Discovering white matter structure beyond fractional anisotropy maps

Introduction Anisotropy in the water diffusion MRI (dMRI) signal can be used to determine local white matter (WM) direction [1] and segment major pathways using tractography [e.g. 2, 3]. It has also been linked, in the form of fractional anisotropy (FA), to WM coherence and therefore, indirectly, to WM integrity [4]. It has been shown, however, that in partial volume (PV) voxels FA is underestimated compared with voxels containing only WM [5]. This can lead to an inaccurate estimation of either tract volume, i.e. discarding voxels containing WM, or coherence, i.e. including false FA values in the tract average. We address this problem by fitting a fully physically plausible, 2-compartment model to the dMRI data that correctly describes PV effects using local diffusion information.