Electrode localization for planning surgical resection of the epileptogenic zone in pediatric epilepsy

Abstract

Purpose: In planning for a potentially curative resection of the epileptogenic zone in patients with pediatric epilepsy, invasive monitoring with intracranial EEG is often used to localize the seizure onset zone and eloquent cortex. A precise understanding of the location of subdural strip and grid electrodes on the brain surface, and of depth electrodes in the brain in relationship to eloquent areas is expected to facilitate pre-surgical planning.

Methods: We developed a novel algorithm for the alignment of intracranial electrodes, extracted from post-operative CT, with pre-operative MRI. Our goal was to develop a method of achieving highly accurate localization of subdural and depth electrodes, in order to facilitate surgical planning. Specifically, we created a patient-specific 3D geometric model of the cortical surface from automatic segmentation of a pre-operative MRI, automatically segmented electrodes from post-operative CT, and projected each set of electrodes onto the brain surface after alignment of the CT to the MRI. Also, we produced critical visualization of anatomical landmarks, e.g., vasculature, gyri, sulci, lesions, or eloquent cortical areas, which enables the epilepsy surgery team to accurately estimate the distance between the electrodes and the anatomical landmarks, which might help for better assessment of risks and benefits of surgical resection.

Results: Electrode localization accuracy was measured using knowledge of the position of placement from 2D intra-operative photographs in ten consecutive subjects who underwent intracranial EEG for pediatric epilepsy. Average spatial accuracy of localization was 1.31 ± 0.69 mm for all 385 visible electrodes in the photos.

Conclusions: In comparison with previously reported approaches, our algorithm is able to achieve more accurate alignment of strip and grid electrodes with minimal user input. Unlike manual alignment procedures, our algorithm achieves excellent alignment without time-consuming and difficult judgements from an operator.

Conflict of interest statement

Conflict of Interest

Vahid Taimouri, Alireza Akhondi-Asl, Xavier Tomas-Fernandez, Jurriaan M. Peters, Sanjay P. Prabhu, Annapurna Poduri, Masanori Takeoka, Tobias Loddenkemper, Ann Marie R. Bergin, Chellamani Harini, Joseph R. Madsen and Simon K. Warfield declare that they have no conflict of interest.

Figures

Fig 1

The sketch (left), radiographic image (right) of an epileptic subject presented in the epilepsy surgery conference. The blue ellipse on the sketch highlights the suspicious focal cortical dysplasia, and the high intensity solid dots in the radiographic image show the locations of the grid/strip electrodes.

Fig 2

The pipeline of our electrode localization on the pre-operative MRI cortical surface.

Fig 3

The extracted electrodes and wires after thresholding the CT image (Left). The narrower wires inside the cranium (Brown) are removed by morphological filtering, and the electrodes (Red) inside the skull are preserved (Right).

Fig 4

Row 1: The brain parcellation segments different anatomical regions. Row 2: A single red 8×2 grid is pushed between the cerebellum and temporal lobe. If we use the total brain cortical surface containing the cerebellum, some of the electrodes are wrongly projected on the cerebellum (left). To solve this, the cerebellum is removed to correct the projection (right). (Row 3): Since the red 8×8 grid is inserted between two hemispheres, its location is not clearly identifiable (left). To project the grid on the right hemisphere, the left hemisphere is removed so that the grid is projected on the parietal lobe of the right hemisphere (right).

Fig 5

The extracted electrodes (blue spheres) in the CT image are displaced in a direction normal to the plastic sheet (brown surface) and then projected onto the smoothed cortical (pink) surface. The extracted electrodes (blue spheres) will be localized onto the MRI cortical surface (green spheres).

Fig 6

(Row 1): Strip electrodes e1, …,e6; (blue circles) are mounted on a narrow rectangular plastic sheet which is bent in one direction during implantation. The hypothetical dashed green line l on the plastic sheet connects the centers of the electrodes (left). Due to the rectangular shape of the plastic sheet, the electrode centers are roughly located on one plane, P, even after the strip is bent; this plane is estimated using the linear least square method (middle). The two normal vectors, nei+ and nei-, are in plane P, and perpendicular to both line l and the plastic sheet, which show the projection directions. (Row 2): illustrates the projection of a 6×1 strip array onto the MRI cortical surface using the proposed method. First, the CT image is registered on the MRI image, and then, the electrode centers on the CT image (blue spheres) are displaced in direction normal to the plastic sheet which can be distinguished as a dark line connecting the high intensity electrodes on the CT image. As seen in the MRI image, the electrodes are accurately projected on the MRI cortical surface before craniotomy (green spheres).

Fig 7

(Row 1) The electrode localization of four subjects on the MRI cortical surfaces are illustrated with craniotomy regions highlighted in blue. (Row 2) The intra-operative photos of the same subjects showing the electrode locations, (Row 3) the projection of the electrodes to the photos shown by blue rings.

Fig 7

(Row 1) The electrode localization of four subjects on the MRI cortical surfaces are illustrated with craniotomy regions highlighted in blue. (Row 2) The intra-operative photos of the same subjects showing the electrode locations, (Row 3) the projection of the electrodes to the photos shown by blue rings.

Fig 8

The primary sketch presented by the neurophysiologists shows the red and green grids covering only the inferior and middle temporal gyri (left). By contrast, our localization method localizes these areas of the brain as well as the superior temporal gyrus (right).

Fig 9

Clinicians can orient to the correct electrode locations (green) using the vasculature (red), reducing the potential for surgical injury to blood vessels during the implantation of electrodes.

Fig 10

An 8×4 grid (green) and a 10×1 depth electrode array (pink) are illustrated along with a lesion (brown) extracted and visualized under a partially transparent cortical surface. The extent of the lesion and its position relative to the electrodes can be analyzed more easily on a partially transparent cortical surface (left). In addition, depth electrodes and the extracted lesion (middle) are likewise illustrated on a partially transparent cortical surface (right). In this way, the epilepsy surgery team can identify the electrode positions relative to the lesion. As seen in the figure to the right, the last two depth electrodes on the strip are actually inside the tumor, which is not evident when the lesion is shown opaquely, as in the middle figure.

Fig 11

The brain parcellation helps neurophysiologists recognize the functional/structural areas where the electrodes are placed (Left). The optic radiation (OR) tractography is illustrated along with the focal cortical dysplasia in red and an 8×2 grid in blue (Middle). The electrode localization and the activation areas in a fMRI experiment on the cortical surface (Right). The functional activation areas and the tractography fibers on the cortical surface may facilitate judgement of the desired surgical resection margin.