Influence of EEG electrodes on the BOLD fMRI signal.

Measurement of the EEG during fMRI scanning can give rise to image distortions due to magnetic susceptibility, eddy currents or chemical shift artifacts caused by certain types of EEG electrodes, cream, leads, or amplifiers. Two different creams were tested using MRS and T2* measurements, and we found that the one with higher water content was superior. This study introduces an index that quantifies the influence of EEG equipment on the BOLD fMRI signal. This index can also be used more generally to measure the changes in the fMRI signal due to the presence of any type of device inside (or outside) of the field of view (e.g., with fMRI and diffuse optical tomography, infrared imaging, transcranial magnetic stimulation, ultrasound imaging, etc.). Quantitative noise measurements are hampered by the normal variability of functional activation within the same subject and by the different slice profiles obtained when inserting a subject multiple times inside a MR imaging system. Our measurements account for these problems by using a matched filtering of cortical surface maps of functional activations. The results demonstrate that the BOLD signal is not influenced by the presence of EEG electrodes when using a properly constructed MRI compatible recording cap.