Exploring the role of vascular factors and tissue properties in pulsatile brain deformation

Introduction Strain tensor imaging (STI) provides precise measurements of brain tissue deformation caused by cerebral arterial pulsations (CAP). This CAP-related brain tissue deformation is expressed in quantitative 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). Method 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. Results Volumetric strain showed significant positive correlations with CBV (globally, cortical gray and white matter) and significant negative correlations with ATT (globally, and in cortical gray and white matter), but not with shear stiffness. Octahedral shear strain showed a significant negative correlation with shear stiffness (globally, in subcortical gray and white matter) and also with ATT (globally, in cortical gray matter). Conclusion Volumetric strain reflects mainly vascular properties (pulse pressure, blood volume), while 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.