Thigh muscle stiffness assessed with magnetic resonance elastography in hyperthyroid patients before and after medical treatment

MR elastography-based slip interface imaging (SII) for functional assessment of myofascial interfaces: A feasibility study

PURPOSE

Abnormal adherence at functional myofascial interfaces is hypothesized as an important phenomenon in myofascial pain syndrome. This study aimed to investigate the feasibility of MR elastography (MRE)-based slip interface imaging (SII) to visualize and assess myofascial mobility in healthy volunteers.

METHODS

SII was used to assess local shear strain at functional myofascial interfaces in the flexor digitorum profundus (FDP) and thighs. In the FDP, MRE was performed at 90 Hz vibration to each index, middle, ring, and little finger. Two thigh MRE scans were performed at 40 Hz with knees flexed and extended. The normalized octahedral shear strain (NOSS) maps were calculated to visualize myofascial slip interfaces. The entropy of the probability distribution of the gradient NOSS was computed for the two knee positions at the intermuscular interface between vastus lateralis and vastus intermedius, around rectus femoris, and between vastus intermedius and vastus medialis.

RESULTS

NOSS map depicted distinct functional slip interfaces in the FDP for each finger. Compared to knee flexion, clearer slip interfaces and larger gradient NOSS entropy at the vastus lateralis-vastus intermedius interface were observed during knee extension, where the quadriceps are not passively stretched. This suggests the optimal position for using SII to visualize myofascial slip interface in skeletal muscles is when muscles are not subjected to any additional force.

CONCLUSION

The study demonstrated that MRE-based SII can visualize and assess myofascial interface mobility in extremities. The results provide a foundation for investigating the hypothesis that myofascial pain syndrome is characterized by changes in the mobility of myofascial interfaces.

Evaluation of the Reproducibility of MR Elastography Measurements of the Lumbar Back Muscles

BACKGROUND

MR elastography (MRE) may provide quantitative imaging biomarkers of lumbar back muscles (LBMs), complementing MRI in spinal diseases by assessing muscle mechanical properties. However, reproducibility analyses for MRE of LBM are lacking.

PURPOSE

To assess technical failure, within-day and inter-day reproducibility, robustness with the excitation source positioning, and inter-observer agreement of MRE of muscles.

STUDY TYPE

Prospective.

SUBJECTS

Seventeen healthy subjects (mean age 28 ± 4 years; 11 females).

FIELD STRENGTH/SEQUENCE

1.5 T, gradient-echo MRE, T1-weighted turbo spin echo.

ASSESSMENT

The pneumatic driver was centered at L3 level. Four MRE were performed during two visits, 2-4 weeks apart, each consisting of two MRE with less than 10 minutes inter-scan interval. At Visit 1, after the first MRE, the coil and driver were removed, then reinstalled. The MRE was repeated. At Visit 2, following the first MRE, only the driver was moved down 5 cm. The MRE was repeated. Two radiologists segmented the multifidus and erector spinae muscles.

STATISTICAL TESTS

Paired t-test, analysis of variance, intraclass correlation coefficients (ICCs). P-values <0.05 were considered statistically significant.

RESULTS

Mean stiffness of LBM ranged from 1.44 to 1.60 kPa. Mean technical failure rate was 2.5%. Inter-observer agreement was excellent (ICC ranging from 0.82 [0.64-0.96] to 0.99 [0.98-0.99] in the multifidus, and from 0.85 [0.69-0.92] to 0.99 [0.97-0.99] in the erector spinae muscles). Within-day reproducibility was fair in the multifidus (ICC: 0.53 [0.47-0.77]) and good in the erector spinae muscles (ICC: 0.74 [0.48-0.88]). Reproducibility after moving the driver was excellent in both multifidus (ICC: 0.85 [0.69-0.93]) and erector spinae muscles (ICC: 0.84 [0.67-0.92]). Inter-day reproducibility was excellent in the multifidus (ICC: 0.76 [0.48-0.89]) and poor in the erector spinae muscles (ICC: 0.23 [-0.61 to 0.63]).

DATA CONCLUSION

MRE of LBM provides measurements of stiffness with fair to excellent reproducibility and excellent inter-observer agreement. However, inter-day reproducibility in the multifidus muscles indicated that the herein used MRE protocol may not be optimal for this muscle.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

Objective Methods of Muscle Tone Diagnosis and Their Application-A Critical Review

"Muscle tone" is a clinically important and widely used term and palpation is a crucial skill for its diagnosis. However, the term is defined rather vaguely, and palpation is not measurable objectively. Therefore, several methods have been developed to measure muscle tone objectively, in terms of biomechanical properties of the muscle. This article aims to summarize these approaches. Through database searches, we identified those studies related to objective muscle tone measurement in vivo, in situ. Based on them, we described existing methods and devices and compared their reliability. Furthermore, we presented an extensive list of the use of these methods in different fields of research. Although it is believed by some authors that palpation cannot be replaced by a mechanical device, several methods have already proved their utility in muscle biomechanical property diagnosis. There appear to be two issues preventing wider usage of these objective methods in clinical practice. Firstly, a high variability of their reliability, and secondly, a lack of valid mathematical models that would provide the observed mechanical characteristics with a clear physical significance and allow the results to be compared with each other.

Imaging biomarkers in the idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIMs) are a group of acquired muscle diseases with muscle inflammation, weakness, and other extra-muscular manifestations. IIMs can significantly impact the quality of life, and management of IIMs often requires a multi-disciplinary approach. Imaging biomarkers have become an integral part of the management of IIMs. Magnetic resonance imaging (MRI), muscle ultrasound, electrical impedance myography (EIM), and positron emission tomography (PET) are the most widely used imaging technologies in IIMs. They can help make the diagnosis and assess the burden of muscle damage and treatment response. MRI is the most widely used imaging biomarker of IIMs and can assess a large volume of muscle tissue but is limited by availability and cost. Muscle ultrasound and EIM are easy to administer and can even be performed in the clinical setting, but they need further validation. These technologies may complement muscle strength testing and laboratory studies and provide an objective assessment of muscle health in IIMs. Furthermore, this is a rapidly progressing field, and new advances are going to equip care providers with a better objective assessment of IIMS and eventually improve patient management. This review discusses the current state and future direction of imaging biomarkers in IIMs.

Nondestructive ultrasound evaluation of microstructure-related material parameters of skeletal muscle: An in silico and in vitro study

Direct and nondestructive assessment of material properties of skeletal muscle in vivo shall advance our understanding of intact muscle mechanics and facilitate personalized interventions. However, this is challenged by intricate hierarchical microstructure of the skeletal muscle. We have previously regarded the skeletal muscle as a composite of myofibers and extracellular matrix (ECM), formulated shear wave propagation in the undeformed muscle using the acoustoelastic theory, and preliminarily demonstrated that ultrasound-based shear wave elastography (SWE) could estimate microstructure-related material parameters (MRMPs): myofiber stiffness μ_f, ECM stiffness μ_m, and myofiber volume ratio V_f. The proposed method warrants further validation but is hampered by the lack of ground truth values of MRMPs. In this study, we presented analytical and experimental validations of the proposed method using finite-element (FE) simulations and 3D-printed hydrogel phantoms, respectively. Three combinations of different physiologically relevant MRMPs were used in the FE simulations where shear wave propagations in the corresponding composite media were simulated. Two 3D-printed hydrogel phantoms with the MRMPs close to those of a real skeletal muscle (i.e., μ_f=2.02kPa, μ_m=52.42kPa, and V_f=0.675,0.832) for ultrasound imaging were fabricated by an alginate-based hydrogel printing protocol that we modified and optimized from the freeform reversible embedding of suspended hydrogels (FRESH) method in literature. Average percent errors of (μ_f,μ_m,V_f) estimates were found to be (2.7%,7.3%,2.4%)in silico and (3.0%,8.0%,9.9%)in vitro. This quantitative study corroborated the potential of our proposed theoretical model along with ultrasound SWE for uncovering microstructural characteristics of the skeletal muscle in an entirely nondestructive way.

