Whole-brain structural connectivity in patients with focal epilepsy studied with MRI diffusion spectrum imaging (DSI)

In patients with focal epilepsy, diffusion MRI often reveals structural changes extending beyond visible lesions or show structural abnormalities even when the clinical scans are normal. MR Tractography based on diffusion tensor imaging allows non-invasive mapping of white matter tracts in vivo. Recently, Diffusion Spectrum Imaging (DSI), based on an increased number of diffusion directions and intensities, has improved the sensitivity of tractography, notably with respect to the problem of fibre crossing and recent development allow acquisition times compatible with clinical application. We used DSI tractography and parcelation of the grey matter in regions of interest to build whole-brain connectivity matrices describing the mutual connections between cortical regions at different spatial resolutions in patients with focal epilepsy and healthy controls. The combination with other imaging modalities (EEG source imaging, PET, SPECT, EEG-fMRI) localising cortical regions corresponding to the irritative zone (IZ) or seizure onset zone (OZ) allows to reconstruct tracts of interest to map the connectivity of the IZ/OZ, as well as physiological close by tracts subserving normal neurological/cognitive functions. We here present our preliminary results on patients with lesional and cryptogenic epilepsy showing that this new analysis strategy allows investigating focal connectivity changes at a whole-brain level with unprecedented sensitivity. This opens new avenues for the understanding of epileptic networks and for clinical management of candidates for epilepsy surgery.