Section: New Results

Towards microstructural based tractography

AxTract: Microstructure-driven tractography based on the Ensemble Average Propagator

Participants : Gabriel Girard [Athena, Inria Sophia-A-M & SCIL Lab., Sherbrooke University] , Rutger Fick, Maxime Descoteaux [SCIL Lab., Sherbrooke University] , Demian Wassermann, Rachid Deriche.

In this work, we propose a novel method to simultaneously trace brain white matter (WM) fascicles and estimate WM microstructure characteristics. Recent advancements in diffusion-weighted imaging (DWI) allow multi-shell acquisitions with b-values of up to 10,000 s/mm2 in human subjects, enabling the measurement of the ensemble average propagator (EAP) at distances as short as 10 micro-meters. Coupled with continuous models of the full 3D DWI signal and the EAP such as Mean Apparent Propagator (MAP) MRI, these acquisition schemes provide unparalleled means to probe the WM tissue in vivo. Presently, there are two complementary limitations in tractography and microstructure measurement techniques. Tractography techniques are based on models of the DWI signal geometry without taking specific hypotheses of the WM structure. This hinders the tracing of fascicles through certain WM areas with complex organization such as branching, crossing, merging, and bottlenecks that are indistinguishable using the orientation-only part of the DWI signal. Microstructure measuring techniques, such as AxCaliber, require the direction of the axons within the probed tissue before the acquisition as well as the tissue to be highly organized. Our contributions are twofold. First, we extend the theoretical DWI models proposed by Callaghan et al. to characterize the distribution of axonal calibers within the probed tissue taking advantage of the MAP-MRI model. Second, we develop a simultaneous tractography and axonal caliber distribution algorithm based on the hypothesis that axonal caliber distribution varies smoothly along a WM fascicle. To validate our model we test it on in-silico phantoms and on the HCP dataset

This work has been published in  [23]

Studying white matter tractography reproducibility through connectivity matrices

Participants : Gabriel Girard [Athena, Inria Sophia-A-M & SCIL Lab., Sherbrooke University] , Kevin Whittingstall [SCIL Lab., Sherbrooke University] , Maxime Descoteaux [SCIL Lab., Sherbrooke University] , Rachid Deriche.

Diffusion-weighted imaging is often used as a starting point for in vivo white matter (WM) connectivity to reconstruct potential WM pathways between brain areas. In this study, we investigate the reproducibility of the connectivity matrix, resulting from different tractography parameters. We vary the number of streamlines used to construct the matrix in cortical to cortical connectivity and analyze its effects. We also compare the effect of probabilistic and deterministic local streamline tractography algorithms, seeding both from the WM and from WM-grey matter interface.

This work has been published in  [31]

Structural connectivity reproducibility through multiple acquisitions

Participants : Gabriel Girard [Athena, Inria Sophia-A-M & SCIL Lab., Sherbrooke University] , Kevin Whittingstall [SCIL Lab., Sherbrooke University] , Maxime Descoteaux [SCIL Lab., Sherbrooke University] , Rachid Deriche.

dMRI is often used to reconstruct white matter pathways between brain areas for in vivo brain connectivity. In this study, we investigate the reproducibility and the specificity of connectivity matrices in cortico-cortical connectivity using probabilistic and deterministic streamline tractography, seeding from both the white matter and the white matter-grey matter interface.

This work has been published in  [30]