Abstracts

Rationale: (Author's Note: Authors Kaufmann and Schmitt contributed equally to this abstract.) Visual field deficit (VFD) after anterior temporal lobe resection (aTLR), can have an impairing extent and persistent complete hemianopsia is a well known risk in partial lesionectomy for occipital lobe epilepsy (OLE). Recently, pre-operative tractography of the optical radiation (OR) has been advised to assess the risk for VFD in patients for aTLR (e.g. Yogarajah et al., Brain 2009: 132; 1656 1668) and for other resective procedures (Winston et al., Epilepsia 2011: in press). Magnetic resonance imaging (MRI) tractography relies heavily on its precise anatomical determination of start and target regions of interest (ROIs), such as the lateral geniculate nucleus (LGN) and the primary visual cortex (V1). However, the postchiasmal visual pathway and the visual cortex varies interindividually, as shown in post mortem and fibre tracking studies.Therefore, a functionally based method is expected to increase the accuracy in the assessment of the VFD risk in the respecitive individuals. In our patient LGN and V1 were identified with fMRI-based retinotopic mapping to yield two ROIs for subsequent tractography with line propagation technique.Methods: The electroclinical and imaging details of a 27 year-old female epilepsy patient with OLE due to a temporo-occipital tumor of maldevelopment origin (dysembryoplastic neuroepithelial tumor, DNET) are discussed under specific consideration of FBT-coordinates of the OR. The MRI protocol included a T1-weighted sagittal 3D scan, a T2-weighted axial 2D scan and a diffusion tensor imaging (DTI) scan with an echo planar spin echosequence (Reese et al., Magn Reson Med. 2003: 49(1);177-82) at 3-Tesla. Start- and target ROIs for fiber tracking of the OR (LGN & V1) were determined with T2*-weighted fMRI-based retinotopic mapping using established techniques (Engel et al., Cereb Cortex 1997; 7(2):181-92). After DTI preprocessing and fiber tracking, paths with similar properties (i.e., trajectories, length) were combined in clusters for visual presentation and determination of coordinates. Results: Retinotopic phase maps allowed for the identification of V1 and LGN (figure 1) facilitating a precise DTI-based reconstruction of the optic radiation (figure 2). The DNET and electrographic seizure onset was distant to the OR. Location and structure of the OR was comparable to the healthy contralateral hemisphere.Conclusions: Using additional 20 min for retinotopic mapping seems to fascilitate the accuracy of pre-operative DTI-fiber-tractography of the OR in a patient with OLE, so that the location of the DNET with respect to the OR can be scrutinized with more precision. The combination of state-of-the-art non-invasive imaging techniques enhances the pre-operative work up in resective epilepsy surgery, provided post-operative results confirm the results.