A Diffeomorphic Vector Field Approach to Analyze the Thickness of the Hippocampus from 7T MRI
Résumé
Ultra-high field (≥ 7-Tesla) MR imaging of the hippocampus enhances the visualization of its internal substructures and of localized changes observed in various neurological disorders. Among these substructures, the cornu Ammonis and subiculum form a contiguous folded ribbon of gray matter. Here, we propose a method to analyze local thickness measurements of this ribbon. Thickness measurement is based upon the estimation of a diffeomorphic vector field that traverses the ribbon and belongs to a reproducing kernel Hilbert space. Its smoothness is controlled via regularizing kernels. The approach is designed to handle specificities of the hippocampus and corresponding image acquisitions: highly convoluted surface, non-closed ribbon, incompletely defined inner/outer boundaries, anisotropic acquisitions. We furthermore propose to conduct group comparisons using a population template built from the central surfaces of individual subjects. We first assessed the robustness of our approach to anisotropy, as well as to inter-rater variability, on a post-mortem scan and on in vivo acquisitions respectively. We then conducted a group study on a dataset of in vivo MRI from temporal lobe epilepsy (TLE) patients and healthy controls. From this, local thinning patterns were unveiled in patients, predominantly ipsilaterally to the seizure focus, and stronger cornu Ammonis thinning was shown contralaterally to the focus in left TLE compared to right TLE. Both results agree with the existing epilepsy literature, showing the ability of our method to detect local alterations of the hippocampus in patient populations. The code is available at https://github.com/aramis-lab/hiplay7-thickness and the postmortem specimen is available at https://doi.org/10.5281/zenodo.3533264.
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