Abstracts

Rationale: Patients with drug refractory focal epilepsy may benefit from neurosurgical treatment. A major concern is to avoid causing new deficits. In patients with temporal lobe epilepsy, diffusion tensor imaging tractography can delineate the optic radiation and predict the risk of a visual field deficit resulting from anterior temporal lobe resection. A smaller group of patients have epileptogenic lesions lying in proximity to the presumed course of the optic radiation as it passes posteriorly to the occipital cortex. Conventional MR imaging does not delineate the optic radiation, so the risk of surgery cannot be predicted, nor can surgery be tailored to minimise the risk. Methods: Conventional structural MR images and diffusion tensor imaging were acquired in 16 patients with lesions lying in proximity to the optic radiation. A probabilistic tractography algorithm was used to delineate the optic radiation and generate a map of connection probabilities. The diffusion data were co-registered with the structural data (T1 volumetric, T2 FLAIR) to demonstrate the relationship between the optic radiation and the lesion. Results were validated using postoperative visual fields and repeat imaging to look at outcome. Results: The relationship between the epileptogenic lesion and the optic radiation was demonstrated in each patient. Visualisation as a 2D overlay on anatomical images and 3D reconstructions is demonstrated. Illustrative cases show the role of the tractography data in each part of the surgical planning process. Some patients declining surgery on the basis of risk to vision may make a different decision if better informed and the surgeon can use the information for surgical planning in those considering or awaiting surgery. Post-operative data on patients including those with and without a visual field deficit, shows a correlation of preoperative data with operative outcome. Conclusions: Diffusion tensor imaging and tractography can demonstrate the relationship between the optic radiation and epileptogenic lesions. This is beneficial in all stages of surgical planning and correlates with outcome. Forthcoming work will aim to further reduce the risk to vision. A new interventional MR suite has been installed, which will enable the display of pre-operative tractography on intraoperative anatomical images to guide the surgeon during the resection. Additional validation using visual-evoked potentials recorded from intracranial electrodes (where available) is being studied.