High-resolution depth electrode localization and imaging in patients with pharmacologically intractable epilepsy

Abstract

Localization and targeting of depth electrodes in specific regions of the human brain is critical for accurate clinical diagnoses and treatment as well as for neuroscientific electrophysiological research. By using high-resolution magnetic resonance imaging combined with 2D computational unfolding, the authors present a method that improves electrode localization in the medial temporal lobe. This method permits visualization of electrode placements in subregions of the hippocampus and parahippocampal gyrus, allowing for greater specificity in relating electrophysiological and anatomical features in the human medial temporal lobe. Such methods may be extended to therapeutic procedures targeting specific neuronal circuitry in subfields of structures deep in the human brain.

Fig. 1

Neuroimaging studies and maps used in localization of microelectrodes to hippocampal subregions. Row 1: High-resolution coronal–oblique MR images (perpendicular to the long axis of the hippocampus) obtained in 3 different patients. “Left” and “right” indicate the cerebral hemispheres. Red target location (arrows) shows location defined by the microtip identified on postimplant CT scans (see Row 2) coregistered to the preimplant MR images. Row 2: Postimplant axial CT scans coregistered to the high-resolution MR images shown in Row 1. In most cases, microelectrodes were clearly visible as artifacts on the CT scan, as indicated with red targets. Microwires occasionally bifurcated within a structure, leading to placement in 2 different hippocampal subfields (Case 4, Row 2). Row 3: Subregion definitions in 3D space based on back-projecting regions from the 2D maps shown in Row 4. Red area, CA2/CA3/dentate gyrus (CA23DG); yellow area, CA1; dark blue area, subiculum (Sub); green area, PRC; white area, ERC; and light blue area, fusiform cortex. The PHC is shown in white when slices are posterior to the ERC-PRC-PHC boundary (for example, Case 5 left and Case 3 left). Row 4: Two-dimensional hippocampal flat maps of each MR imaging sequence shown partially in Row 1. Top of map shows the most anterior aspect of the hippocampus, and bottom of map shows the most posterior end. Electrode locations in flat space, defined by coregistration from 3D neuroimages shown in Rows 1 and 2, are indicated with red targets. Black lines indicate subregion boundaries, and dotted black line indicates location of medial fusiform vertex. Electrode names indicate putative placement. Thus, “RAH” and “LAH” indicate putative placement in the right and left anterior hippocampus, respectively, and “LMH” indicates putative placement in the left medial hippocampus. The label “RAH₁” indicates a bifurcated electrode placed in the superior portion of the right anterior hippocampus, and “RAH₂” denotes a bifurcated electrode placed in the inferior portion of the right anterior hippocampus. Also visible in some cases are electrodes placed in the right and left ERC (designated “REC” and “LEC”) and in the right and left PHC (labeled “RPG” and “LPG”). CADG = CA/dentate gyrus.