Surface-based tracking for short association fibre tractography

Short association fibres (SAF) of the human brain are estimated to represent over a half of the total white matter volume, and their involvement has been implicated in a range of neurological and psychiatric conditions. This population of fibres, however, remains relatively understudied in the neuroimaging literature. Some of the challenges pertinent to the mapping of SAF include their variable anatomical course and close proximity to the cortical mantle, leading to partial volume effects and exacerbating the influence of the gyral bias. This work considers the choice of scanner, acquisition, voxel size, seeding strategy and filtering techniques to propose a whole-brain, surface-based tractography approach with the aim of providing a method for investigating SAF ≤30-40 mm. The framework is designed to: (1) ensure a greater cortical surface coverage through spreading streamline seeds more uniformly; (2) introduce precise filtering mechanics which are particularly important when dealing with small, morphologically diverse structures; and (3) allow the use of surface-based registration for dataset comparisons which can be superior to volume-based registration in the cortical vicinity. The indexation of surface vertices at each streamline end enables direct interfacing between streamlines and the cortical surface without dependence on the voxel grid. SAF tractograms generated using recent test- retest data from our institution are carefully characterised and measures of consistency using streamline-, voxel- and surface-wise comparisons calculated to inform researchers and serve as a benchmark for future methodological developments.