Mesoscale diffusion magnetic resonance imaging of the ex vivo human hippocampus

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.

Ex vivo mesoscopic diffusion MRI correlates with seizure frequency in patients with uncontrolled mesial temporal lobe epilepsy

“…T 2 maps were computed for signal measurements. Using the same geometry as the T 2 -weighted images, DTI images were acquired with a 3D Pulsed Gradient Spin Echo (PGSE) sequence (TR = 1,100 ms, TE = 25 ms, diffusion duration δ = 4 ms, diffusion spacing Δ = 15 ms, diffusion time t D = 13.6 ms, 12 non-colinear diffusion directions, b-value = 4,000, 100 μm isotropic resolution; 63 hr scanning time) (Ly et al, 2020 (Jiang, van Zijl, Kim, Pearlson, & Mori, 2006). Scalar indices of diffusion, notably fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) were calculated (Basser, Mattiello, & LeBihan, 1994) to determine how epilepsy affected hippocampal tissue microstructure.…”

“…There is generally a good correspondence between hippocampal cell layers observed on high resolution MRI and histology (Adler et al, 2018;Coras, Milesi, et al, 2014;Ly et al, 2020;Modo et al, 2016;Schoene-Bake et al, 2014;Wieshmann et al, 1999). High spatial resolution imaging at the mesoscale (0.1-1 mm) is required to resolve hippocampal layers and sub-fields (Modo et al, 2016;Rondinoni, Magnun, Vallota da Silva, Heinsen, & Amaro Jr., in press).…”

“…To achieve a mesoscale resolution and dissect fiber tracts connecting different layers of the hippocampus, ex vivo imaging is currently required due to the long scanning time required to acquire high‐resolution 3‐dimensional images with sufficient diffusion‐encoding directions (Ly et al, 2020). We previously determined that, for instance, the granule cell layer (GCL) of the dentate gyrus (DG) is only 0.2 mm thick, hence requiring at least a resolultion of 0.1 mm to achieve coverage by 2 imaging voxels for tractography (Ly et al, 2020; Modo et al, 2016). Due to the laminar organization of the hippocampus along its axis, prior studies have achieved excellent images with very high in‐plane resolution (Coras, Milesi, et al, 2014; Shepherd, Ozarslan, Yachnis, King, & Blackband, 2007).…”

“…However, the disproportionaly low‐resolution slice thickness precludes a robust and valid tracing of fiber tracts connecting different hippocampal layers. An isotropic voxel dimension of 0.1 mm is required to probe intra‐hippocampal connectivity (Ly et al, 2020). A major advantage of ex vivo samples is that extended scanning times can be implemented to achieve sufficient signal‐to‐noise (SNR) at a mesoscale resolution (Beaujoin et al, 2018; Coras, Milesi, et al, 2014; Dell'Acqua, Bodi, Slater, Catani, & Modo, 2013; Ly et al, 2020; Modo et al, 2016).…”

“…An isotropic voxel dimension of 0.1 mm is required to probe intra‐hippocampal connectivity (Ly et al, 2020). A major advantage of ex vivo samples is that extended scanning times can be implemented to achieve sufficient signal‐to‐noise (SNR) at a mesoscale resolution (Beaujoin et al, 2018; Coras, Milesi, et al, 2014; Dell'Acqua, Bodi, Slater, Catani, & Modo, 2013; Ly et al, 2020; Modo et al, 2016). As there is a high correspondence between in vivo and ex vivo measurement of the hippocampal formation (Wisse et al, 2017), ex vivo investigations are advantageous to discover novel imaging targets, as these avoid motion artifacts, afford longer scanning times, permit repeat scanning of the same sample and allow histological comparisons for validation of imaging results (Rondinoni et al, in press).…”