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.

Registering imaged ECoG electrodes to human cortex: A geometry-based technique

“…In a method proposed by Yang and colleagues (2012), electrodes were registered along the rays originating from a fixed central point, equivalent to modeling the deformation as compressing towards the chosen fixed point. While this technique was efficient in localizing electrodes in a lateral grid, this modeling approach is ineffective for strips of electrodes and in areas of high curvature, including the temporal and occipital poles among others.…”

“…The floating spatial configuration of all of the implanted electrodes can be determined with minimum radiation exposure from orthogonal skull films (Grzeszczuk et al 1992), but this does not correct for post-surgical brain-shift. Alternatively, to determine the location and configuration of electrodes, some institutions obtain post-implant MRIs to localize the electrodes on the cortical surface (Morris et al 2004; Yang et al 2012). However, the metallic composition of the electrodes causes significant spatial distortion of the MR images, and degradation of the images in the region of the electrodes.…”

Slow Cortical Dynamics and the Accumulation of Information over Long Timescales

“…Electrodes were localized on the individual cortical surfaces using a combination of manual identification in the T1images, intraoperative photographs, and a custom MATLAB tool based on the known physical dimensions of the grids and strips (Yang et al, 2012). Subsequently, the individual-subject T1 images were nonlinearly registered to an MNI template using the DARTEL algorithm via SPM (Ashburner, 2007), and the same transformation was applied to map individual electrode coordinates into MNI space.…”

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

“…The intracranial electrodes were localized and reconstructed from pre-and post-op MRIs using previously developed procedures at NYUMC (Yang et al, 2012), and their coordinates were registered into post-op images to read off the electric potentials in the brain predicted by the model. Specifically, the voltages on the FEM nodes at the brain that are closest to the electrodes were read out, and are ready to be compared against the actual measurements ('Validation criteria').…”