In vivo wideband MR elastography for assessing age-related viscoelasticity changes of the human brain.

Whole-brain stiffness declined with age, with the strongest effect observed at low frequencies compared to mid and high frequencies.

• Low frequencies (5-16 Hz): -0.24%/year decline in shear wave speed (p = 0.038)
• Mid frequencies (20-35 Hz): -0.12%/year decline (p = 0.040)
• High frequencies (40-50 Hz): -0.10%/year decline (p = 0.123, not statistically significant)
• Study included 24 healthy adults split into young (23-39 years) and older (50-63 years) groups
• Wave fields were acquired at 13 frequencies spanning 5-50 Hz using a dual-actuator wideband MRE protocol

Younger adults showed significantly higher baseline stiffness compared to older adults as measured by the power-law rheological model.

• Younger adults showed 8.96% higher baseline stiffness in the power-law model compared to older brains (p = 0.013)
• Shear wave speed maps were generated as a proxy for stiffness
• SWS dispersion was modeled using Newtonian, Kelvin-Voigt, and power-law rheological models

Younger adults showed significantly higher viscosity compared to older adults according to both Newtonian and Kelvin-Voigt models.

• Younger adults showed 8.15-8.39% higher viscosity according to the Newtonian and Kelvin-Voigt models (p < 0.05)
• Both Newtonian and Kelvin-Voigt models yielded consistent viscosity differences between age groups

Deep gray matter showed an age-related increase in the power-law exponent, suggesting a transition toward more fluid-like properties with aging.

• Deep gray matter showed an increase in power-law exponent of +0.001/year (p = 0.046)
• White matter and cortical gray matter exhibited similar age-related stiffness decreases
• The increase in power-law exponent in deep gray matter is interpreted as indicating a transition toward more fluid-like properties associated with aging

A dual-actuator wideband MRE protocol was developed that acquires wave fields at 13 frequencies spanning 5-50 Hz in vivo.

• The protocol spans a frequency range of 5-50 Hz, substantially wider than conventional single-frequency or narrow-band MRE approaches
• Wave fields were acquired at 13 discrete frequencies
• The protocol was applied in 24 healthy adults in vivo
• Most conventional MRE implementations use a single frequency or narrow frequency band, limiting analysis of frequency-dependent viscoelasticity

Extending brain MRE into the low frequency regime potentially enhances sensitivity to solid-fluid interactions in brain tissue.

• Low frequency MRE may serve as an early biomechanical marker of microstructural brain changes due to aging and neurodegeneration
• The strongest age-related effects were observed at low frequencies (5-16 Hz)
• Low frequency sensitivity is interpreted as reflecting microstructural alterations and potentially fluid-solid interactions in brain tissue