Abstracts

Rationale: Understanding the characteristics of the structural connection between the hippocampi in temporal lobe epilepsy (TLE) may provide insight into the seizure propagation mechanism in these patients. In this study we quantified the hippocampal structural connectivity strength (SCS) in TLE using Magnetic Resonance Imaging (MRI), and compared it to several subject characteristics to determine the effect of TLE on this pathway.Methods: Five patients with TLE and five age- and sex-matched healthy controls were recruited thus far. Structural T1-weighted MRI was acquired on a 3T Philips Achieva scanner (TE=4.6ms, TR=8.9ms, matrix=256x256mm, voxel size=1mm isotropic, 150 slices). Diffusion-weighted MRI was also acquired (TE=65ms, TR=8500ms, matrix=96x96mm, voxel size=2.5mm isotropic, 50 slices, 92 diffusion directions with b-value=1600 s/mm²). Image analysis was performed using the tools from FSL (http://www.fmrib.ox.ac.uk/fsl), SPM (http://www.fil.ion.ucl.ac.uk/spm/) and FreeSurfer (http://ftp.nmr.mgh.harvard.edu/). A probabilistic tractography method (Behrens, et al) was used to assess the anatomical connection of two hippocampi. First, the T1-weighted image was segmented to obtain the regions of interest (ROIs) using FreeSurfer. Then, ROIs were mapped into native diffusion space using SPM8. The diffusion-weighted images were registered to a non-diffusion-weighted image to correct eddy current distortions and head motions in FSL. Finally, ROIs served as seed and target masks and, therefore, the probabilistic tractography was implemented in two directions to get fiber pathways. The other gray matter and cerebrospinal fluid (CSF) were the exclusion masks. The streamline number (the number of fiber pathways) and voxel number in each seed region were recorded. The structural connectivity strength (SCS) between region A and B is defined as follows: SCS(A,B)= (c(A->B)+c(B->A))/2, c(A->B)= the streamline number/the voxel number in region A reaching region B. The SCS of TLE patients and controls was compared. The relationship between SCS and age, hippocampus volume, and duration of disease were investigated. Results: Figure 1 shows the SCS of patients is lower than that of the matched controls. As the age increased, after age 30, the SCS increased linearly. There was no linear correlation between SCS and hippocampal volume (Figure 2), nor duration of disease. Conclusions: The SCS measures the average connection strength of white matter between two ROIs, which may reflect the damage of white matter pathways due to recurrent seizures. The SCS results showed the white matter connection of two hippocampi in TLE patients was reduced as compared with controls . In addition, SCS may be significantly influenced by age, but not by the hippocampal volume. More subjects will be recruited to confirm these findings. Further investigation of SCS may contribute to a better prediction of surgical therapy outcome. R01 NS075270 (Morgan) Behrens, T. E., et al. NeuroImage 34, 144 155 (2007).