The biophysics of crossing fibers

Abstract

Crossing fibers are increasingly acknowledged to be a significant problem, affecting both tractography and tensor-derived measures of anisotropy. Moreover, typical human scans have been shown to contain a significant proportion of voxels with crossing fibers. Thankfully, there is information in the diffusion-weighted signal that can be used to resolve these crossing fibers. This chapter examines the general biophysical considerations relevant to the crossing fiber problem and describes a number of algorithms that have recently been proposed to estimate fiber orientations. It introduces a classification of these algorithms based on their conceptual framework, along with a brief description of their distinctive features where relevant. DWI acquisition requirements are different for resolving crossing fibers than for diffusion tensor imaging, generally requiring higher b-values and a larger number of directions, depending on the algorithm used. Crossing fibers have a profound impact on scalar measures of white matter “integrity” (e.g.m anisotropy), and make it extremely difficult, if not impossible, to obtain scalar markers of white matter status that are truly independent of crossing fibers.