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Reverberant elastography provides fast and robust estimates of shear modulus; however, its reliance on multiple mechanical drivers hampers clinical utility. In this work, we hypothesize that for constrained organs such as the brain, reverberant elastography can produce accurate magnetic resonance elastograms with a single mechanical driver. To corroborate this hypothesis, we performed studies on healthy volunteers (n=3); and a constrained calibrated brain phantom containing spherical inclusions with diameters ranging from 4-18 mm. In both studies (i.e., phantom and clinical), imaging was performed at frequencies of 50 Hz and 70 Hz. We used the accuracy and contrast-to-noise ratio performance metrics to evaluate reverberant elastograms relative to those computed using the established subzone inversion method. Errors incurred in reverberant elastograms varied from 1.3% to 16.6% when imaging at 50 Hz and 3.1% and 16.8 % when imaging at 70 Hz. In contrast, errors incurred in subzone elastograms ranged from 1.9% to 13% at 50 Hz and 3.6% to 14.9% at 70 Hz. The contrast-to-noise ratio of reverberant elastograms ranged from 63.1 dB to 73 dB compared to 65 dB to 66.2 dB for subzone elastograms. The average global brain stiffness estimated from reverberant and subzone elastograms was 2.36 ± 0.07 kPa and 2.38 ± 0.11 kPa, respectively, when imaging at 50 Hz and 2.70 ± 0.20 kPa and 2.89 ± 0.60 kPa respectively, when imaging at 70 Hz. The results of this investigation demonstrate that reverberant elastography can produce accurate, high-quality elastograms of the brain with a single mechanical driver.

This study combined multi-excitation MR Elastography (ME-MRE) with a transversely isotropic inversion nonlinear inversion algorithm and diffusion tensor imaging (DTI) fiber directions to investigate anisotropic white matter tract integrity in aging. Participants were 5 older adults and 17 young adults. MRE outcomes include substrate shear modulus, μ, shear anisotropy, φ, tensile anisotropy, ζ, and DTI measures were also examined. Shear and tensile anisotropies were significantly different in the Corona Radiata and Superior Longitudinal Fasciculus. Diffusion measures were significantly different between groups in most tracts. These results suggest sensitivity of anisotropic properties to biological changes along white matter tracts in aging.

Objective: In-vivo imaging assessments of skeletal muscle structure and function allow for longitudinal quantification of tissue health. Magnetic resonance elastography (MRE) non-invasively quantifies tissue mechanical properties, allowing for evaluation of skeletal muscle biomechanics in response to loading, creating a better understanding of muscle functional health. Approach: In this study, we analyze the anisotropic mechanical response of calf muscles using MRE with a transversely isotropic, nonlinear inversion algorithm (TI-NLI) to investigate the role of muscle fiber stiffening under load. We estimate anisotropic material parameters including fiber shear stiffness (μ1), substrate shear stiffness (μ2), shear anisotropy (ϕ), and tensile anisotropy (ζ) of the gastrocnemius muscle in response to both passive and active tension. Main Results: In passive tension, we found a significant increase in μ1, ϕ, and ζ with increasing muscle length. While in active tension, we observed increasing μ2 and decreasing ϕ and ζ during active dorsiflexion and plantarflexion – indicating less anisotropy – with greater effects when the muscles act as agonist. Significance: The study demonstrates the ability of this anisotropic MRE method to capture the multifaceted mechanical response of skeletal muscle to tissue loading from muscle lengthening and contraction.

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In this study, we utilized MR elastography (MRE) and transversely isotropic inversion to estimate anisotropic material parameters of muscle during contraction states, including passive muscle lengthening and isometric contraction. We collected MRE and DTI data on six subjects and took averages of the parameters within the medial and lateral heads of the gastrocnemius muscle and compared the results during the contraction states. We found significant differences between the estimates during the various states of contraction. Here we demonstrate that MRE and TI-NLI can generate anisotropic parameter estimates for muscle tissue while capturing changes to functional aspects of the tissue.

This study explores reverberant shear wave elastography to create accurate magnetic resonance elastograms. The reverberant elastography technique utilizes the complex wave field originating from multiple point sources or reflected from various angles and superimposed with each other. The study was conducted on a calibrated brain phantom. Results showed that reverberant elastography produced accurate elastograms with an accuracy range of 84-97% and contrast-to-noise ratios of 24 dB, compared to an accuracy range of 86-97.7% and contrast-to-noise ratios of 25 dB for the established subzone inversion method.