Quantification of Subdural Electrode Shift Between Initial Implantation, Postimplantation Computed Tomography, and Subsequent Resection Surgery

Abstract

Background: Subdural electrodes are often implanted for localization of epileptic regions. Postoperative computed tomography (CT) is typically acquired to locate electrode positions for planning any subsequent surgical resection. Electrodes are assumed to remain stationary between CT acquisition and resection surgery.

Objective: To quantify subdural electrode shift that occurred between the times of implantation (Crani 1), postoperative CT acquisition, and resection surgery (Crani 2).

Methods: Twenty-three patients in this case series undergoing subdural electrode implantation were evaluated. Preoperative magnetic resonance and postoperative CT were acquired and coregistered, and electrode positions were extracted from CT. Intraoperative positions of electrodes and the cortical surface were digitized with a coregistered stereovision system. Movement of the exposed cortical surface was also tracked, and change in electrode positions was calculated relative to both the skull and the cortical surface.

Results: In the 23 cases, average shift of electrode positions was 8.0 ± 3.3 mm between Crani 1 and CT, 9.2 ± 3.7 mm between CT and Crani 2, and 6.2 ± 3.0 mm between Crani 1 and Crani 2. The average cortical shift was 4.7 ± 1.4 mm with 2.9 ± 1.0 mm in the lateral direction. The average shift of electrode positions relative to the cortical surface between Crani 1 and Crani 2 was 5.5 ± 3.7 mm.

Conclusion: The results show that electrodes shifted laterally not only relative to the skull, but also relative to the cortical surface, thereby displacing the electrodes from their initial placement on the cortex. This has significant clinical implications for resection based upon seizure activity and functional mapping derived from intracranial electrodes.

FIGURE 1.

Cortical surface (A and C) and electrode grids (B and D) acquired from iSV during Crani 1 (A and B) and Crani 2 (C and D), respectively, overlaid with pMR.

FIGURE 2.

Electrode positions obtained from iSV and CT overlaid on the segmented brain. Red circles show electrodes extracted from the Crani 1 procedure and blue crosses show electrode positions derived from the Crani 2 procedure. Green triangles indicate their locations in the CT scans. Each electrode was labeled with other identifying information, eg, “t1” represents contact number 1 on the grid covering the temporal lobe (t: temporal; f: frontal; o: orbital-frontal).

FIGURE 3.

Measurement of cortical shift using optical flow motion tracking. A, Red-green overlay of projection images of the cortical surface from Crani 1 (red) and Crani 2 (green). White arrows point to misalignment between features, and indicate brain shift that occurred laterally. B, Red-green overlay of the projection images after optical flow registration. Features are well aligned. Blue vectors show lateral shift of the cortical surface. C, Shows reconstructed 3D cortical surfaces from Crani 1 (bottom) and Crani 2 (top), respectively. Yellow vectors denote 3D displacements of the cortical surface.

FIGURE 4.

Illustration of the overall and relative lateral shift of an electrode. Red lines represent features on the exposed cortical surface such as blood vessels, which shifted from their positions in Crani 1 (left) to new positions in Crani 2 (right). Circles represent an example electrode whose center (blue dot) aligns with the vessel intersection in Crani 1 (left) which shifts to its new location in Crani 2 (right), and no longer aligns with the vessel intersection. The shift of the electrode relative to the cortical surface is calculated by subtracting the shift of the cortical surface at the electrode center (ie, the vessel junction in this example) from the overall displacement between the 2 circles.

FIGURE 5.

Box plots of electrode and cortical shifts. Columns 1 to 3 display electrode shift as measured by iSV between Crani 1 and CT, between CT and Crani 2, between Crani 1 and Crani 2, respectively. Column 4 shows cortical shift, and column 5 plots the electrode shift relative to the cortical surface. The overall magnitude of 3D shift and the components along surface normal and lateral directions are represented in blue, red, and green, respectively. In each box plot, central line corresponds to the median, edges of the box indicate the 25th (Q1) and 75th (Q3) percentiles, whiskers extend to the most extreme points that are not outliers, and the outliers are plotted individually as black circles. Outliers were determined as points larger than Q3 + 1.5 × (Q3 – Q1) or smaller than Q1 – 1.5 × (Q3 – Q1).

FIGURE 6.

Average lateral electrode shift relative to the skull (blue) and relative to the cortical surface (red) in each patient case.