Effect of durations and pressures of cupping therapy on muscle stiffness of triceps

Cupping therapy has been used for the alleviation of muscle soreness in athletes. However, clinical studies of cupping therapy show conflicting results. Lack of standardized guidelines of the dose-response relationship of cupping therapy, such as appropriate cupping duration and negative pressure, limits the adoption of cupping therapy in clinical practice. The objectives of this study were to investigate the effect of various pressures and durations of cupping therapy on reducing muscle stiffness. The 2 × 2 factorial design with the repeated measures and counterbalanced design was used to test four cupping protocols, including two negative pressures at -225 and -300 mmHg and two durations at 5 and 10 min, in 12 healthy young people. B-mode and elastographic ultrasound was used to assess muscle stiffness of the triceps before and after cupping therapy. The region of interest of elastographic image was divided into the superficial and deep layers for assessing the effect of cupping therapy on stiffness of various depths of the triceps. Normalized stiffness was calculated as a ratio of pre-cupping stiffness divided by post-cupping stiffness of each participant. The two-way analysis of variance (ANOVA) was used to examine the main effects of the pressure and duration factors and the interaction effect between the pressure and duration factors. The results showed that there were no interactions between the pressure and duration factors (overall layer p = 0.149, superficial layer p = 0.632, and deep layer p = 0.491). The main effects of duration of the overall, superficial and deep layers were p = 0.538, p = 0.097 and p = 0.018, respectively. The results showed that 10-min cupping at -300 mmHg is more effective on reducing stiffness of the deep layer of the triceps compared to 5-min cupping (p = 0.031). This study provides the first evidence that the dose of cupping therapy could significantly affect changes of triceps stiffness and the deep layer of the muscle is more sensitive to cupping therapy compared to the superficial and overall layers.

Characterizing Musculoskeletal Tissue Mechanics Based on Shear Wave Propagation: A Systematic Review of Current Methods and Reported Measurements

Developing methods for the non-invasive characterization of the mechanics of musculoskeletal tissues is an ongoing research focus in biomechanics. Often, these methods use the speed of shear wave propagation to characterize tissue mechanics (e.g., shear wave elastography and shear wave tensiometry). The primary purpose of this systematic review was to identify, compare, and contrast current methods for exciting and measuring shear wave propagation in musculoskeletal tissues. We conducted searches in the Web of Science, PubMed, and Scopus databases for studies published from January 1, 1900, to May 1, 2020. These searches targeted both shear wave excitation using acoustic pushes and mechanical taps, and shear wave speed measurement using ultrasound, magnetic resonance imaging, accelerometers, and laser Doppler vibrometers. Two reviewers independently screened and reviewed the articles, identifying 524 articles that met our search criteria. Regarding shear wave excitation, we found that acoustic pushes are useful for exciting shear waves through the thickness of the tissue of interest, and mechanical taps are useful for exciting shear waves in wearable applications. Regarding shear wave speed measurement, we found that ultrasound is used most broadly to measure shear waves due to its ability to study regional differences and target specific tissues of interest. The strengths of magnetic resonance imaging, accelerometers, and laser Doppler vibrometers make them advantageous to measure shear wave speeds for high-resolution shear wave imaging, wearable measurements, and non-contact ex vivo measurements, respectively. The advantages that each method offers for exciting and measuring shear waves indicate that a variety of systems can be assembled using currently available technologies to determine musculoskeletal tissue material behavior across a range of innovative applications.

Magnetic Resonance Elastography Reconstruction for Anisotropic Tissues

Elastography has become widely used clinically for characterising changes in soft tissue mechanics that are associated with altered tissue structure and composition. However, some soft tissues, such as muscle, are not isotropic as is assumed in clinical elastography implementations. This limits the ability of these methods to capture changes in anisotropic tissues associated with disease. The objective of this study was to develop and validate a novel elastography reconstruction technique suitable for estimating the linear viscoelastic mechanical properties of transversely isotropic soft tissues. We derived a divergence-free formulation of the governing equations for acoustic wave propagation through a linearly transversely isotropic viscoelastic material, and transformed this into a weak form. This was then implemented into a finite element framework, enabling the analysis of wave input data and tissue structural fibre orientations, in this case based on diffusion tensor imaging. To validate the material constants obtained with this method, numerous in silico phantom experiments were run which encompassed a range of variations in wave input directions, material properties, fibre structure and noise. The method was also tested on ex vivo muscle and in vivo human volunteer calf muscles, and compared with a previous curl-based inversion method. The new method robustly extracted the transversely isotropic shear moduli (G_⊥^', G_∥^', G^″) from the in silico phantom tests with minimal bias, including in the presence of experimentally realistic levels of noise in either fibre orientation or wave data. This new method performed better than the previous method in the presence of noise. Anisotropy estimates from the ex vivo muscle phantom agreed well with rheological tests. In vivo experiments on human calf muscles were able to detect increases in muscle shear moduli with passive muscle stretch. This new reconstruction method can be applied to quantify tissue mechanical properties of anisotropic soft tissues, such as muscle, in health and disease.

Noninvasive Assessment of In Vivo Passive Skeletal Muscle Mechanics as a Composite Material Using Biomedical Ultrasound

OBJECTIVE

This study develops a biomedical ultrasound imaging method to infer microstructural information (i.e., tissue level) from imaging mechanical behavior of skeletal muscle (i.e., organ level).

METHODS

We first reviewed the constitutive model of skeletal muscle by regarding it as a transversely isotropic (TI) hyperelastic composite material, for which a theoretical formula was established among shear wave speed, deformation, and material parameters (MPs) using the acoustoelasticity theory. The formula was evaluated by finite element (FE) simulations and experimentally examined using ultrasound shear wave imaging (SWI) and strain imaging (SI) on in vivo passive biceps brachii muscles of two healthy volunteers. The imaging sequence included 1) generation of SW in multiple propagation directions while resting the muscle at an elbow angle of 90; 2) generation of SW propagating along the myofiber direction during continuous uniaxial muscle extension by passively changing the elbow angle from 90 to 120. Ultrasound-quantified SW speeds and muscle deformations were fitted by the theoretical formula to estimate MPs of in vivo passive muscle.

RESULTS

Estimated myofiber stiffness, stiffness ratio of myofiber to extracellular matrix (ECM), ECM volume ratio all agreed with literature findings.

CONCLUSION

The proposed mathematical formula together with our in-house ultrasound imaging method enabled assessing microstructural material properties of in vivo passive skeletal muscle from organ-level mechanical behavior in an entirely noninvasive way.

