EEG‐fMRI using z‐shimming in patients with temporal lobe epilepsy

Abstract

Purpose: To use z-shimming, a technique that reduces signal loss due to susceptibility artifacts that can result in reduced or absent activation in electroencephalography (EEG) functional MRI (fMRI) sessions in patients with temporal lobe epilepsy (TLE), to determine whether it would result in an increased ability to detect significant regions of blood oxygenation level-dependent (BOLD) signal change. Materials and Methods:Eight patients with TL EEG spikes underwent an EEG-fMRI scanning session using z-shimming. The signal intensities in the z-shimmed images were compared with those in the standard images. BOLD activation maps were created from the two sets of images using the timings of the spikes observed on the EEG. Results:The mean signal increase in the TLs as a result of z-shimming was 45.9% Ϯ 4.5%. The percentage of TL voxels above a brain intensity threshold rose from 66.1% Ϯ 7.6% to 77.6% Ϯ 5.7%. This appreciable increase in signal did not lead to any significant differences in the statistical maps created with the two sets of functional images. Conclusion:The results suggest that loss of signal is not the limiting factor for the detection of spike-related BOLD signal changes in patients with TLE activity.

The value of EEG‐fMRI and EEG source analysis in the presurgical setup of children with refractory focal epilepsy

“…In contrast, ESI seems to be a powerful and reliable tool in determining the epileptogenic zone in patients with refractory epilepsy. Previous studies have compared ESI and EEG‐fMRI analysis (Bagshaw et al., 2006; Benar et al., 2006; Sperli et al., 2006; Boor et al., 2007; Grova et al., 2008; Brodbeck et al., 2009; Groening et al., 2009; Brodbeck et al., 2010; Vulliemoz et al., 2010), consistently showing the superiority of the ESI results similar to the results of the present study. The main problem with these comparative studies remains—that the ESI and EEG–fMRI analyses were not performed on simultaneously acquired datasets and that there are intrinsically different underlying neurophysiologic mechanisms for each analysis.…”

“…Thirdly, as revealed by recent studies, EEG‐fMRI techniques seem to have limited accuracy and reliability in patients with temporal lobe epilepsies (TLEs) (Bagshaw et al., 2006; Kobayashi et al., 2006c; Salek‐Haddadi et al., 2006; Grouiller et al., 2011). Image distortion and signal dropout are caused notably by air–tissue interfaces (basal temporal lobe, orbitofrontal regions) and thus influence the probability of obtaining a good response in patients with temporal lobe epilepsy (Bagshaw et al., 2004).…”

Simultaneous intracranial EEG and fMRI of interictal epileptic discharges in humans

“…In addition to the fMRI artefacts due to the depth electrode, the detection of IEDrelated hippocampal BOLD changes with scalp EEG-fMRI is limited by the difficulty of recording temporal mesial IED even when using fMRI protocols optimised for the basal temporal lobe (Bagshaw, Torab et al 2006;. In patient 2, we found a highly significant correlation between hippocampal IED and BOLD signal changes from within the same region.…”

Localization of Epileptic Foci Based on Simultaneous EEG–fMRI Data

“…However, the question is whether this signal loss will be a limiting factor to identify the spike-related BOLD signal changes in patients with epilepsy. To find the actual effect of z-shimming in the results of identifying the spike-related BOLD responses, Bagshaw et al (25) designed an experiment in which eight patients with temporal lobe epilepsy (TLE) underwent an EEG-fMRI session, and zshimming was applied to their BOLD images. After comparing the intensities between z-shimmed and standard images and creating BOLD activation maps from the two sets of functional images using the times of spikes extracted from the EEG, it was found that the mean signal of the temporal lobes (TLs) increased 45.9 ± 4.5% as a result of z-shimming.…”