Abstracts

Rationale: Disruption of connectivity in certain disease states, including epilepsy, can be demonstrated using functional connectivity MRI (fcMRI), based on task-free, resting state fMRI. In healthy controls, fcMRI reveals strong interhemispheric connectivity between homotopic (geometrically corresponding) regions across the brain. A technique to quantify functional homotopy is voxel-mirrored homotopic connectivity (VMHC), providing a voxel-wise connectivity computed between mirroring voxels in each hemisphere. Functional MRI, both task based and resting state, is routinely acquired along with other diagnostic techniques (EEG, FDG-PET, DTI and intracranial EEG) as a part of presurgical evaluation for drug resistant epilepsy at our Institution. We employed the VMHC technique to evaluate a series of pediatric patients with MTS in order to characterize the homotopic connectivity. Methods: In this retrospective study, we assessed homotopic connectivity in a group of 9 pediatric MTS patients evaluated during a four-year period. The fcMRI scans in both awake and anesthetized patients utilized Siemens 3T imaging (EPIBOLD sequence, TR=2s, TE=27ms, flip angle = 90°, 4mm isotropic voxels). Awake patients were instructed to rest with eyes open. Following linear coregistration of structural T1 images to standard (MNI) space, and fcMRI preprocessing in standard space (1000 Functional Connectomes Project scripts; AFNI and FSL software), we completed the VMHC analysis with the Resting-State fMRI Data Analysis Toolkit (REST v. 1.8, Song et al. 2011). This study was approved by the Seattle Children's Hospital Institutional Review Board. Results: In 8 out of 9 MTS cases collected from 2010 to 2014, we observed a reduction of VMHC in parts of temporal lobe (see Fig 1), with the precise location varying between patients. In addition, in individual MTS cases we observed areas of prominent VMHC reduction in the parietal lobe (2 cases) and frontal lobe (2 cases). Comparing averaged VMHC maps for patients and controls supports the observations of reduced VMHC primarily in temporal lobe in MTS patients. Conclusions: Analysis in a small group of pediatric MTS patients revealed that atypical reduction in homotopic connectivity in the temporal lobe occurred frequently. In addition, we observed reduction in the parietal and frontal lobe in some individual cases. Variability of homotopic connectivity in healthy subjects may reflect the dynamic nature of BOLD signal as well as variation in individual anatomy, motion artifact, or other factors. In MTS patients, it may also reflect the dynamic nature of epileptogenic activity. VMHC can only be used meaningfully in patients without significant distortion of brain anatomy. This makes it difficult to apply VMHC to patients with lesional epilepsy and abnormalities of smaller structures. This technique may have application to cortical areas that are grossly symmetric, and by extension well suited to presurgical evaluations of patients with nonlesional focal epilepsy.