SIGNIFICANCE

Noninvasive assessment of both micro and macro properties of in vivo skeletal muscle will advance our understanding of complex muscle dynamics and facilitate treatment and rehabilitation planning.

Axially- and torsionally-polarized radially converging shear wave MRE in an anisotropic phantom made via Embedded Direct Ink Writing

Magnetic Resonance Elastography (MRE) is a non-invasive imaging method to quantitatively map the shear viscoelastic properties of soft tissues. In this study, Embedded Direct Ink Writing is used to fabricate a muscle mimicking anisotropic phantom that may serve as a standard for imaging studies of anisotropic materials. The technique allowed us to obtain a long shelf life silicone-based phantom expressing transverse isotropic mechanical properties. Another goal of the present investigation is to introduce a torsionally-polarized, radially-converging shear wave actuation method for MRE. The implemented design for this novel setup was first validated via its application to isotropic and homogeneous gelatin phantoms. Then, a comparison of the resulting complex wave images from axially- and torsionally-polarized MRE on the developed anisotropic phantom and on a skeletal muscle murine sample is presented, highlighting the value of using multiple actuation and motion encoding polarization directions when studying anisotropic materials.

Fall-Associated Drugs in Community-Dwelling Older Adults: Results from the ActiFE Ulm Study

OBJECTIVES

Many studies describing an association of drugs with falls focus mostly on drugs acting in the central nervous system. We aim to analyze the association of all drugs taken with falls in older adults.

DESIGN

Prospective population-based study (ActiFE study).

SETTING AND PARTICIPANTS

A total of 1377 community-dwelling older adults with complete recording of falls and baseline information on drug intake.

METHODS

Negative binomial regression was used to analyze the association of 34 drug classes with a 12-month incidence rate ratio (IRR) of falls adjusting for age, sex, comorbidities, gait speed, balance, chair rise, kidney function, liver disease, and smoking.

RESULTS

Participants took a median 3 drugs (interquartile range 1, 5), with 34.5% (n = 469) having ≥5 drugs. The median IRR for a fall per person-year was overall 0.72 [95% confidence interval (CI) 0.60-0.83] and 2.22 (95% CI 1.90-2.53) among those who experienced ≥1 fall. The following drug classes showed significant associations: antiparkinsonian medication [IRR 2.68 (95% CI 1.59-4.51)], thyroid therapy [IRR 1.40 (95% CI 1.08-1.81)], and systemic corticosteroids [IRR 0.33 (95% CI 0.13-0.81)]. Among fall-risk-increasing drugs only antiepileptics [IRR 2.16 (95% CI 1.10-4.24)] and urologicals [IRR 2.47 (95% CI 1.33-4.59)] were associated with falls in those participants without a prior fall history at baseline.

CONCLUSION AND IMPLICATIONS

Additional drug classes, such as antiparkinsonian medication, thyroid therapy, and systemic corticosteroids, might be associated with falls in older adults, possibly representing pharmacological effects on the musculoskeletal and central nervous systems. Further evaluations in larger study populations are recommended.

Application of ultrasound elastography in the evaluation of muscle strength in a healthy population

Background

To investigate the validity of shear wave elastography (SWE) for the evaluation of muscle strength compared with isokinetic muscle testing, and to assess the influence of demographic factors such as height, weight, and body mass index (BMI) on the shear wave velocity (SWV).

Methods

Sixty healthy volunteers were consecutively enrolled. SWE was used to measure the SWV of the right quadriceps femoris in a relaxed position, in a tensive position, and under loads of 1 and 2 kg. Muscle strength parameters including peak torque (PT), PT to body weight ratio (PT/BW), and total work (TW) were evaluated using isokinetic muscle testing. The SWV of the rectus femoris in different positions were compared using the Friedman test and the Kruskal-Wallis test, and the SWV and muscle strength parameters were compared between different genders and age groups using the Mann-Whitney U test. Additionally, Spearman's correlation coefficient was used to evaluate the correlation between SWV and muscle strength, as well as the possible effects of height, weight, and BMI on SWV.

Results

As the load increased, the SWV of the rectus femoris increased (P<0.001). In the relaxed position, there was no significant correlation between the SWV and the results of isokinetic muscle testing. With increasing load, the SWV and the results of isokinetic muscle testing were not significantly correlated (r=-0.256--0.392, P<0.05). In the 1 kg load position, height and weight were not significantly correlated with SWV (r=-0.261--0.393, P<0.05). In the relaxed position, there were no significant differences in the maximum, minimum, or mean SWV of the rectus femoris between different genders and age groups (P>0.05). However, under a 1 kg load, the maximum, minimum, and mean SWV of the females in this study were significantly higher than those of the males (4.49±0.60 vs. 3.98±0.68 m/s; 2.55±0.61 vs. 2.20±0.63 m/s; and 3.51±0.60 vs. 3.06±0.58 m/s; P=0.003, 0.028, and 0.004, respectively). Furthermore, there were significant differences in the maximum and mean velocities between the groups aged 20-34 and 35-60 years (4.11±0.62 vs. 4.47±0.70 m/s; 3.17±0.53 vs. 3.52±0.69 m/s; P=0.045 and 0.044, respectively).

Conclusions

Ultrasound elastography (UE) shows potential for the measurement of muscle strength. The SWV of muscles demonstrate an increasing trend with the increase of impedance. Additionally, age and gender have a significant effect on SWV, while the effects of height, weight, and BMI require further investigation.

Evaluation of Postoperative Changes in Patellar and Quadriceps Tendons after Total Knee Arthroplasty-A Comprehensive Analysis by Shear Wave Elastography, Power Doppler and B-mode Ultrasound

RATIONALE AND OBJECTIVES

Up to now, the diagnosis of tendinopathies is based on conventional B-mode-ultrasound (B-US), Power Doppler-ultrasound (PD-US), and magnetic resonance imaging. In the past decade, Shear Wave Elastography (SWE) has been introduced in tendon imaging, for example in athletes or patients suffering from tendinopathy. SWE allows real-time quantification of tissue stiffness, and, by this, the assessment of the mechanical properties of a tendon and its changes during acute disease and tendon healing. So far there are no ultrasound-based studies that have evaluated postoperative tendon changes, anatomical and mechanical properties and tendon healing of the patellar, and quadriceps tendon following Total Knee Arthroplasty (TKA). The purpose of this prospective study was two-fold: first to analyze morphologic, vascular, and mechanical properties of patellar and quadriceps tendons in patients following TKA; and, second to evaluate possible changes thereof and their visibility in the course of time.

MATERIALS AND METHODS

Observational cross-sectional, IRB-approved study in 63 postoperative patients with a total of 76 total knee arthroplasties (50 unilateral, 13 bilateral) and 50 nonoperated knees for comparison, resulting in 152 postoperative patellar- and quadriceps and 100 nonoperated patellar- and quadriceps-tendons for comparative analysis. For further examination, we divided the 63 patients into two groups according to the duration since surgery (group A < 24 months; group B > 24 months). All patients completed a standardized questionnaire, furthermore the Knee Society score and the Knee Society function score. The amount of experienced pain was assessed using the ordinal numeric rating scale and the presence of anterior knee pain was examined. Subsequently every participant underwent a standardized multimodal ultrasound protocol consisting of B-US, PD-US, and SWE of the left and right patellar and quadriceps tendons.

