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Updated: Jul 29, 2026
High-resolution In Vivo Manual Segmentation Protocol for Human Hippocampal Subfields Using 3T Magnetic Resonance Imaging
Published on: November 10, 2015
Mesoscale diffusion magnetic resonance imaging of the ex vivo human hippocampus
Maria Ly1, Lesley Foley2, Ashwinee Manivannan3
1Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
Abstract:
Mesoscale diffusion magnetic resonance imaging (MRI) endeavors to bridge the gap between macroscopic white matter tractography and microscopic studies investigating the cytoarchitecture of human brain tissue. To ensure a robust measurement of diffusion at the mesoscale, acquisition parameters were arrayed to investigate their effects on scalar indices (mean, radial, axial diffusivity, and fractional anisotropy) and streamlines (i.e., graphical representation of axonal tracts) in hippocampal layers. A mesoscale resolution afforded segementation of the pyramidal cell layer (CA1-4), the dentate gyrus, as well as stratum moleculare, radiatum, and oriens. Using ex vivo samples, surgically excised from patients with intractable epilepsy (n = 3), we found that shorter diffusion times (23.7 ms) with a b-value of 4,000 s/mm2 were advantageous at the mesoscale, providing a compromise between mean diffusivity and fractional anisotropy measurements. Spatial resolution and sample orientation exerted a major effect on tractography, whereas the number of diffusion gradient encoding directions minimally affected scalar indices and streamline density. A sample temperature of 15°C provided a compromise between increasing signal-to-noise ratio and increasing the diffusion properties of the tissue. Optimization of the acquisition afforded a system's view of intra- and extra-hippocampal connections. Tractography reflected histological boundaries of hippocampal layers. Individual layer connectivity was visualized, as well as streamlines emanating from individual sub-fields. The perforant path, subiculum and angular bundle demonstrated extra-hippocampal connections. Histology of the samples confirmed individual cell layers corresponding to ROIs defined on MR images. We anticipate that this ex vivo mesoscale imaging will yield novel insights into human hippocampal connectivity.
Insights
Mesoscale diffusion MRI reveals detailed human hippocampal connectivity by optimizing acquisition parameters. This technique bridges macroscale tractography and microscale cytoarchitecture, offering new insights into brain structure.
Area of Science:
- Neuroimaging
- Neuroanatomy
- Biophysics
Background:
- Mesoscale diffusion MRI aims to link macroscopic white matter tractography with microscopic brain tissue cytoarchitecture.
- Understanding hippocampal connectivity is crucial for neuroscience research.
Purpose of the Study:
- To optimize mesoscale diffusion MRI acquisition parameters for robust measurements of scalar indices and streamlines in hippocampal layers.
- To investigate the effects of spatial resolution, sample orientation, diffusion gradient directions, and temperature on diffusion MRI data.
- To visualize intra- and extra-hippocampal connections at the mesoscale.
Main Methods:
- Acquisition parameters (diffusion time, b-value, spatial resolution, sample orientation, gradient directions, temperature) were systematically varied.
- Ex vivo human hippocampal samples from epilepsy patients (n=3) were used.
- Diffusion MRI data were analyzed for scalar indices (mean, radial, axial diffusivity, fractional anisotropy) and streamline tractography.
- Histology was performed to confirm MR-defined regions of interest and cell layers.
Main Results:
- Shorter diffusion times (23.7 ms) and a b-value of 4,000 s/mm² offered a compromise for mesoscale measurements.
- Spatial resolution and sample orientation significantly impacted tractography; gradient directions had minimal effect on scalar indices and streamline density.
- A sample temperature of 15°C balanced signal-to-noise ratio and diffusion properties.
- Tractography accurately reflected histological boundaries of hippocampal layers, visualizing intra- and extra-hippocampal connections like the perforant path.
Conclusions:
- Optimized mesoscale diffusion MRI provides a systems-level view of hippocampal connectivity.
- This technique successfully visualizes individual layer connectivity and extra-hippocampal pathways.
- Ex vivo mesoscale imaging holds promise for novel insights into human hippocampal circuitry and its relation to neurological conditions.
