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Purpose Magnetic resonance elastography (MRE) maps the viscoelastic properties of soft tissues for diagnostic purposes. However, different MRE inversion methods yield different results, which hinder comparison of values, standardization, and establishment of quantitative MRE markers. Here, we introduce an expandable, open‐access, webserver‐based platform that offers multiple inversion techniques for multifrequency, 3D MRE data. Methods The platform comprises a data repository and standard MRE inversion methods including local frequency estimation (LFE), direct‐inversion based multifrequency dual elasto‐visco (MDEV) inversion, and wavenumber‐based (k‐) MDEV. The use of the platform is demonstrated in phantom data and in vivo multifrequency MRE data of the kidneys and brains of healthy volunteers. Results Detailed maps of stiffness were generated by all inversion methods showing similar detail of anatomy. Specifically, the inner renal cortex had higher shear wave speed (SWS) than renal medulla and outer cortex without lateral differences. k‐MDEV yielded higher SWS values than MDEV or LFE (full kidney/brain k‐MDEV: 2.71 ± 0.19/1.45 ± 0.14 m/s, MDEV: 2.14 ± 0.16/0.99 ± 0.11 m/s, LFE: 2.12 ± 0.15/0.89 ± 0.06 m/s). Conclusion The freely accessible platform supports the comparison of MRE results obtained with different inversion methods, filter thresholds, or excitation frequencies, promoting reproducibility in MRE across community‐developed methods.

Purpose To develop and test real‐time MR elastography for viscoelastic parameter quantification in skeletal muscle during dynamic exercises. Methods In 15 healthy participants, 6 groups of lower‐leg muscles (tibialis anterior, tibialis posterior, peroneus, extensor digitorum longus, soleus, gastrocnemius) were investigated by real‐time MR elastography using a single‐shot, steady‐state spiral gradient‐echo pulse sequence and stroboscopic undersampling of harmonic vibrations at 40 Hz frequency. One hundred and eighty consecutive maps of shear‐wave speed and loss angle (φ) covering 30.6 s of total acquisition time at 5.9‐Hz frame rate were reconstructed from 360 wave images encoding 2 in‐plane wave components in an interleaved manner. The experiment was carried out twice to investigate 2 exercises—isometric plantar flexion and isometric dorsiflexion—each performed over 10 s between 2 resting periods. Results Activation of lower‐extremity muscles was associated with increasing viscoelastic parameters shear‐wave speed and φ, both reflecting properties related to the transverse direction relative to fiber orientation. Major viscoelastic changes were observed in soleus muscle during plantar flexion (shear‐wave speed: 20.0% ± 3.6%, φ: 41.3% ± 12.0%) and in the tibialis anterior muscle during dorsiflexion (41.8% ± 10.2%, φ: 27.9% ± 2.8%; all P < .0001). Two of the muscles analyzed were significantly activated by plantar flexion and 4 by dorsiflexion based on shear‐wave speed, whereas φ changed significantly in 5 muscles during both exercises. Conclusion Real‐time MR elastography allows mapping of dynamic, nonperiodic viscoelasticity changes in soft tissues such as voluntary muscle with high spatial and temporal resolution. Real‐time MR elastography thus opens new horizons for the in vivo study of physiological processes in soft tissues toward functional elastography.

Ultrasound elastography quantitatively measures tissue stiffness and is widely used in clinical practice to diagnose various diseases including liver fibrosis and portal hypertension. The stiffness of soft organs has been shown to be sensitive to blood flow and pressure-related diseases such as portal hypertension. Because of the intricate coupling between tissue stiffness of abdominal organs and perfusion-related factors such as vascular stiffness or blood volume, simple breathing maneuvers have altered the results of liver elastography, while other organs such as the spleen are understudied. Therefore, we investigated the effect of a standardized Valsalva maneuver on liver stiffness and, for the first time, on spleen stiffness using time-harmonic elastography (THE). THE acquires full-field-of-view stiffness maps based on shear wave speed (SWS), covers deep tissues, and is potentially sensitive to SWS changes induced by altered abdominal pressure in the hepatosplenic system. SWS of the liver and the spleen was measured in 17 healthy volunteers under baseline conditions and during the Valsalva maneuver. With the Valsalva maneuver, SWS in the liver decreased by 2.2% (from a median of 1.36 m/s to 1.32 m/s; p = 0.021), while SWS in the spleen decreased by 5.2% (from a median of 1.63 m/s to 1.51 m/s; p = 0.00059). Furthermore, we observed that the decrease was more pronounced the higher the baseline SWS values were. In conclusion, the results confirm our hypothesis that the Valsalva maneuver decreases liver and spleen stiffness, showing that THE is sensitive to perfusion pressure-related changes in tissue stiffness. With its extensive organ coverage and high penetration depth, THE may facilitate translation of pressure-sensitive ultrasound elastography into clinical routine.

Purpose The zebrafish (Danio rerio) has become an important animal model in a wide range of biomedical research disciplines. Growing awareness of the role of biomechanical properties in tumor progression and neuronal development has led to an increasing interest in the noninvasive mapping of the viscoelastic properties of zebrafish by elastography methods applicable to bulky and nontranslucent tissues. Methods Microscopic multifrequency MR elastography is introduced for mapping shear wave speed (SWS) and loss angle (φ) as markers of stiffness and viscosity of muscle, brain, and neuroblastoma tumors in postmortem zebrafish with 60 µm in‐plane resolution. Experiments were performed in a 7 Tesla MR scanner at 1, 1.2, and 1.4 kHz driving frequencies. Results Detailed zebrafish viscoelasticity maps revealed that the midbrain region (SWS = 3.1 ± 0.7 m/s, φ = 1.2 ± 0.3 radian [rad]) was stiffer and less viscous than telencephalon (SWS = 2.6 ± 0. 5 m/s, φ = 1.4 ± 0.2 rad) and optic tectum (SWS = 2.6 ± 0.5 m/s, φ = 1.3 ± 0.4 rad), whereas the cerebellum (SWS = 2.9 ± 0.6 m/s, φ = 0.9 ± 0.4 rad) was stiffer but less viscous than both (all p < .05). Overall, brain tissue (SWS = 2.9 ± 0.4 m/s, φ = 1.2 ± 0.2 rad) had similar stiffness but lower viscosity values than muscle tissue (SWS = 2.9 ± 0.5 m/s, φ = 1.4 ± 0.2 rad), whereas neuroblastoma (SWS = 2.4 ± 0.3 m/s, φ = 0.7 ± 0.1 rad, all p < .05) was the softest and least viscous tissue. Conclusion Microscopic multifrequency MR elastography‐generated maps of zebrafish show many details of viscoelasticity and resolve tissue regions, of great interest in neuromechanical and oncological research and for which our study provides first reference values.