RESULTS

Using the different US-modalities, operated patellar, and quadriceps tendons (n = 152) were significantly more frequent classified as pathological (B-US) (p < 0.001), the mean Ohberg score was significantly higher (PD-US) (p < 0.001), and the tendons were significantly softer (SWE) than their nonoperated counterparts (n = 100). Mean SWE-value of postoperative patellar tendons was 45.66 ± 14.84 kPa versus 60.08 ± 19.13 kPa in nonoperated knees (p < 0.001). Mean SWE-value of postoperative quadriceps tendons was 35.73 ± 15.66 kPa versus 52.69 ± 16.20 kPa in nonoperated knees (p < 0.001). Comparing the two postoperative groups (group A and B), we recognized a significant decrease of pathologically classified patellar and quadriceps tendons (B-US and PD-US) in group B. The early postoperatively reduced SWE values slightly increased during the course of time.

CONCLUSION

After TKA, patellar, and quadriceps tendons show significant measurable alterations in B-US, PD-US, and SWE. Especially a significant decrease of tendon stiffness in operated knees, as assessed by SWE, might be a surrogate marker for changed mechanical properties. These alterations improve, the longer ago the surgery was. The quantitative information obtained by SWE could be of particular interest in follow-up and therapy monitoring after TKA. Knowledge about tendon stiffness and it's varieties in different population groups (e.g. athletes, elderly, postoperative patients) is crucial to sonographically rate a tendon as "healthy" or "diseased."

Exploration of New Contrasts, Targets, and MR Imaging and Spectroscopy Techniques for Neuromuscular Disease - A Workshop Report of Working Group 3 of the Biomedicine and Molecular Biosciences COST Action BM1304 MYO-MRI

Neuromuscular diseases are characterized by progressive muscle degeneration and muscle weakness resulting in functional disabilities. While each of these diseases is individually rare, they are common as a group, and a large majority lacks effective treatment with fully market approved drugs. Magnetic resonance imaging and spectroscopy techniques (MRI and MRS) are showing increasing promise as an outcome measure in clinical trials for these diseases. In 2013, the European Union funded the COST (co-operation in science and technology) action BM1304 called MYO-MRI (www.myo-mri.eu), with the overall aim to advance novel MRI and MRS techniques for both diagnosis and quantitative monitoring of neuromuscular diseases through sharing of expertise and data, joint development of protocols, opportunities for young researchers and creation of an online atlas of muscle MRI and MRS. In this report, the topics that were discussed in the framework of working group 3, which had the objective to: Explore new contrasts, new targets and new imaging techniques for NMD are described. The report is written by the scientists who attended the meetings and presented their data. An overview is given on the different contrasts that MRI can generate and their application, clinical needs and desired readouts, and emerging methods.

Shear wave velocity is sensitive to changes in muscle stiffness that occur independently from changes in force

Clinical assessments for many musculoskeletal disorders involve evaluation of muscle stiffness, although it is not yet possible to obtain quantitative estimates from individual muscles. Ultrasound elastography can be used to estimate the material properties of unstressed, homogeneous, and isotropic materials by tracking the speed of shear wave propagation; these waves propagate faster in stiffer materials. Although elastography has been applied to skeletal muscle, there is little evidence that shear wave velocity (SWV) can directly estimate muscle stiffness since this tissue violates many of the assumptions required for there to be a direct relationship between SWV and stiffness. The objective of this study was to evaluate the relationship between SWV and direct measurements of muscle force and stiffness in contracting muscle. Data were collected from six isoflurane-anesthetized cats. We measured the short-range stiffness in the soleus via direct mechanical testing in situ and SWV via ultrasound imaging. Measurements were taken during supramaximal activation at optimum muscle length, with muscle temperature varying between 26°C and 38°C. An increase in temperature causes a decrease in muscle stiffness at a given force, thus decoupling the tension-stiffness relationship normally present in muscle. We found that increasing muscle temperature decreased active stiffness from 4.0 ± 0.3 MPa to 3.3 ± 0.3 MPa and SWV from 16.9 ± 1.5 m/s to 15.9 ± 1.6 m/s while force remained unchanged (mean ± SD). These results demonstrate that SWV is sensitive to changes in muscle stiffness during active contractions. Future work is needed to determine how this relationship is influenced by changes in muscle structure and tension.NEW & NOTEWORTHY Shear wave ultrasound elastography is a noninvasive tool for characterizing the material properties of muscle. This study is the first to compare direct measurements of stiffness with ultrasound measurements of shear wave velocity (SWV) in a contracting muscle. We found that SWV is sensitive to changes in muscle stiffness, even when controlling for muscle tension, another factor that influences SWV. These results are an important step toward developing noninvasive tools for characterizing muscle structure and function.

Contemporary image-based methods for measuring passive mechanical properties of skeletal muscles in vivo

Skeletal muscles' primary function in the body is mechanical: to move and stabilize the skeleton. As such, their mechanical behavior is a key aspect of their physiology. Recent developments in medical imaging technology have enabled quantitative studies of passive muscle mechanics, ranging from measurements of intrinsic muscle mechanical properties, such as elasticity and viscosity, to three-dimensional muscle architecture and dynamic muscle deformation and kinematics. In this review we summarize the principles and applications of contemporary imaging methods that have been used to study the passive mechanical behavior of skeletal muscles. Elastography measurements can provide in vivo maps of passive muscle mechanical parameters, and both MRI and ultrasound methods are available (magnetic resonance elastography and ultrasound shear wave elastography, respectively). Both have been shown to differentiate between healthy muscle and muscles affected by a broad range of clinical conditions. Detailed muscle architecture can now be depicted using diffusion tensor imaging, which not only is particularly useful for computational modeling of muscle but also has potential in assessing architectural changes in muscle disorders. More dynamic information about muscle mechanics can be obtained using a range of dynamic MRI methods, which characterize the detailed internal muscle deformations during motion. There are several MRI techniques available (e.g., phase-contrast MRI, displacement-encoded MRI, and "tagged" MRI), each of which can be collected in synchrony with muscle motion and postprocessed to quantify muscle deformation. Together, these modern imaging techniques can characterize muscle motion, deformation, mechanical properties, and architecture, providing complementary insights into skeletal muscle function.

Soft tissue rheology and its implications for elastography: Challenges and opportunities

Magnetic resonance elastography and related shear wave ultrasound elastography techniques can be used to estimate the mechanical properties of soft tissues in vivo by using the relationships between wave propagation and the elastic properties of materials. These techniques have found numerous clinical and research applications, tracking changes in tissue properties as a result of disease or other interventions. Most dynamic elastography approaches estimate tissue elastic (or viscoelastic) properties from a simplified version of the equations for the propagation of acoustic waves through a homogeneous linear (visco)elastic medium. However, soft tissue rheology is complex and departs significantly from this idealized picture. In particular, soft tissues are nonlinearly viscoelastic, inhomogeneous and often anisotropic, and their apparent stiffness can vary with the current loading state. All of these features have implications for the reliability and reproducibility of elastography measurements, from data acquisition to analysis and interpretation. New developments in inversion algorithms for elastography are beginning to offer solutions to account for the complex rheology of tissues, including inhomogeneity and anisotropy. There remains considerable potential to further refine elastography to capture the full spectrum of tissue rheology, and thus to better understand the underlying tissue microstructural changes in a broad range of clinical disorders.

