Accuracy of frameless image-guided implantation of depth electrodes for intracranial epilepsy monitoring

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Various techniques are available for stereotactic implantation of depth electrodes for intracranial epilepsy monitoring. The goal of this study was to evaluate the accuracy and effectiveness of frameless MRI-guided depth electrode implantation.


Using a frameless MRI-guided stereotactic approach (Stealth), depth electrodes were implanted in patients via burr holes or craniotomy, mostly into the medial temporal lobe. In all cases in which it was possible, postoperative MR images were coregistered to planning MR images containing the marked targets for quantitative analysis of intended versus actual location of each electrode tip. In the subset of MR images done with sufficient resolution, qualitative assessment of anatomical accuracy was performed. Finally, the effectiveness of implanted electrodes for identifying seizure onset was retrospectively examined.


Sixty-eight patients underwent frameless implantation of 413 depth electrodes (96% to mesial temporal structures) via burr holes by one surgeon at 2 institutions. In 36 patients (203 electrodes) planning and postoperative MR images were available for quantitative analysis; an additional 8 procedures with 19 electrodes implanted via craniotomy for grid were also available for quantitative analysis. The median distance between intended target and actual tip location was 5.19 mm (mean 6.19 ± 4.13 mm, range < 2 mm–29.4 mm). Inaccuracy for transtemporal depths was greater along the electrode (i.e., deep), and posterior, whereas electrodes inserted via an occipital entry deviated radially. Failure to localize seizure onset did not result from implantation inaccuracy, although 2 of 62 patients (3.2%)—both with electrodes inserted occipitally—required reoperation. Complications were mostly transient, but resulted in long-term deficit in 2 of 68 patients (3%).


Despite modest accuracy, frameless depth electrode implantation was sufficient for seizure localization in the medial temporal lobe when using the orthogonal approach, but may not be adequate for occipital trajectories.

ABBREVIATIONS AH = anterior hippocampus; MH = midhippocampus; PH = posterior hippocampus.

Article Information

Correspondence Robert E. Gross: Emory University School of Medicine, Atlanta, GA.

INCLUDE WHEN CITING Published online March 22, 2019; DOI: 10.3171/2018.12.JNS18749.

Disclosures Dr. Gross is a consultant for Medtronic, Inc., on work unrelated to this research. Dr. Mayo is a consultant for the American Institute of Biological Sciences.

© AANS, except where prohibited by US copyright law.



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    Flowchart of patients included in the study. * One patient had a temporal burr hole for depth electrodes contralateral to craniotomy for grid and depth electrodes. EU = Emory University; proc = procedures; UU = University of Utah.

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    Photographs showing frameless implantation of depth electrodes via transtemporal (orthogonal) approach. A: The setup is depicted, showing the reference frame on the left attached to the operating room table by one Vertek arm, and on the right the Guideframe DT is held within the Precision stereoguide, attached to the second Vertek arm. B: A closer view showing the depth electrode inserted through the slotted cannula within the Guideframe DT. The depth electrode is inserted through a temporal burr hole. C: The burr hole has 4 tie-down holes drilled in it. Monocryl sutures (arrowhead) have been passed through the tie-down holes to anchor the electrodes for the duration of the invasive monitoring session. To allow the anchoring suture to be broken to release the depth electrode for bedside removal, a heavy nylon suture (e.g., 1-0) is tied within the monocryl (arrow points to the tail of the nylon suture). The tails of the nylon are subsequently passed through the same hole in the skin as the depth electrode by using a passing needle. By pulling on the nylon tails the monocryl is broken, allowing the depth electrode to be pulled through the skin. D: The final arrangement of 4 depth electrodes tied down within the burr hole. Modified from Gross RE, Rowland NC, Sung EK, LaBorde DV, Suleiman SL. (2012). Anchoring Depth Electrodes for Bedside Removal: A “Break-Away” Suturing Technique for Intracranial Monitoring. Neurosurgery, 71, ons52–ons57, by permission of Oxford University Press. Figure is available in color online only.

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    Analysis of depth electrode insertion accuracy. A: Screenshot of StealthStation software used for depth electrode placement analysis showing the planned orthogonal electrode trajectories to the amygdala, AH (pes), MH, and PH. The images are aligned on the anterior hippocampal (pes) trajectory (arrowheads). B: For implantation the Guideframe is aligned with the planned trajectory using “trajectory views” (arrowheads) and “guidance view” (lower right frame within panel B). The “distance to target” from the bottom of the Guideframe is indicated by the software (here, 53.7 mm). C: Postoperative scan showing the electrode susceptibility artifacts, aligned to the anterior hippocampal electrode (arrowheads) and coregistered to the preoperative planning Stealth MR image. (The larger artifacts visible are from strip electrode contacts.) D: To measure the implantation error directly, a trajectory (arrowheads) was made from the planned electrode target to a point 5 mm (see text) proximal to the end of the electrode artifact. The distance-to-target measurement then indicates this error (here, 4.0 mm for the posterior hippocampal depth electrode).

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    Distribution of depth electrode insertion accuracy measurements. A: Histogram of the average calculated distances between the actual and intended locations of each depth electrode, by insertion approach (occipital, temporal [orthogonal], and parietal). B: Graph of the average calculated distances between the actual and intended locations of the depth electrodes for each patient, along with the minimum and maximum distances. Where the line comes to the upper edge of the graph, the actual maximal measurement is not shown (> 15 mm). Numbers on the x axes designate patient numbers 1–32, etc. C: Scatterplot of the distribution of depth electrodes relative to their intended locations. The figure demonstrates a statistically significant bias in the medial (p < 0.01) and posterior (p < 0.001) directions.

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    Flowchart showing outcomes of intracranial depth electrodes implanted via burr holes. The outcome of invasive monitoring for all patients is shown for bilateral (A) and unilateral (B) depth electrode insertion. ATL = anterior temporal lobectomy; GTC = generalized tonic-clonic seizure; RNS = responsive neurostimulation; SAH = selective amygdalohippocampectomy; SANTE = stimulation of the anterior nucleus of the thalamus for epilepsy; VNS = vagus nerve stimulator.


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