Structural Layers of Ex Vivo Rat Hippocampus at 7T MRI

Jeanine Manuella Kamsu,Franck Lamberton,Patrick Courtheoux,Maelle Coquemont,Jean-Marc Constans,Stephane Besnard,Pierre Denise,Bruno Philoxene,Xi-Nian Zuo

doi:10.1371/journal.pone.0076135

Abstract

Magnetic resonance imaging (MRI) applied to the hippocampus is challenging in studies of the neurophysiology of memory and the physiopathology of numerous diseases such as epilepsy, Alzheimer’s disease, ischemia, and depression. The hippocampus is a well-delineated cerebral structure with a multi-layered organization. Imaging of hippocampus layers is limited to a few studies and requires high magnetic field and gradient strength. We performed one conventional MRI sequence on a 7T MRI in order to visualize and to delineate the multi-layered hippocampal structure ex vivo in rat brains. We optimized a volumic three-dimensional T2 Rapid Acquisition Relaxation Enhancement (RARE) sequence and quantified the volume of the hippocampus and one of its thinnest layers, the stratum granulare of the dentate gyrus. Additionally, we tested passive staining by gadolinium with the aim of decreasing the acquisition time and increasing image contrast. Using appropriated settings, six discrete layers were differentiated within the hippocampus in rats. In the hippocampus proper or Ammon’s Horn (AH): the stratum oriens, the stratum pyramidale of, the stratum radiatum, and the stratum lacunosum moleculare of the CA1 were differentiated. In the dentate gyrus: the stratum moleculare and the stratum granulare layer were seen distinctly. Passive staining of one brain with gadolinium decreased the acquisition time by four and improved the differentiation between the layers. A conventional sequence optimized on a 7T MRI with a standard receiver surface coil will allow us to study structural layers (signal and volume) of hippocampus in various rat models of neuropathology (anxiety, epilepsia, neurodegeneration).

Highlights

The hippocampal formation is a complex cerebral structure involved in episodic memory and spatial memory processes [1,2]
Magnetic Resonance Imaging (MRI) applied to the hippocampus is challenging in studies on the neurophysiology of memory and the physiopathology of numerous diseases such as epilepsy [4,5], ischemia [6], Alzheimer’s disease [7], and the types of damage observed by MRI remain nonspecific and poorly delineated, in rodent brains
MRI parameters Our chosen 3D Rapid Acquisition Relaxation Enhancement (RARE) sequence parameters offered the best compromise between spatial resolution, signal-to-noise ratio (SNR) and CNR, and acquisition time: T2GR weighted sequence (TR)/Effective TE = 1700ms/63 ms with four averages, a RARE Factor of 6, a total scan time of 12 h 30 min, a field of view at 4 x 1.6 x 1.6 cm, and a matrix of 512 x 200 x 200 with a spatial resolution of 80 x 80 x 80 μm3 (Table 1)

Summary

Introduction

The hippocampal formation is a complex cerebral structure involved in episodic memory and spatial memory processes [1,2]. Given that specific regions or layers of the hippocampus are affected early in the development of some pathologies, such as the CA1 region with Alzheimer’s disease [7], or the granular layer of the dentate gyrus with depression [8], it has become imperative to obtain more precise images of the hippocampal structure in order to improve our understanding and the early detection of microstructural changes which could occur in the hippocampus [9]. This can be achieved through images with high spatial resolution which allow us to distinguish hippocampal structural layers

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