Anisotropic composite material phantom to improve skeletal muscle characterization using magnetic resonance elastography

The presence and progression of neuromuscular pathology, including spasticity, Duchenne's muscular dystrophy and hyperthyroidism, has been correlated with changes in the intrinsic mechanical properties of skeletal muscle tissue. Tools for noninvasively measuring and monitoring these properties, such as Magnetic Resonance Elastography (MRE), could benefit basic research into understanding neuromuscular pathologies, as well as translational research to develop therapies, by providing a means of assessing and tracking their efficacy. Dynamic elastography methods for noninvasive measurement of tissue mechanical properties have been under development for nearly three decades. Much of the technological development to date, for both Ultrasound (US)-based and Magnetic Resonance Imaging (MRI)-based strategies, has been grounded in assumptions of local homogeneity and isotropy. Striated skeletal and cardiac muscle, as well as brain white matter and soft tissue in some other organ regions, exhibit a fibrous microstructure which entails heterogeneity and anisotropic response; as one seeks to improve the accuracy and resolution in mechanical property assessment, heterogeneity and anisotropy need to be accounted for in order to optimize both the dynamic elastography experimental protocol and the interpretation of the measurements. Advances in elastography methodology at every step have been aided by the use of tissue-mimicking phantoms. The aim of the present study was to develop and characterize a heterogeneous composite phantom design with uniform controllable anisotropic properties meant to be comparable to the frequency-dependent anisotropic properties of skeletal muscle. MRE experiments and computational finite element (FE) studies were conducted on a novel 3D-printed composite phantom design. The displacement maps obtained from simulation and experiment show the same elliptical shaped wavefronts elongated in the plane where the structure presents higher shear modulus. The model exhibits a degree of anisotropy in line with literature data from skeletal muscle tissue MRE experiments. FE simulations of the MRE experiments provide insight into proper interpretation of experimental measurements, and help to quantify the importance of heterogeneity in the anisotropic material at different scales.

Effects of an acute bout of dynamic stretching on biomechanical properties of the gastrocnemius muscle determined by shear wave elastography

AIMS

The aim of this study was to examine the acute effects of dynamic stretching (DS) exercise on passive ankle range of motion (RoM), resting localized muscle stiffness, as measured by shear wave speed (SWS) of medial gastrocnemius muscle, fascicle strain, and thickness.

METHODS/RESULTS

Twenty-three participants performed a DS protocol. Before and after stretching, SWS was measured in the belly of the resting medial gastrocnemius muscle (MGM) using shear wave elastography. DS produced small improvements in maximum dorsiflexion (+1.5° ±1.5; mean difference ±90% confidence limits) and maximum plantarflexion (+2.3° ±1.8), a small decrease in fascicle strain (-2.6% ±4.4) and a small increase in SWS at neutral resting angle (+11.4% ±1.5). There was also a small increase in muscle thickness (+4.1mm ±2.0).

CONCLUSIONS

Through the use of elastography, this is the first study to suggest that DS increases muscle stiffness, decreases fascicle strain and increases muscle thickness as a result of improved RoM. These results can be beneficial to coaches, exercise and clinical scientists when choosing DS as a muscle conditioning or rehabilitation intervention.

RIGID MUSCULOSKELETAL MODELS OF THE HUMAN BODY SYSTEMS: A REVIEW

Advancing the knowledge of the biomechanics of the human body is essential to improve the clinical decision-makings of musculoskeletal disorders in the framework of in silico medicine. An impressive number of research projects focused on the development of rigid-body musculoskeletal models have been conducted over the world thanks to the new research directives. However, the application of these models in clinical practices remains a challenging issue. The objective of this review paper was to present the most current rigid-body musculoskeletal models of the human body systems and to analyze their trends and weaknesses for clinical applications. Then, recommendations were proposed for future researches toward fully clinical decision support. A systematic review process was performed. Well-selected studies related to the most current rigid-body 3D musculoskeletal models for each body system component (jaw, cervical spine, upper limbs, lumbar spine, and lower limbs) were summarized and explored. Trends in rigid musculoskeletal modeling are highlighted as personalization, new imaging techniques for specific joint kinematics, and computational efficiency. Weaknesses are highlighted as modeling assumptions, use of generic model, lack of modeling consensus, model validation, and parameter and model uncertainties. Future directions related to joint and muscle modeling, neuro-musculoskeletal modeling, model validation, data and model uncertainty quantification are recommended.

Shear Wave Elastography Is a Reliable and Repeatable Method for Measuring the Elastic Modulus of the Rectus Femoris Muscle and Patellar Tendon

OBJECTIVES

The purpose of this study was to determine intraobserver, interobserver, and inter-day reliability levels for stiffness measurements of the patellar tendon and rectus femoris muscle using shear wave elastography (SWE).

METHODS

This study was conducted on 12 healthy male individuals. Two examiners measured mean shear wave velocity values of the patellar tendons and rectus femoris muscles of both extremities using a 9L4 (4-9 MHz) transducer and an Acuson S3000 ultrasound system (Siemens Medical Solutions, Mountain View, CA). The elasticity images were acquired by the Virtual Touch tissue imaging quantification technique (Siemens Medical Solutions). Measurements were repeated 20 minutes and 1 week after the first measurements. The reliability of SWE measurements was assessed by means of the intraclass correlation coefficient (ICC).

RESULTS

The 12 participants ranged in age from 19 to 33 years (mean age ± SD, 25.33 ± 4.56 years). For the patellar tendon stiffness measurements with SWE, it was found that intraobserver reliability (ICC, 0.91-0.92) and interday reliability (ICC, 0.81-0.83) were excellent, and interobserver reliability (ICC, 0.71) was good. For the rectus femoris muscle stiffness measurements with SWE, it was found that the intraobserver reliability (ICC, 0.93-0.94), interday reliability (ICC, 0.81-0.91), and interobserver reliability (ICC, 0.95) were perfect.

CONCLUSIONS

Shear wave elastography using the Virtual Touch tissue imaging quantification technique is a reliable and repeatable technique for patellar tendon and rectus femoris stiffness measurements according to intraobserver, interday, and interobserver ICC values.

A MRI-Compatible Combined Mechanical Loading and MR Elastography Setup to Study Deformation-Induced Skeletal Muscle Damage in Rats

Deformation of skeletal muscle in the proximity of bony structures may lead to deep tissue injury category of pressure ulcers. Changes in mechanical properties have been proposed as a risk factor in the development of deep tissue injury and may be useful as a diagnostic tool for early detection. MRE allows for the estimation of mechanical properties of soft tissue through analysis of shear wave data. The shear waves originate from vibrations induced by an external actuator placed on the tissue surface. In this study a combined Magnetic Resonance (MR) compatible indentation and MR Elastography (MRE) setup is presented to study mechanical properties associated with deep tissue injury in rats. The proposed setup allows for MRE investigations combined with damage-inducing large strain indentation of the Tibialis Anterior muscle in the rat hind leg inside a small animal MR scanner. An alginate cast allowed proper fixation of the animal leg with anatomical perfect fit, provided boundary condition information for FEA and provided good susceptibility matching. MR Elastography data could be recorded for the Tibialis Anterior muscle prior to, during, and after indentation. A decaying shear wave with an average amplitude of approximately 2 μm propagated in the whole muscle. MRE elastograms representing local tissue shear storage modulus G_d showed significant increased mean values due to damage-inducing indentation (from 4.2 ± 0.1 kPa before to 5.1 ± 0.6 kPa after, p<0.05). The proposed setup enables controlled deformation under MRI-guidance, monitoring of the wound development by MRI, and quantification of tissue mechanical properties by MRE. We expect that improved knowledge of changes in soft tissue mechanical properties due to deep tissue injury, will provide new insights in the etiology of deep tissue injuries, skeletal muscle damage and other related muscle pathologies.

