In vivo mapping of macroscopic neuronal projections in the mouse hippocampus using high-resolution diffusion MRI

Highlights

• High-resolution diffusion MRI of the mouse hippocampus

• The organization of the neuronal networks can be visualized in vivo.

• Tractography and viral tracer results showed comparable spatial patterns.

• Tractography-based projection densities correlate with tracer-based results.

Abstract

Recent developments in diffusion magnetic resonance imaging (MRI) make it a promising tool for non-invasive mapping of the spatial organization of axonal and dendritic networks in gray matter regions of the brain. Given the complex cellular environments, in which these networks reside, evidence on the capability of diffusion MRI-based tractography to study these networks is still lacking. In this study, we used a localized diffusion MRI approach to acquire high spatial and angular resolution images of the live mouse hippocampus. The diffusion MRI and tractography results were compared with histology and the Allen mouse brain connectivity atlas using a multi-step image registration pipeline. The results demonstrated that in vivo diffusion MRI data at 0.1 mm isotropic resolution revealed the organization of axonal and dendritic networks in the hippocampus and the tractography results shared remarkable similarity with the viral tracer data in term of their spatial projection patterns. Quantitative analysis showed significant correlations between tractography- and tracer-based projection density measurements in the mouse hippocampus. These findings suggest that high-resolution diffusion MRI and tractography can reveal macroscopic neuronal projections in the mouse hippocampus and are important for future development of advanced tractography methods.

Introduction

Mapping structural connectivity of the brain is critical for understanding its structural and functional organization. An array of imaging techniques has been used to dissect structural connectivity from the synaptic level using electron micrograph (Helmstaedter et al., 2013, Takemura et al., 2013) up to the system level using magnetic resonance imaging (MRI) (Lazar, 2010, Van Essen et al., 2013). The last 15 years have witnessed rapid development in diffusion MRI-based tract reconstruction, or tractography (Mori and van Zijl, 2002, Tournier et al., 2007, Tuch et al., 2002, Wedeen et al., 2008). To date, it is the primary tool for non-invasive mapping of large white matter tracts in the brain and an important component of the Human Connectome Project (Toga et al., 2012, Van Essen et al., 2013).

With rapid advances in MRI techniques, high-resolution diffusion MRI data from the human brain are increasingly available (Gaggl et al., 2014, Setsompop et al., 2013, Uğurbil et al., 2013), and increasing efforts have been made towards tracing small neuronal connections deep in gray matter regions, e.g., small white matter tracts and axonal projections in the hippocampus and cortex (Dell'Acqua et al., 2013, Gomez et al., 2015, Kurniawan et al., 2014, Yassa et al., 2010, Zeineh et al., 2012), in order to map the organization of various anatomical units in these regions. Along these fine connections, tractography faces a vastly different landscape from those in large white matter tracts, as the proximity of small axonal bundles to neurons, astrocytes, and adjacent axonal bundles makes it challenging to determine their trajectories and microstructural properties. Even though the strengths and limitations of diffusion MRI tractography have been investigated in large white matter tracts with tracer-based histological data (Choe et al., 2012, Dauguet et al., 2007, Dyrby et al., 2007, Leergaard et al., 2010, Seehaus et al., 2013, Thomas et al., 2014), evidence on its capability to resolve small neuronal connections in gray matter regions remains scarce.

In this study, we examined the mouse hippocampus and the spatial organization of its axonal and dendritic networks using in vivo high-resolution diffusion MRI. We chose the hippocampus because it plays an important role in memory, spatial navigation, and emotion (Eichenbaum et al., 1996, Strange et al., 2014), and contains an intrinsic network between its subfields with distinct functions (van Strien et al., 2009). Modem viral tracing techniques, such as those used by the Allen Mouse Brain Connectivity Atlas (AMBCA) (Oh et al., 2014), have generated extensive tracer-based neuronal connectivity data in the mouse brain, including the hippocampus. With our recently developed localized high-resolution diffusion MRI technique (Wu et al., 2014) that can “zoom-in” a selected region in the live mouse brain to reach a high spatial resolution, and advanced image processing techniques to co-register histological and MRI data, we examined the capability of high-resolution diffusion MRI by direct comparison of diffusion MRI tractography data and co-registered anterograde viral tracing data from AMBCA.