Localization of dense intracranial electrode arrays using magnetic resonance imaging

Abstract

Intracranial electrode arrays are routinely used in the pre-surgical evaluation of patients with medically refractory epilepsy, and recordings from these electrodes have been increasingly employed in human cognitive neurophysiology due to their high spatial and temporal resolution. For both researchers and clinicians, it is critical to localize electrode positions relative to the subject-specific neuroanatomy. In many centers, a post-implantation MRI is utilized for electrode detection because of its higher sensitivity for surgical complications and the absence of radiation. However, magnetic susceptibility artifacts surrounding each electrode prohibit unambiguous detection of individual electrodes, especially those that are embedded within dense grid arrays. Here, we present an efficient method to accurately localize intracranial electrode arrays based on pre- and post-implantation MR images that incorporates array geometry and the individual's cortical surface. Electrodes are directly visualized relative to the underlying gyral anatomy of the reconstructed cortical surface of individual patients. Validation of this approach shows high spatial accuracy of the localized electrode positions (mean of 0.96 mm ± 0.81 mm for 271 electrodes across 8 patients). Minimal user input, short processing time, and utilization of radiation-free imaging are strong incentives to incorporate quantitatively accurate localization of intracranial electrode arrays with MRI for research and clinical purposes. Co-registration to a standard brain atlas further allows inter-subject comparisons and relation of intracranial EEG findings to the larger body of neuroimaging literature.

Fig. 1

Complete procedure for localization of grids from pre- and post-implant MR images. Post-implant MR image (B) is co-registered to the pre-implant MR image (A) using a rigid-body transformation. Widespread artifacts referred to as “black holes” surround each electrode of the dense grid, prohibiting unambiguous identification of all electrodes. Therefore, the co-registered image (D) is used to manually determine the xyz coordinates of two electrodes that are easily identifiable (yellow lines guide this procedure on simultaneous sagittal, axial, and coronal sections). These coordinates are in the same space as the smoothed pial surface reconstruction (C). The remaining electrodes are interpolated on a flat surface traversing the pial surface, referred to as the map plane (E). The two manually-localized electrodes on diagonal corners (blue) are on the cortical surface while the remaining electrodes (black) are either above or below the surface. Note that the entire lateral surface of the cortical hemisphere is shown here for illustrative purposes. The coordinates of the remaining electrodes are calculated using the inverse of the gnomonic projection to “fold” the grid onto the smoothed pial surface. Visualization is made on the subject-specific gyral surface (F).

Fig. 2

Localized electrodes in a patient with bilateral grids and multiple strips implanted. Accurate localization of all electrodes is possible even in cases with overlap in the coverage of the grids and strips.

Fig. 3

Localized depth electrodes visualized in synchronized coronal, saggital, and axial planes, in addition to in 3D with the subject-specific pial surface rendered partially transparent.

Fig. 4

Localized grids in the 3D MRI space with the craniotomy regions highlighted in yellow (row A), raw intra-operative photographs (row B), and the same photographs with the localized grid electrodes, projected onto the photographs using the camera projection, shown as black dots (row C) for patients 1 through 8. Note that 8×8 grids were cut into 2x8 and 6×8 grids in patients 5 and 7.

Fig. 5

Validation results. Box plots are shown of the Euclidean distances between localized grid electrodes, projected onto the intra-operative photographs using the camera projection, and the corresponding “ground truth” electrodes visible in the photographs. These measures are shown for all electrodes (n=271) across eight patients, and separately for electrodes implanted in each patient. Validation was performed using either all visible electrodes (A) or a subset of electrodes (n=10) at the center of the craniotomy (B) as control points in the camera projection. Red lines indicate median distance, boxes 50% of the distribution, and dotted lines maximum and minimum distances. Outliers are indicated by red crosses, and are defined by points outside q₃±1.5 (q₃–q₁), where q₁ and q₃ are the 25th and 75th percentiles, respectively.