Three-dimensional visualization of subdural electrodes for presurgical planning

Abstract

Background: Accurate localization and visualization of subdural electrodes implanted for intracranial electroencephalography in cases of medically refractory epilepsy remains a challenging clinical problem.

Objective: We introduce a technique for creating accurate 3-dimensional (3D) brain models with electrode overlays, ideal for resective surgical planning.

Methods: Our procedure uses postimplantation magnetic resonance imaging (MRI) and computed tomographic (CT) imaging to create 3D models of compression-affected brain combined with intensity-thresholded CT-derived electrode models using freely available software. Footprints, or "shadows," beneath electrodes are also described for better visualization of sulcus-straddling electrodes. Electrode models were compared with intraoperative photography for validation.

Results: Realistic representations of intracranial electrode positions on patient-specific postimplantation MRI brain renderings were reliably created and proved accurate when compared with photographs. Electrodes placed interhemispherically were also visible with our rendering technique. Electrode shadows were useful in locating electrodes that straddle sulci.

Conclusion: We present an accurate method for visualizing subdural electrodes on brain compression effected 3D models that serves as an ideal platform for surgical planning.

FIGURE 1

Illustration of the creation and implementation of electrode shadows on patient S1. A, electrodes from binarized CT data are shown in yellow. Thin red outline indicates the pial brain surface, and the thin green outline indicates the white matter/gray matter boundary. B, illustration of the benefit of volumetrically smoothing electrodes before projection. The partially inflated pial brain surface is shown with a yellow line that intersects with all electrodes. C, electrode shadows cast onto the pial surface without electrode smoothing. D, smoothed CT electrode projected as surface shadows cast upon the pial brain surface. E, electrode placement cartoon indicating electrode names and numbers in patient S1. F, combination of 3D representation of binarized electrodes (teal) from A and electrode shadows from C for final visualization with 3Dslicer.

FIGURE 2

Illustration of the use of electrode shadows in determining sulcal-straddling electrodes. Top, cortical electrode model of patient S5. Middle, zoomed in view of electrodes with shadows, suspected sulcal-straddling electrodes circled in red. Bottom, same view without shadows.

FIGURE 3

Three-dimensional electrode representations in 3 patients (without electrode shadows) cast on the brain surface in patient S1 (A), patient S2 (C), and patient S3 (E). Corresponding digital intraoperative photographs of brain anatomy before electrode explantation and resection of epileptic zones are shown in B, D, and F, respectively. Yellow lines indicate major sulcal patterns for identification and comparison of electrode positions. Red lines indicate the edges of dural flaps that cover some electrodes in the digital photographs. Two asterisks (**) in A and B indicate the temporal-occipital electrode strip shown in Figure 2E for patient S3. Three asterisks (***) indicate a suction instrument holding down the grid for photography in A and B, a single asterisk (*) indicates a metal marker in the grid provided by the vendor, rather than a spurious electrode.

FIGURE 4

Final cortical surface electrode models for all 6 patients (S1 to S6) illustrating grid and strip electrode locations and their corresponding projected shadows. Patient S4 illustrates the ability to visualize medial electrodes between hemispheres, as a partial grid of electrodes is shown placed over corpus collosal defect. Orbiting electrodes from the right hemisphere are visible in patient S4’s model. In the reconstruction of patient S3, electrode shadows were unable to be generated. Larger grounding electrodes attached to the skull are visible in patients S1 (posterior), S2 (lateral), S3 (lateral), S5 (posterior), and S6 (dorsal). Brain compression caused by the grid is clearly visible in the lower panels of S1, S2, and S3.

FIGURE 5

Comparison of 3D representations of the same cortex (patient S6). A, 3D volume rendering of the right hemisphere produced by skull stripping and 3D rendering with the Analyze software package, compared with D produced with Freesurfer software (ventral surface below). B and E show projections of electrodes onto the corresponding brain surfaces. Gyral and sulcal patterns are clearly visible in D and E, but are distorted in A and B by metallic electrode susceptibility artifacts. C and F show the corresponding cortical reconstructions of the unimplanted left hemisphere.