Freehand placement of depth electrodes using electromagnetic frameless stereotactic guidance

Technical note

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The presurgical evaluation of patients with epilepsy often requires an intracranial study in which both subdural grid electrodes and depth electrodes are needed. Performing a craniotomy for grid placement with a stereotactic frame in place can be problematic, especially in young children, leading some surgeons to consider frameless stereotaxy for such surgery. The authors report on the use of a system that uses electromagnetic impulses to track the tip of the depth electrode. Ten pediatric patients with medically refractory focal lobar epilepsy required placement of both subdural grid and intraparenchymal depth electrodes to map seizure onset. Presurgical frameless stereotaxic targeting was performed using a commercially available electromagnetic image-guided system. Freehand depth electrode placement was then performed with intraoperative guidance using an electromagnetic system that provided imaging of the tip of the electrode, something that has not been possible using visually or sonically based systems. Accuracy of placement of depth electrodes within the deep structures of interest was confirmed postoperatively using CT and CT/MR imaging fusion. Depth electrodes were appropriately placed in all patients. Electromagnetic-tracking–based stereotactic targeting improves the accuracy of freehand placement of depth electrodes in patients with medically refractory epilepsy. The ability to track the electrode tip, rather than the electrode tail, is a major feature that enhances accuracy. Additional advantages of electromagnetic frameless guidance are discussed.

Article Information

Address correspondence to: Carter D. Wray, M.D., Seattle Children's Hospital, Division of Pediatric Neurology, MS B 5552, PO Box 5371, Seattle, Washington 98145. email:

© AANS, except where prohibited by US copyright law.



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    Case 9 (18-year-old patient with an insular cavernoma). A preoperative T2-weighted MR image with a coregistered depth electrode from a CT scan obtained immediately after electrode implantation using electromagnetic frameless stereotactic guidance.



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