3D visualization of subdural electrode shift as measured at craniotomy reopening

Abstract

Purpose: Subdural electrodes are implanted for recording intracranial EEG (iEEG) in cases of medically refractory epilepsy as a means to locate cortical regions of seizure onset amenable to surgical resection. Without the aid of imaging-derived 3D electrode models for surgical planning, surgeons have relied on electrodes remaining stationary from the time between placement and follow-up resection. This study quantifies electrode shift with respect to the cortical surface occurring between electrode placement and subsequent reopening.

Methods: CT and structural MRI data were gathered following electrode placement on 10 patients undergoing surgical epilepsy treatment. MRI data were used to create patient specific post-grid 3D reconstructions of cortex, while CT data were co-registered to the MRI and thresholded to reveal electrodes only. At the time of resective surgery, the craniotomy was reopened and electrode positions were determined using intraoperative navigational equipment. Changes in position were then calculated between CT coordinates and intraoperative electrode coordinates.

Results: Five out of ten patients showed statistically significant overall magnitude differences in electrode positions (mean: 7.2mm), while 4 exhibited significant decompression based shift (mean: 4.7mm), and 3 showed significant shear displacement along the surface of the brain (mean: 7.1mm).

Discussion: Shift in electrode position with respect to the cortical surface has never been precisely measured. We show that in 50% of our cases statistically significant shift occurred. These observations demonstrate the potential utility of complimenting electrode position measures at the reopening of the craniotomy with 3D electrode and brain surface models derived from post-implantation CT and MR imaging for better definition of surgical boundaries.

Figure 1

(a) Patients S1—S5 grid placement and shift maps. Top row shows CT-derived electrode models overlaid on post-grid MRI-derived 3D brain surface models. Row 2 shows reconstructed grid sheet models overlaid on the 3D brain model. Sheets are limited to those electrodes whose positions were visible and available for measurement in the OR (at the discretion of the surgeon). Yellow sheet indicates CT-derived electrode positions, Red indicates Stealth OR coordinate-derived sheet, and Orange indicates overlap. Rows 3 and 4 show coronal and axial slices indicating location of red and yellow sheets above. Slices shown are included in the 3D rendering in Rows 1 and 2 (b) Patients S6—S10 grid placement and shift maps.

Figure 2

Illustration depicting the method of electrode shift measurement. Magnitude of shift is measured as the absolute shift. Compression is a measured as the difference between the brain's center of the mass to the CT-derived electrode position, and the brain's center to the Stealth-derived electrode position. Shear is defined as the remaining shift along the brain's surface.

Figure 3

Box plot showing electrode shift measurements for each patient (S1—S10, horizontal axis). Vertical axis indicates shift measurement in mm. Magnitude (M), Compression (C), and Shear (S) are shown. Median measure is indicated by horizontal line within the box, solid black squares indicate the mean, (+) indicate ground electrodes anchored to the skull, indicative of intrinsic error in the measures. Open circles indicate outliers as defined by measurements lying outside 99.5% confidence interval from the mean. Error bars indicate the most extreme value not considered as an outlier. Shaded boxes indicate group mean error in ground electrode comparison for magnitude, compression and shear. **Indicates significantly different from ground electrodes, p < 0.01, one way ANOVA.

Figure 4

(A) Grid shift map for patient S4. Teal electrodes indicate where electrodes were measured via CT. Red dots indicate electrode location as measured in the operating room at the time of resection with navigational unit. Green lines indicate major sulcal patterns visible in the photograph (B). Blue ovals indicate two example electrodes that have shifted across sulci. (B) Photograph of electrodes at the time of resection with green lines analogous to (A) overlaid. (C) Photograph without overlays.