Introduction: The hippocampus is commonly implicated in drug-resistant epilepsy with characteristic involvement of specific sectors of the cornu ammonis (CA) . It can be divided longitudinally into the head, body, and tail; and unfolded along the medial-to-lateral axis into specific subfields: the subiculum, CA sectors, and dentate gyrus. The increased signal at ultra-high magnetic field strengths (= 7 Tesla; 7T) allows these substructures to be visualized in continuity at submillimeter resolution. We propose to use 7T magnetic resonance imaging (MRI) to identify intracerebral electrode locations within hippocampal substructures.
Methods: 53 patients with drug-resistant epilepsy were identified undergoing first-time electrode implantation. Post-operative computed tomography scans were registered with the pre-operative MRI permitting localization of the electrodes in MRI space. Electrode contacts within the hippocampus were semi-automatically labeled . Each patient MRI scan was subsequently aligned with a recently developed 7T template space (0.6 mm isotropic voxel size) . Transformation of electrode locations into our recently developed "unfolded" coordinate space  permitted labelling of hippocampal substructures (Figure 1).
Results: Bilateral electrode contacts were superimposed onto the unfolded coordinate space (Figure 2). Out of a total of 178 implanted hippocampal electrodes (88 left; 49.4%), 25 (14.0%) were predominantly in the subiculum, 85 (47.8%) were in CA1, 23 (12.9%) were in CA2, 18 (10.1%) were in CA3/CA4, and 27 (15.2%) were in the dentate gyrus. Along the longitudinal axis of the hippocampus, electrodes were most commonly implanted in the body (92; 51.7%) followed by the head (86; 48.3%).
Conclusions: Here, we demonstrate the use of 7T MRI to assist with identifying the location of electrode implantations within hippocampal substructures. While limitations with existing electrode technology may prevent our ability to observe electrographic differences based on location, our findings suggest that at least from an imaging perspective, specific targeting of hippocampal substructures is feasible using ultra-high field MRI.
Patient Care: Ultra-high field MRI enables improved visualization of brain structures including the hippocampus. This technology may facilitate improved diagnosis and more accurate image-based targeting.
Learning Objectives: By the conclusion of this session, participants should be able to:
1. Identify the major substructures of the hippocampus from MRI.
2. Discuss how ultra-high field MRI improves visualization of hippocampal substructures.
3. Consider the utility of the proposed coordinate space for labelling electrode locations within hippocampal substructures.
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