Abstracts

Rationale: It is well established that hemispheric asymmetries in resting brain activity can help identify epileptiform areas using FDG-PET. Here we explore the utility of functional magnetic resonance imaging (fMRI) to detect such areas via inter-hemispheric asymmetries. fMRI is potentially superior to FDG-PET in that it is less invasive, has greater spatial resolution, and can be analyzed in multiple ways that may have independent diagnostic utility. We investigated four such statistics derived from the fMRI BOLD signal: regional homogeneity (ReHo), amplitude of low frequency fluctuations (ALFF), fractional ALFF (fALFF), and degree centrality (DC). ReHo measures the degree to which the activity in a voxel is similar to its neighbors. ALFF measures the amplitude of the fMRI signal, while fALFF is a normalized variant of ALFF. DC measures the mean functional connectivity between a voxel and the rest of the brain. Based on prior work by others, we expected ReHo to be increased and the other statistics to be decreased in epileptiform areas relative to their contralateral homologues. Methods: fMRI data were acquired from 10 individuals with focal epilepsy being evaluated for surgery prior to intracranial electrode implantation. Half had medial temporal lobe epilepsy. Participants rested for 5-10 minutes while data were acquired using an EPI gradient echo sequence on a 3T scanner. Data were processed using the Configurable Pipeline for the Analysis of Connectomes software. The aforementioned statistics were derived from the fMRI data for each voxel. Statistics were converted to z-scores and warped to a symmetric template brain. Differences between each voxel and its inter-hemisphere analog were then computed and warped back to the individual's brain. The seizure onset zone (SOZ) and irritative zone (IZ) were identified by epileptologist readings of subsequent intracranial electrode recordings. Electrode locations were identified in the pre-implant MRI data by co-registering a post-implant CT scan to the pre-implant MRI with correction for brain shift. All voxels within 5 mm of an electrode's center were assigned that electrode's category (i.e., SOZ, IZ, or non-epileptiform). Results: As expected ReHo was reliably increased (p=.02) and ALFF decreased (p=.04) in the SOZ relative to the contralateral homologous region. Surprisingly, SOZ fALFF and DC values did not reliably differ across hemisphere (p>.32). None of the fMRI statistics showed reliable asymmetries in the IZ (p>.13). Conclusions: This initial investigation of resting fMRI asymmetries suggests that the SOZ is marked by relatively increased regional homogeneity and decreased activity relative to its homologous contralateral area. If these differences prove reliable, they could have significant clinical utility given the ease of resting fMRI data acquisition.