Inter-Subject Correlations between DTI Indices and Tissue Fractions in Human Brain

Introduction: Voxel-based analysis of diffusion tensor imaging (DTI) data has been widely used to access the alterations of white matter (WM) integrity at a group level [1]. However, the histological mechanism underlying the observed DTI index (FA/MD) changes is still poorly understood, posing a major difficulty in interpreting DTI findings [2]. For example, it remains unclear to what extent the inter-subject variations of FA measurements should be explained by the intra-voxel WM tissue fraction rather than the underlying WM integrity changes. A previous preliminary study has shown correlations of DTI indices with the tissue fractions estimated from a set of inversion-recovery (IR) procedures, although the results were limited to a 5-slice partial brain in a single subject [3]. Here we promote this investigation to a group-level whole-brain analysis by using a voxel-based DTI approach and a fast fractional signal mapping from inversion recovery (FRASIER) [4]. The major goals of this study are (I) to test whether significant correlations exist in WM between DTI and tissue fractions across the subjects, and (II) to identify the regions where the correlations between DTI and tissue fractions most likely occur. We expect the outcomes to provide useful information on whether and where we should take into account the effects of tissue fraction variations in a DTI group analysis.