Detection of Hippocampal Subfield Asymmetry at 7T With Automated Segmentation in Epilepsy Patients With Normal Clinical Strength MRIs

Abstract

While the etiology of hippocampal sclerosis (HS) in epilepsy patients remains unknown, distinct phenotypes of hippocampal subfield atrophy have been associated with different clinical presentations and surgical outcomes. The advent of novel techniques including ultra-high field 7T magnetic resonance imaging (MRI) and automated subfield volumetry have further enabled detection of hippocampal pathology in patients with epilepsy, however, studies combining both 7T MRI and automated segmentation in epilepsy patients with normal-appearing clinical MRI are limited. In this study, we present a novel application of the automated segmentation of hippocampal subfields (ASHS) software to determine subfield volumes of the CA1, CA2/3, CA4/DG, and the subiculum using ultra high-field 7T MRI scans, including T1-weighted MP2RAGE and T2-TSE sequences, in 27 patients with either mesial temporal lobe epilepsy (mTLE) or neocortical epilepsy (NE) compared to age and gender matched healthy controls. We found that 7T improved visualization of structural abnormalities not otherwise seen on clinical strength MRIs in patients with unilateral mTLE. Additionally, our automated segmentation algorithm was able to detect structural differences in volume and asymmetry across hippocampal subfields in unilateral mTLE patients compared to controls. Specifically, amongst unilateral mTLE patients with longer disease durations, volume loss was observed in the ipsilateral CA1 and CA2/3 subfields and contralateral CA1. There were no differences in subfield volumes in patients with NE compared to controls. We report the first application of 7T with automated segmentation to characterize the relationship between disease duration burden and asymmetry across specific hippocampal subfields in this population. Disease duration was found to have a statistically significant positive relationship with subfield asymmetry within the unilateral mTLE cohort. These findings highlight the ability of 7T MRI and automated segmentation to provide novel qualitative and quantitative information in epilepsy patients who are otherwise MRI-negative at clinical field strengths.

Morphometric and microstructural characteristics of hippocampal subfields in mesial temporal lobe epilepsy and their correlates with mnemonic discrimination

“…Of these, the DG and CA1 are the structures with the greatest reduction with respect to their counterparts in the contralesional hippocampus. Other structural MRI studies have also identified these two substructures as the most affected in this disease ( 53 , 54 ), also when compared with patients with non-epileptic controls and non-HS MTLE ( 53 , 55 , 56 ). This is also consistent with data from the ILAE histopathological classification study of MTLE-HS.…”

Mapping hippocampal and thalamic atrophy in epilepsy: A 7‐T magnetic resonance imaging study

“…Prior work at 3 T has demonstrated structural alterations in the hippocampus and thalamus of patients with epilepsy. 18,[36][37][38][39] Independently, hippocampal [10][11][12][13] and thalamic 20 alterations have also been reported at 7 T. We expand on this literature by leveraging 7-T structural neuroimaging of both the hippocampus and the thalamus. We found that all ipsilateral hippocampal subfields (CA1, CA2, CA3, DG) as well as some ipsilateral (CM, MD) and contralateral (AV, Pul, MD) thalamic nuclei were significantly atrophied in our epilepsy cohort relative to controls.…”

“…[5][6][7] Volumetric changes, quantified through 3-T structural neuroimaging, have been demonstrated between controls and patients with epilepsy across these subfields. 8,9 More recently, ultra-high-field 7-T scanners with hippocampus-specific imaging protocols have been used to provide greater contrast in these subfields, allowing for the interrogation of more subtle volumetric, [10][11][12][13] functional, 14 and even neurotransmitter expression 15 changes.…”

Stress-related reduction of hippocampal subfield volumes in major depressive disorder: A 7-Tesla study

“…Use of ultra-high field MRI scanners, such as those operating at 7-Tesla (7T), can allow for increased visualization of boundaries and delineation of subfields than conventional clinical strength scanners due to superior contrast and resolution at 7T ( 31 ). These high-field advantages can reveal structural subtleties which are below the threshold of detectability at clinical field strengths ( 32 ). There is a limited number of 7T studies examining hippocampal subfields in MDD and their segmentation methods include either manual tracings ( 33 ) or automated segmentation with FreeSurfer version 6.0 ( 29 , 34 , 35 ).…”