Effect of Aging on the Viscoelastic Properties of Hippocampal Subfields Assessed with High-Resolution MR Elastography.

Age-related memory impairments have been linked to differences in structural brain parameters, including the integrity of the hippocampus (HC) and its distinct hippocampal subfields (HCsf). Imaging methods sensitive to the underlying tissue microstructure are valuable in characterizing age-related HCsf structural changes that may relate to cognitive function. Magnetic resonance elastography (MRE) is a noninvasive MRI technique that can quantify tissue viscoelasticity and may provide additional information about aging effects on HCsf health. Here, we report a high-resolution MRE protocol to quantify HCsf viscoelasticity through shear stiffness, μ, and damping ratio, ξ, which reflect the integrity of tissue composition and organization. HCsf exhibit distinct mechanical properties-the subiculum had the lowest μ and both subiculum and entorhinal cortex had the lowest ξ. Both measures correlated with age: HCsf μ was lower with age (P < 0.001) whereas ξ was higher (P = 0.002). The magnitude of age-related differences in ξ varied across HCsf (P = 0.011), suggesting differential patterns of brain aging. This study demonstrates the feasibility of using MRE to assess HCsf microstructural integrity and suggests incorporation of these metrics to evaluate HC health in neurocognitive disorders.