to simulate an active current dipole. An MR compatible EEG cap with 64 Ag/AgCl electrodes (EasyCap, Herrsching, Germany) was placed on the phantom which was then placed in the MR scanner.