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The hippocampus as never seen before

Researchers at the Institut Frédéric-Joliot have provided the mapping of the hippocampus at an unequalled resolution using high field MRI. In addition to its substructures, this has revealed its internal connections and its cellular composition.
Published on 21 March 2018

The hippocampus is a structure nestled deep within the brain, known for its leading role in memory and learning mechanisms. It is also the first area to undergo degradation in Alzheimer's disease. While a number of studies have established human hippocampus atlases, few studies using MRI have been conducted so far in order to characterize its organization at the mesoscopic scale (e.g. equal to about one hundred micrometers of resolution). As part of the HBP[1] project, researchers at the Institut Frédéric-Joliot, in collaboration with the Jülich Research Center in Germany, observed a post-mortem human hippocampus with NeuroSpin's 11.7T preclinical MRI. This provided images of unequalled resolution, based on new approaches for characterizing its cellular composition.

"We benefited from a very high field MRI which, combined with very high gradients[2] (780 mT/m), allowed us to obtain anatomical images with a 150-µm resolution and a resolution of 300 µm using diffusion MRI microscopy, 30 times more accurate than the imagers used in hospitals," explain Justine Beaujoin and Cyril Poupon, researchers at the Institut Frédéric-Joliot and the principal authors of this study. In addition to the segmentation of substructures in the human hippocampus, the goal of this ultra-modern technology is to reveal the neural interconnections within the hippocampus and to determine its cellular characteristics, such as whether this region is rich in neurons or dendrites. "Our images enable obtaining a very precise segmentation of the internal regions of the hippocampus and the ability to measure with great reliability the connections which link them," add the two researchers. "They also make it possible to characterize the cellular composition of these regions."

This study performed on a post-mortem anatomical sample demonstrates the full potential of the future 11.7T clinical MRI that is being installed at NeuroSpin. "High-definition visualizing of the hippocampus, the first area of the brain to be affected in patients with Alzheimer's disease, will undoubtedly lead to major advances in diagnosis and treatment development," states Cyril Poupon. And to conclude: "In the short term, the study of the hippocampus is ongoing in Germany, where researchers from Jülich are using polarized optical imaging to map the connections of the hippocampus with an in-plane resolution reaching 1.3 µm so as to compare the results obtained from the two imaging approaches. This is a monumental task. In addition, with these preliminary results, we have already started similar work in collaboration with Christophe Destrieux at the Université de Tours, this time using our 11.7T MRI to scan an entire brain at a 100-µm resolution for anatomy and a 200-µm resolution for diffusion MRI, which will require nearly one year for acquisition. These data will be unique in the world and will undoubtedly allow new discoveries to be made about the organization of the human brain at the cellular level."

[1] The Human Brain Project is an international collaboration dedicated to the modelling of the human brain in which the CEA's Institut Frédéric-Joliot participates. Jean-François Mangin, Director of the UNATI, is the coordinator for the institute's teams.

[2] Gradients are spatial field variations that make it possible to encode the position of measurements, as well as encode the movement of water molecules in tissues.

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