Functional Brain Signal Processing: EEG & fMRI Lesson 17 Kaushik Majumdar Indian Statistical Institute Bangalore Center M.Tech. (CS), Semester III, Course B50

Simultaneous EEG and fMRI Recording

Imaging, Ballistocardigraphic (BCG) and Ocular Artifacts Mantini et al., 2007 BCG artifacts create a 10 Hz rhythm in EEG.

Artifacts Detail a = fat suppression pulse, b = slice selection RF, c, d, h are spoilers, e = slice selection gradient, f = dephasing and rephasing gradient, g = read out gradient. g has frequency 833 Hz here. Anami et al., 2003

Removal of BCG or Pulse Artifacts Allen et al., 1998 Template matching

Gradient Artifact and Its Removal Niazy et al., 2005

Four Stage Removal of Gradient Artifacts 1. Slice timing trigger alignment – synchronization of EEG and fMRI system clocks. 2. Local slice artifact template subtraction. 3. Residual artifact removal by optimal basis set (OBS). Niazy et al., 2005

Four Stage Removal of Gradient Artifacts (cont) 4. Adaptive noise cancelation (ANC). Widrow et al., 1975

Simultaneous EEG-fMRI Recording Wires must not loop around to avoid generation of current (Lenz’s law) under high magnetic field. EEG electrodes and wires must not contain magnetic materials like steel, nickel.

Simultaneous EEG-fMRI recording (cont) Movements in body, wires, electrodes and equipments must be minimized as far as possible. Head to be fixed by air cushion. Electric wires are also to be fixed to avoid any possible movements.

Safety Issues Specific absorption rate (SAR) Heat is generated due to eddy current in electrode tips under high magnetic field. Heat generation (carbon fiber electrodes are better than metal electrodes).

Bipolar Montage with Twisted Wires In twisted wire pair electric field produced in one due to high magnetic field is canceled out by opposing field in the other. Goldman et al., 2000

References D. Mantini et al., Complete artifact removal for EEG recorded during continuous fMRI using independent component analysis, NeuroImage, 34(2): 598 – 607, K. Anami et al., Stepping stone sampling for retrieving artifact-free electroencephalogram during functional magnetic resonance imaging, NeuroImage, 19: 281 – 295, 2003.

References (cont) P. J. Allen et al., Identification of EEG events in MR scanner: the problem of pulse artifact and a method for its subtraction, NeuroImage, 8: 229 – 239, Fig. 2. P. K. Niazy et al., Removal of FMRI environment artifacts from EEG data using optimal basis sets, NeuroImage, 28: 720 – 737, Four stage FSTAR algorithm.

Artifact Removal Software MATLAB based artifact removal tool box for simultaneous EEG-fMRI recording, ent&view=article&id=53&Itemid=78. ent&view=article&id=53&Itemid=78

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