Magnetic resonance elastography (MRE) for measurement of muscle stiffness of the shoulder: feasibility with a 3 T MRI system

BACKGROUND

Magnetic resonance elastography (MRE) at 3 T MR has the potential to improve the objective detection of skeletal muscle stiffness.

PURPOSE

To determine the feasibility of MRE using 3 T MR for measurement of the stiffness of shoulder muscles in subjects.

MATERIAL AND METHODS

This study prospectively evaluated 16 healthy subjects (mean age, 29.8 years; range, 25-51 years). MRE was acquired with 3 T MR through the use of a 2D-gradient-echo-based MRE sequence at two different excitation frequencies (90 and 120 Hz). The mean stiffness values (MSV) of the trapezius and infraspinatus muscles were measured by two radiologists. Differences between the MSV in the x, y, and z motion-sensitization directions were assessed. Inter-observer agreement was also measured.

RESULTS

The MSV of the trapezius muscle were 2.72 kPa ± 0.6 (SD) at 90 Hz and 4.66 kPa ± 1.2 at 120 Hz, while the MSV for the infraspinatus muscle were 3.2 kPa ± 0.52 at 90 Hz and 4.38 kPa ± 0.92 at 120 Hz. The MSV for both muscles were significantly higher at 120 Hz than at 90 Hz (P < 0.05). The MSV in the three different directions were significantly different from each other in the infraspinatus muscle (P < 0.05). Levels of inter-observer agreement regarding MSV were good to excellent for both the trapezius (intraclass correlation coefficient [ICC] = 0.979-0.996) and infraspinatus muscles (ICC = 0.614-0.943).

CONCLUSION

MRE at 3 T is a feasible technique for the evaluation of shoulder muscle stiffness. Extended application of skeletal muscle MRE at 3 T will contribute to the evaluation and treatment of skeletal muscle disorders.

An automatic differentiation-based gradient method for inversion of the shear wave equation in magnetic resonance elastography: specific application in fibrous soft tissues

Quantitative and accurate measurement of in vivo mechanical properties using dynamic elastography has been the scope of many research efforts over the past two decades. Most of the shear-wave-based inverse approaches for magnetic resonance elastography (MRE) make the assumption of isotropic viscoelasticity. In this paper, we propose a quantitative gradient method for inversion of the shear wave equation in anisotropic media derived from a full waveform description using analytical viscoelastic Green formalism and automatic differentiation. The abilities and performances of the proposed identification method are first evaluated on numerical phantoms calculated in a transversely isotropic medium, and subsequently on experimental MRE data measured on an isotropic hydrogel phantom, on an anisotropic cryogel phantom and on an ex vivo fibrous muscle. The experiments are carried out by coupling circular shear wave profiles generated by acoustic radiation force and MRE acquisition of the wave front. Shear modulus values obtained by our MRE method are compared to those obtained by rheometry in the isotropic hydrogel phantom, and are found to be in good agreement despite non-overlapping frequency ranges. Both the cryogel and the ex vivo muscle are found to be anisotropic. Stiffness values in the longitudinal direction are found to be 1.8 times and 1.9 times higher than those in the transverse direction for the cryogel and the muscle, respectively. The proposed method shows great perspectives and substantial benefits for the in vivo quantitative investigation of complex mechanical properties in fibrous soft tissues.

Nonlinear multiscale regularisation in MR elastography: Towards fine feature mapping

Fine-featured elastograms may provide additional information of radiological interest in the context of in vivo elastography. Here a new image processing pipeline called ESP (Elastography Software Pipeline) is developed to create Magnetic Resonance Elastography (MRE) maps of viscoelastic parameters (complex modulus magnitude |G^*| and loss angle ϕ) that preserve fine-scale information through nonlinear, multi-scale extensions of typical MRE post-processing techniques.

METHODS

A new MRE image processing pipeline was developed that incorporates wavelet-domain denoising, image-driven noise estimation, and feature detection. ESP was first validated using simulated data, including viscoelastic Finite Element Method (FEM) simulations, at multiple noise levels. ESP images were compared with MDEV pipeline images, both in the FEM models and in three ten-subject cohorts of brain, thigh, and liver acquisitions. ESP and MDEV mean values were compared to 2D local frequency estimation (LFE) mean values for the same cohorts as a benchmark. Finally, the proportion of spectral energy at fine frequencies was quantified using the Reduced Energy Ratio (RER) for both ESP and MDEV.

RESULTS

Blind estimates of added noise (σ) were within 5.3% ± 2.6% of prescribed, and the same technique estimated σ in the in vivo cohorts at 1.7 ± 0.8%. A 5 × 5 × 5 truncated Gabor filter bank effectively detects local spatial frequencies at wavelengths λ ≤ 10px. For FEM inversions, mean |G^*| of hard target, soft target, and background remained within 8% of prescribed up to σ=20%, and mean ϕ results were within 10%, excepting hard target ϕ, which required redrawing around a ring artefact to achieve similar accuracy. Inspection of FEM |G^*| images showed some spatial distortion around hard target boundaries and inspection of ϕ images showed ring artefacts around the same target. For the in vivo cohorts, ESP results showed mean correlation of R=0.83 with MDEV and liver stiffness estimates within 7% of 2D-LFE results. Finally, ESP showed statistically significant increase in fine feature spectral energy as measured with RER for both |G^*| (p<1×10^-9) and ϕ (p<1×10^-3).

CONCLUSION

Information at finer frequencies can be recovered in ESP elastograms in typical experimental conditions, however scatter- and boundary-related artefacts may cause the fine features to have inaccurate values. In in vivo cohorts, ESP delivers an increase in fine feature spectral energy, and better performance with longer wavelengths, than MDEV while showing similar stability and robustness.

General review of magnetic resonance elastography

Magnetic resonance elastography (MRE) is an innovative imaging technique for the non-invasive quantification of the biomechanical properties of soft tissues via the direct visualization of propagating shear waves in vivo using a modified phase-contrast magnetic resonance imaging (MRI) sequence. Fundamentally, MRE employs the same physical property that physicians utilize when performing manual palpation - that healthy and diseased tissues can be differentiated on the basis of widely differing mechanical stiffness. By performing “virtual palpation”, MRE is able to provide information that is beyond the capabilities of conventional morphologic imaging modalities. In an era of increasing adoption of multi-parametric imaging approaches for solving complex problems, MRE can be seamlessly incorporated into a standard MRI examination to provide a rapid, reliable and comprehensive imaging evaluation at a single patient appointment. Originally described by the Mayo Clinic in 1995, the technique represents the most accurate non-invasive method for the detection and staging of liver fibrosis and is currently performed in more than 100 centers worldwide. In this general review, the mechanical properties of soft tissues, principles of MRE, clinical applications of MRE in the liver and beyond, and limitations and future directions of this discipline -are discussed. Selected diagrams and images are provided for illustration.

