Exploring the Role of Vascular Factors and Tissue Properties in Pulsatile Brain Deformation

Strain tensor imaging (STI) provides precise measurements of brain tissue deformation caused by cerebral arterial pulsations (CAPs). This CAP-related brain tissue deformation is expressed in rotation-invariant strain metrics, such as volumetric strain and octahedral shear strain, which hold promise as quantitative markers of the (mechanical) properties of both the intracerebral vasculature and the intervascular tissue components. However, the extent to which these strain metrics can be specifically linked to the underlying anatomical, vascular, and tissue properties remains largely unknown. This study aims to explore the relationship between STI metrics and independent markers of pulse pressure (arterial transit time, ATT), vascular function (cerebral blood volume, CBV; cerebral blood flow, CBF; mean transit time, MTT), and tissue properties (shear stiffness). Volumetric and octahedral shear strain were computed from previously obtained 7T displacement data (approximately 2-mm isotropic resolution) of eight healthy subjects (27 ± 7 years). Shear stiffness maps were generated from the same displacement data set using poroviscoelastic intrinsic MR elastography. Regional values of CBV, CBF, MTT, and ATT were obtained from standard-space atlases. Linear mixed-effects models were used to investigate potential regional relationships between specific strain metrics and the corresponding tissue, pulse pressure, or vascular markers. Volumetric strain showed significant positive correlations with CBV (globally and in cortical gray and white matters) and significant negative correlations with ATT (globally and in cortical gray and white matters), but not with shear stiffness. Octahedral shear strain showed a significant negative correlation with shear stiffness (globally and in subcortical gray and white matters) and also with ATT (globally and in cortical gray matter). Volumetric strain reflects mainly vascular properties (pulse pressure and blood volume), whereas octahedral shear strain is more sensitive to tissue properties. These findings provide a foundation for future studies that investigate the physiological characteristics reflected by these strain metrics and their intricate interplay.