Mapping the macrostructure and microstructure of the in vivo human hippocampus using diffusion MRI

Area of Science:

• Neuroimaging
• Computational Neuroscience
• Human Anatomy

Background:

• The hippocampus's mesoscopic subfields have unique microstructures crucial for function.
• Current neuroimaging struggles to fully characterize hippocampal microstructure.
• Understanding subfield variations is key for cognitive research and disease diagnosis.

Purpose of the Study:

• To characterize hippocampal microstructure using in vivo diffusion MRI and a novel surface-based approach.
• To identify distinct microstructural distributions of neurite density, dispersion, myelin content, and diffusion metrics.
• To correlate macrostructural and microstructural features for a comprehensive understanding of hippocampal anatomy.

Main Methods:

• Utilized diffusion magnetic resonance imaging (dMRI) with a novel surface-based analysis.
• Applied the neurite orientation dispersion and density imaging (NODDI) model for grey matter.
• Employed logistic regression and non-negative matrix factorization (OPNNMF) for data analysis.

Main Results:

• Distinct distributions of neurite density, dispersion, and myelin content (T1w/T2w ratio) were found across hippocampal subfields.
• Neurite dispersion was highest in CA1 and subiculum; density and myelin were highest in subiculum and CA3.
• Macrostructural measures aligned with ex vivo anatomy, and combined approaches revealed co-varying micro- and macrostructural regions.

Conclusions:

• In vivo dMRI combined with computational methods can effectively capture hippocampal microstructure.
• Identified microstructural variations correlate with known cyto- and myeloarchitectural differences.
• These findings provide valuable features for understanding cognition and diagnosing neurological diseases.