7T MR Diffusion-tensor Imaging and Tractography of a 1-week Postmortem Fixed Human Brainstem-cerebellum: Novel Imaging Methodology Producing Extraordinary Images

Introduction: Diffusion-tensor imaging (DTI) is an extension of diffusion-weighted MR imaging (DWI) that, when applied to the brain, can estimate the pattern, orientation, and directionality of white matter tracts. Although classically used on in-vivo tissue (animal models or human subjects), DTI can be expanded to ex vivo (post-mortem/cadaveric) tissue due to preservation of fractional anisotropy in fixed brains. Ex vivo tissue is optimal for DTI and white matter tractography because it allows longer scan times, enabling high spatial resolution for reconstructing small and complex tracts. High resolution tractography has not been produced using standard post-mortem cadaveric brain tissue until now.

Methods: One formalin-fixed whole brainstem-cerebellum specimen (85 y/o M) was imaged. Time from death to fixation was 7 days. Before imaging, specimen was re-hydrated in normal saline solution for 7d and later degassed for 12hr in an inert, non-proton, oxidatively stable solution with continuous vibration. 7T MRI was used for anatomic and DTI scanning, acquisition time was 3hr and 49hr, respectively. Diffusion maps were generated with DIPY. Whole brainstem tractography displayed using DSI studio.

Results: Clear tractography was successful using a 7d postmortem brainstem-cerebellum specimen. White matter fibers traveling from mesencephalon to medulla (corticospinal, corticobulbar, medial longitudinal fasciculus tracts), horizontally traversing the pons (superior and inferior transpontine fibers), fasciculi of superior/middle/inferior cerebellar peduncles, and decussation fibers are visible. K-means clustering shows anatomically accurate fiber distribution with clear left-right and superior-inferior preference clusters generated. Topology informed approach reveals a high number of anatomically plausible tracts when adding a seed region to top and region of interest at bottom.

Conclusion : Highly detailed and precise white matter tractography were created from standard post-mortem specimens, even when fixed 1-week after death. This methodology and results are ground-breaking for DTI and tractography research because standard cadaveric specimens obtained from donor organizations can be reliably used.