Atlasing white matter pathways using diffusion-weighted imaging with a focus on human association tracts in the external and extreme capsulesdécembre 2015 Directeur(s) de thèse : Laurent PETIT Résumé de thèse
The importance of the brain’s connections for function is increasingly emphasized especially for higher cognitive functions, like language. But the lack of a proper technique to address the macroscopic connections in the human brain has prevented its advancement and much of our knowledge on the anatomy of fiber pathways has not changed since the 19th century. Controversy surrounding the anatomical course and connections of many fiber pathways persists. Using diffusion imaging tractography, the connectional anatomies of key fiber pathways are re-evaluated in a large group of healthy subjects. We use a novel approach to tract segmentation that aims to reproduce the method introduced by dissectionists, defining a tract as a bundle of fibers, with minimal a priori on their terminations. Focusing on the association pathways of the external and extreme capsules, namely the inferior fronto-occipital (IFOF) and uncinate fasciculi (UF), we review the literature on these tracts, provide detailed descriptions of their connectional anatomies and present new insights regarding their asymmetry and internal organization. We also highlight the necessity of using advanced tractography methods for anatomical studies. In a first study, we confirm more extensive projections within the cortex than previously thought in both tracts. In addition, we present new results regarding asymmetrical branches within the tracts, laying the groundwork for a second study. The second study focusses on resolving a centuries old debate regarding the UF. We clarify its elusive boundary with the IFOF and revealed distinct anatomical connectivity and asymmetries with respect to its subcomponents. These results shed new light on the IFOF and UF, which have been implicated in the language circuitry. The clear and detailed descriptions of the structural connections of these fiber pathways will be crucial for understanding their precise roles in mediating brain function.