Development of a phantom mimicking the functional and structural behaviors of the thigh muscles characterized with magnetic resonance elastography technique

Magnetic Resonance Elastography (MRE) is a non invasive technique based on the propagation of shear waves in soft tissues providing the quantification of the mechanical properties [1]. MRE was successfully applied to healthy and pathological muscles. However, the MRE muscle methods must be further improved to characterize the deep muscles. A way will be to develop phantom mimicking the muscle behavior in order to set up new MRE protocol. Thus, the purpose of this study is to create a phantom composed of a similar skeletal muscle architecture (fiber, aponorosis) and equivalent elastic properties as a function of the muscle state (passive or active). Two homogeneous phantoms were manufactured with different concentrations of plastisol to simulate the elastic properties in relaxed (50% of plastisol) and contracted (70% of plastisol) muscle conditions. Moreover, teflon tubing pipes (D = 0.9 mm) were thread in the upper part of the phantom (50%) to represent the muscle fibers and a plastic sheet (8 × 15 cm) was also included in the middle of the phantom to mimic the aponeurosis structure. Subsequently, MRE tests were performed with two different pneumatic drivers, tube and round, (f = 90Hz) to analyze the effect of the type of driver on the wave propagation. Then, the wavelength was measured from the phase images to obtain the elastic properties (shear modulus). Both phantoms revealed elastic properties which were in the same range as in vivo muscle in passive (μ(50%) = 2.40 ± 0.18 kPa ) and active (6.24 ± 0.21 kPa) states. The impact of the type of driver showed higher values (about 1.2kPa) with the tube. The analysis of the wave behavior revealed a sliding along the plastic sheet as it was observed for in vivo muscle study. The wave was also sensitive to the presence of the fibers where gaps were identified. The present study demonstrates the ability of the phantom to mimic the structural and functional properties of the muscle.

Magnetic resonance imaging of skeletal muscle disease

Neuromuscular diseases often exhibit a temporally varying, spatially heterogeneous, and multifaceted pathology. The goals of this chapter are to describe and evaluate the use of quantitative magnetic resonance imaging (MRI) methods to characterize muscle pathology. The following criteria are used for this evaluation: objective measurement of continuously distributed variables; clear and well-understood relationship to the pathology of interest; sensitivity to improvement or worsening of clinical status; and the measurement properties of accuracy and precision. Two major classes of MRI methods meet all of these criteria: (1) MRI methods for measuring muscle contractile volume or cross-sectional area by combining structural MRI and quantitative fat-water MRI; and (2) an MRI method for characterizing the edema caused by inflammation, the measurement of the transverse relaxation time constant (T2). These methods are evaluated with respect to the four criteria listed above and examples from neuromuscular disorders are provided. Finally, these methods are summarized and synthesized and recommendations for additional quantitative MRI developments are made.

Quantifying the Elastic Property of Nine Thigh Muscles Using Magnetic Resonance Elastography

BACKGROUND

Pathologies of the muscles can manifest different physiological and functional changes. To adapt treatment, it is necessary to characterize the elastic property (shear modulus) of single muscles. Previous studies have used magnetic resonance elastography (MRE), a technique based on MRI technology, to analyze the mechanical behavior of healthy and pathological muscles. The purpose of this study was to develop protocols using MRE to determine the shear modulus of nine thigh muscles at rest.

METHODS

Twenty-nine healthy volunteers (mean age = 26 ± 3.41 years) with no muscle abnormalities underwent MRE tests (1.5 T MRI). Five MRE protocols were developed to quantify the shear moduli of the nine following thigh muscles at rest: rectus femoris (RF), vastus medialis (VM), vastus intermedius (VI), vastus lateralis (VL), sartorius (Sr), gracilis (Gr), semimembranosus (SM), semitendinosus (ST), and biceps (BC). In addition, the shear modulus of the subcutaneous adipose tissue was analyzed.

RESULTS

The gracilis, sartorius, and semitendinosus muscles revealed a significantly higher shear modulus (μ_Gr = 6.15 ± 0.45 kPa, μ_ Sr = 5.15 ± 0.19 kPa, and μ_ ST = 5.32 ± 0.10 kPa, respectively) compared to other tissues (from μ_ RF = 3.91 ± 0.16 kPa to μ_VI = 4.23 ± 0.25 kPa). Subcutaneous adipose tissue had the lowest value (μ_adipose tissue = 3.04 ± 0.12 kPa) of all the tissues tested.

CONCLUSION

The different elasticities measured between the tissues may be due to variations in the muscles' physiological and architectural compositions. Thus, the present protocol could be applied to injured muscles to identify their behavior of elastic property. Previous studies on muscle pathology found that quantification of the shear modulus could be used as a clinical protocol to identify pathological muscles and to follow-up effects of treatments and therapies. These data could also be used for modelling purposes.

Reliable protocol for shear wave elastography of lower limb muscles at rest and during passive stretching

Development of shear wave elastography gave access to non-invasive muscle stiffness assessment in vivo. The aim of the present study was to define a measurement protocol to be used in clinical routine for quantifying the shear modulus of lower limb muscles. Four positions were defined to evaluate shear modulus in 10 healthy subjects: parallel to the fibers, in the anterior and posterior aspects of the lower limb, at rest and during passive stretching. Reliability was first evaluated on two muscles by three operators; these measurements were repeated six times. Then, measurement reliability was compared in 11 muscles by two operators; these measurements were repeated three times. Reproducibility of shear modulus was 0.48 kPa and repeatability was 0.41 kPa, with all muscles pooled. Position did not significantly influence reliability. Shear wave elastography appeared to be an appropriate and reliable tool to evaluate the shear modulus of lower limb muscles with the proposed protocol.

Measurement of passive skeletal muscle mechanical properties in vivo: recent progress, clinical applications, and remaining challenges

The ability to measure and quantify the properties of skeletal muscle in vivo as a method for understanding its complex physiological and pathophysiological behavior is important in numerous clinical settings, including rehabilitation. However, this remains a challenge to date due to the lack of a "gold standard" technique. Instead, there are a myriad of measuring techniques each with its own set of pros and cons. This review discusses the current state-of-the-art in elastography imaging techniques, i.e., ultrasound and magnetic resonance elastography, as applied to skeletal muscle, and briefly reviews other methods of measuring muscle mechanical behavior in vivo. While in vivo muscle viscoelastic properties can be measured, these techniques are largely limited to static or quasistatic measurements. Emerging elastography techniques are able to quantify muscle anisotropy and large deformation effects on stiffness, but, validation and optimization of these newer techniques is required. The development of reliable values for the mechanical properties of muscle across the population using these techniques are required to enable them to become more useful in rehabilitation and other clinical settings.

