Real‐time MR elastography for viscoelasticity quantification in skeletal muscle during dynamic exercises

Abstract

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.

Real-Time Multifrequency MR Elastography of the Human Brain Reveals Rapid Changes in Viscoelasticity in Response to the Valsalva Maneuver

“…Single-frequency rt-MRE using a 2D single-shot gradient echo MRE pulse sequence with spiral readout was recently introduced for directly mapping skeletal muscle function ( Schrank et al, 2020a ). For rt-MMRE we used a similar prototype—a single-shot, gradient-echo sequence with dual-density spiral readout, which samples multifrequency vibrations in a stroboscopic fashion as illustrated in Figure 2 .…”

“…The same ROI was also used to determine magnetization signal-to-noise ratio (SNR) and wave displacement SNR (WSNR) for every time frame. WSNR was derived using the blind noise estimation method proposed by Donoho et al (1995) as outlined and previously applied to MRE data in Bertalan et al (2019b) and Schrank et al (2020a) This noise estimation method is suited for wave image analysis since the spatial frequencies of MRE waves and noise are well separated in the wavelet domain ( Selesnick et al, 2005 ; Barnhill et al, 2017 ).…”

“…Faster techniques including time-harmonic ultrasound elastography ( Tzschatzsch et al, 2018 ; Kreft et al, 2020 ) and real-time MRE (rt-MRE) ( Schrank et al, 2020a ) have been introduced recently. While cerebral ultrasound elastography is limited by acoustic windows and cannot generate detailed maps, rt-MRE has the potential to map viscoelasticity with both high spatial resolution and high frame rates.…”

“…While cerebral ultrasound elastography is limited by acoustic windows and cannot generate detailed maps, rt-MRE has the potential to map viscoelasticity with both high spatial resolution and high frame rates. However, feasibility of rt-MRE has as yet only been demonstrated with a small field of view in the lower extremities ( Schrank et al, 2020a ) and has never been tested in the brain.…”

Image-based biomechanical models of the musculoskeletal system

“…Further, MRE has demonstrated to detect mechanical property changes of the disc, with shear stiffness measurements having shown to highly correlate with intervertebral disc degeneration [68]. Preliminary phantom and in vivo studies showed the feasibility of this technique to assess mechanical and functional properties of skeletal muscles under passive (rest) conditions [69] and after activation with dynamic exercises [70]. This data might be used as a reference in future studies on muscle disorders in addition to other quantitative MR imaging biomarkers of muscle structure and function [71].…”