Alignment of cardiac-induced brain tissue strain with global boundary conditions and local microstructure: potential effects of anisotropy.

Abstract

Strain tensor imaging allows for the construction of the full 3D strain tensor in the human brain from precise measurements of systolic cardiac-induced tissue deformation. Such tensors can be decomposed into principal strains, where the first principal strain (FPS) describes the direction of maximum stretch while the third principal strain (TPS) describes the direction of maximum shortening. This technique offers an opportunity to study the mechanical properties of brain tissue in vivo. It allows us to explore how strain directions are influenced by global boundary conditions and local microstructure. Additionally, it helps to determine whether human brain tissue exhibits mechanical anisotropy. We obtained strain tensor and diffusion tensor imaging (DTI) data across 8 healthy subjects from a previous 7T MRI study. The strain tensor was constructed from the DENSE displacement measurements. We compared the measured strain directions with two conceptual models to assess the impact of global boundary conditions and local brain microstructure on the measured strain. The boundary condition-based model effectively explained the measured FPS directions across all subjects (mean R²; 0.61±0.03), indicating that global boundary conditions largely dictate the direction of the stretching that occurs during cerebral arterial pulsations. The TPS demonstrated a tendency to align perpendicularly to the DTI. These results highlight a potential indicator of mechanical anisotropy along white matter tracks using a novel approach, adding a new perspective to the ongoing discussion of the brain's structural characteristics.