The white matter fiber tract deforms most in the perpendicular direction duringin vivovolunteer impacts

Abstract

White matter (WM) tract-related strains are increasingly used to quantify brain mechanical responses, but their dynamics in live human brains during in vivo impact conditions remain largely unknown. Existing research primarily looked into the normal strain along the WM fiber tracts (i.e., tract-oriented normal strain), but it is rarely the case in reality that the fiber tract only endures tract-oriented normal strain. In this study, we aim to extend the measurement of WM deformation by quantifying the normal strain perpendicular to the fiber tract (i.e., tract-perpendicular normal strain) and the shear strain along and perpendicular to the fiber tract (i.e., tract-oriented shear strain and tract-perpendicular shear strain, respectively). To achieve this, we combine the three-dimensional strain tensor from the tagged magnetic resonance imaging (tMRI) with the diffuse tensor imaging (DTI) from an open-access dataset, including 44 volunteer impacts under neck rotations (N = 30, peak angular acceleration: 228.6 (8.3) rad/s2) and neck extensions (N = 14, peak angular acceleration: 185.5 (59.2) rad/s2). The strain tensor is rotated to the coordinate system with one axis aligned with DTI-revealed fiber orientation and then four tract-related strain measures are calculated. The results show that tract-perpendicular normal strain peaks are the largest among the four strain types and tract-oriented normal strains are the lowest. The mean peak values (standard deviation) for tract-oriented normal strain, tract-perpendicular normal strain, tract-oriented shear strain, and tract-perpendicular shear strain are 0.020 (0.005), 0.028 (0.007), 0.024 (0.006) and 0.023 (0.006) under the neck rotation, and 0.013 (0.002), 0.019 (0.004), 0.016 (0.003), and 0.017 (0.003) under the neck extension, respectively. The distribution of tract-related strains is affected by the head loading mode. Our study presents a comprehensive in vivo strain quantification towards a multifaceted understanding of WM dynamics. For the first time, we find the WM fiber tract deforms most in the perpendicular direction, illuminating new fundamental of brain mechanics. The reported strain results can be used to evaluate the biofidelity of computational head models, especially those intended to predict brain strains under non-injurious conditions.