High-resolution magnetic resonance elastography reveals differences in subcortical gray matter viscoelasticity between young and healthy older adults

Abstract

Volumetric structural magnetic resonance imaging (MRI) is commonly used to determine the extent of neuronal loss in aging, indicated by cerebral atrophy. The brain, however, exhibits other biophysical characteristics such as mechanical properties, which can be quantified with magnetic resonance elastography (MRE). MRE is an emerging noninvasive imaging technique for measuring viscoelastic tissue properties, proven to be sensitive metrics of neural tissue integrity, as described by shear stiffness, μ and damping ratio, ξ parameters. The study objective was to evaluate global and regional MRE parameter differences between young (19-30 years, n = 12) and healthy older adults (66-73 years, n = 12) and to assess whether MRE measures provide additive value over volumetric magnetic resonance imaging measurements. We investigated the viscoelasticity of the global cerebrum and 6 regions of interest (ROIs) including the amygdala, hippocampus, caudate, pallidum, putamen, and thalamus. In older adults, we found a decrease in μ in all ROIs, except for the hippocampus, indicating widespread brain softening; an effect that remained significant after controlling for ROI volume. In contrast, the relative viscous-to-elastic behavior of the brain ξ did not differ between age groups, suggesting a preservation of the organization of the tissue microstructure. These data support the use of MRE as a novel imaging biomarker for characterizing age-related differences to neural tissue not captured by volumetric imaging alone.

Keywords: Brain; Elasticity imaging techniques; Elastography; Healthy aging; Magnetic resonance elastography (MRE); Subcortical gray matter; Viscoelasticity.

Fig. 1

Schematic representation of the experimental pneumatic actuator design for brain tissue vibration (Klatt et al., 2015). Compressed air is transmitted through a plastic tube from an active driver, situated in the MRI control room, to a passive soft pillow-like driver placed beneath the head (Resoundant, Mayo Clinic, Rochester, MN, USA). The vibrations induce a gentle nodding motion of the head.

Fig. 2

Mean and standard deviation for (A) volume (cm³), (B) shear stiffness, μ (kPa), and (C) damping ratio, ξ. N = 12 in each group.

Fig. 3

Mean shear stiffness μ properties of the cerebrum (Ce) for young and older adults, showing widespread softer brains in older age (p < 0.001). MRE parameter maps have been transformed into standard MNI space, with anatomical information overlaid for illustration purposes. 3D rendering of the MNI template shows the location of the three representative slices. Abbreviation: MNI, Montreal Neurological Institute; MRE, magnetic resonance elastography.

Fig. 4

Mean shear stiffness μ properties of SGM structures (Ca, Caudate; Pa, Pallidum; Pu, Putamen; Th, Thalamus) for young and older adults, in standard MNI space. μ of these structures remain significantly different between age groups after correcting for ROI volume, with all being softer in older adults. *** denotes p < .001 and ** denotes p < .01 significance levels. Abbreviation: SGM, subcortical gray matter; ROI, region of interest.