Histological validation of per-bundle water diffusion metrics within a region of fiber crossing following axonal degeneration - Synthetic data

Analysis of dMRI of the optic chiasm using CSD revealed unexpectedly high AFD of the intact bundle in the experimental animals. To evaluate whether this finding is an artifact produced by CSD, we used analytic synthetic data. Using the multi-tensor model, two sets of synthetic dMRI data were created consisting of 100 voxels, each with two tensors crossing at right angles. For each voxel, one tensor was modelled as an “intact” fiber bundle, following the DTI metrics observed in our experimental data (λ∥=0.64 and λ⟂=0.12 ×10-3 mm²/s), while the other tensor was modelled as an “injured” fiber bundle (λ∥=0.42 and λ⟂=0.22 ×10-3 mm²/s). Volume fractions were set as f=0.5 for both tensors. The multi-shell scheme to generate the synthetic signals consisted of 20 b=0 s/mm², and 160 DWI volumes, consisting of 80 diffusion gradient orientations, each with two distinct b values, namely b=1000 and 3000 s/mm². Signal-to-noise ratio was set at 20. Diffusion-weighted parameters are shown in the corresponding *protocol.txt files, organized as in the MRtrix gradient scheme (https://mrtrix.readthedocs.io/en/latest/concepts/dw_scheme.html), namely one row per volume, each containing the vector [x y z b], where x, y, and z, correspond to the components of the diffusion gradient direction, and b is the b value in s/mm^2. Refers to: Rojas-Vite, Gilberto, Coronado-Leija, Ricardo, Narvaez-Delgado, Omar, Ramirez-Manzanares, Alonso, Marroquin, Jose-Luis, Noguez-Imm, Ramses, Aranda, Marcos L, Scherrer, Benoit, Larriva-Sahd, Jorge, Concha, Luis: Histological validation of per-bundle water diffusion metrics within a region of fiber crossing following axonal degeneration , Available at bioRxiv, doi.org/10.1101/571539, 2019.