ESTIMATION OF MUSCLE FORCE DERIVED FROM IN VIVO MR ELASTOGRAPHY TESTS: A PRELIMINARY STUDY

The objective is to estimate the vastus medialis (VM) muscle force from multifrequency magnetic resonance elastography (MMRE) tests and two different rheological models (Voigt and springpot). Healthy participants (N = 13) underwent multifrequency (70, 90 and 110 Hz) magnetic resonance elastography MMRE tests. Thus, in vivo experimental elastic (μ) properties of the VM in passive and active (20% MVC) conditions were characterized. Moreover, the muscle viscosity (η) was determined with Voigt and springpot rheological models, in both muscle states. Subsequently, the VM muscle forces were calculated with a generic musculoskeletal model (OpenSIM) where the active and passive shear moduli (μ) were implemented. The viscosity measured with the two rheological models increased when the muscle is contracted. During the stance and the swing phases, the VM tensile forces decrease and the VM force was lower with the springpot model. It can be noted that during the swing phase, the muscle forces estimated from springpot model showed a higher standard deviation compared to the Voigt model. This last result may indicate a strong sensitivity of the muscle force to the change of active and passive contractile components in the swing phase of gait. This study provides for the first time an estimation of the muscle tensile forces for lower limb, during human motion, from in vivo experimental muscle mechanical properties. The assessment of individualized muscle forces during motion is valuable for finite element models, increasing the patient specific parameters. This novel muscle database will be of use for the clinician to better elucidate the muscle pathophysiology and to better monitor the effects of the muscle disease.

IN VIVOCHARACTERIZATION OF THE MUSCLE VISCOELASTICITY IN PASSIVE AND ACTIVE CONDITIONS USING MULTIFREQUENCY MR ELASTOGRAPHY

This study aims to develop a viscoelastic database for muscles (VM: vastus medialis and Sr: sartorius) and subcutaneous adipose tissue with multifrequency magnetic resonance elastography (MMRE) coupled with rheological models. MMRE was performed on 13 subjects, at 70-90-110 Hz, to experimentally assess the elastic properties (μ) of passive and active (20% MVC) muscles. Then, numerical shear modulus (μ) and viscosity (η) were calculated using three rheological models (Voigt, Zener, Springpot). The elastic properties, obtained with the Springpot model, were closer to the experimental data for the different physiological tissues (μSpringpot_VM_Passive= 3.67 ± 0.71 kPa, μSpringpot_Sr= 6.89 ± 1.27 kPa, μSpringpot_Adipose Tissue= 1.61 ± 0.37 kPa) and at different muscle states (μSpringpot_VM_20%MVC= 11.29 ± 1.04 kPa). The viscosity parameter increased with the level of contraction (η_VM_ Passive_Springpot= 4.5 ± 1.64 Pa.s versus η_VM_20% MVC_Springpot= 12.14 ± 1.47 Pa.s ) and varied with the type of muscle. (η_VM_ Passive_Springpot= 4.5 ± 1.64 Pa.s versus η_Sr_Springpot= 6.63 ± 1.27 Pa.s). Similar viscosities were calculated for all tissues and rheological models. These first physiologically realistic viscoelastic parameters could be used by the physicians to better identify and monitor the effects of muscle disorder, and as a database for musculoskeletal model.

Length and activation dependent variations in muscle shear wave speed

Muscle stiffness is known to vary as a result of a variety of disease states, yet current clinical methods for quantifying muscle stiffness have limitations including cost and availability. We investigated the capability of shear wave elastography (SWE) to measure variations in gastrocnemius shear wave speed induced via active contraction and passive stretch. Ten healthy young adults were tested. Shear wave speeds were measured using a SWE transducer positioned over the medial gastrocnemius at ankle angles ranging from maximum dorsiflexion to maximum plantarflexion. Shear wave speeds were also measured during voluntary plantarflexor contractions at a fixed ankle angle. Average shear wave speed increased significantly from 2.6 to 5.6 m s(-1) with passive dorsiflexion and the knee in an extended posture, but did not vary with dorsiflexion when the gastrocnemius was shortened in a flexed knee posture. During active contractions, shear wave speed monotonically varied with the net ankle moment generated, reaching 8.3 m s(-1) in the maximally contracted condition. There was a linear correlation between shear wave speed and net ankle moment in both the active and passive conditions; however, the slope of this linear relationship was significantly steeper for the data collected during passive loading conditions. The results show that SWE is a promising approach for quantitatively assessing changes in mechanical muscle loading. However, the differential effect of active and passive loading on shear wave speed makes it important to carefully consider the relevant loading conditions in which to use SWE to characterize in vivo muscle properties.

Review of MR elastography applications and recent developments

The technique of MR elastography (MRE) has emerged as a useful modality for quantitatively imaging the mechanical properties of soft tissues in vivo. Recently, MRE has been introduced as a clinical tool for evaluating chronic liver disease, but many other potential applications are being explored. These applications include measuring tissue changes associated with diseases of the liver, breast, brain, heart, and skeletal muscle including both focal lesions (e.g., hepatic, breast, and brain tumors) and diffuse diseases (e.g., fibrosis and multiple sclerosis). The purpose of this review article is to summarize some of the recent developments of MRE and to highlight some emerging applications.

Multifrequency inversion in magnetic resonance elastography

Time-harmonic shear wave elastography is capable of measuring viscoelastic parameters in living tissue. However, finite tissue boundaries and waveguide effects give rise to wave interferences which are not accounted for by standard elasticity reconstruction methods. Furthermore, the viscoelasticity of tissue causes dispersion of the complex shear modulus, rendering the recovered moduli frequency dependent. Therefore, we here propose the use of multifrequency wave data from magnetic resonance elastography (MRE) for solving the inverse problem of viscoelasticity reconstruction by an algebraic least-squares solution based on the springpot model. Advantages of the method are twofold: (i) amplitude nulls appearing in single-frequency standing wave patterns are mitigated and (ii) the dispersion of storage and loss modulus with drive frequency is taken into account by the inversion procedure, thereby avoiding subsequent model fitting. As a result, multifrequency inversion produces fewer artifacts in the viscoelastic parameter map than standard single-frequency parameter recovery and may thus support image-based viscoelasticity measurement. The feasibility of the method is demonstrated by simulated wave data and MRE experiments on a phantom and in vivo human brain. Implemented as a clinical method, multifrequency inversion may improve the diagnostic value of time-harmonic MRE in a large variety of applications.

Supersonic shear imaging provides a reliable measurement of resting muscle shear elastic modulus

The aim of the present study was to assess the reliability of shear elastic modulus measurements performed using supersonic shear imaging (SSI) in nine resting muscles (i.e. gastrocnemius medialis, tibialis anterior, vastus lateralis, rectus femoris, triceps brachii, biceps brachii, brachioradialis, adductor pollicis obliquus and abductor digiti minimi) of different architectures and typologies. Thirty healthy subjects were randomly assigned to the intra-session reliability (n = 20), inter-day reliability (n = 21) and the inter-observer reliability (n = 16) experiments. Muscle shear elastic modulus ranged from 2.99 (gastrocnemius medialis) to 4.50 kPa (adductor digiti minimi and tibialis anterior). On the whole, very good reliability was observed, with a coefficient of variation (CV) ranging from 4.6% to 8%, except for the inter-operator reliability of adductor pollicis obliquus (CV = 11.5%). The intraclass correlation coefficients were good (0.871 ± 0.045 for the intra-session reliability, 0.815 ± 0.065 for the inter-day reliability and 0.709 ± 0.141 for the inter-observer reliability). Both the reliability and the ease of use of SSI make it a potentially interesting technique that would be of benefit to fundamental, applied and clinical research projects that need an accurate assessment of muscle mechanical properties.

Stiffness imaging of the kidney and adjacent abdominal tissues measured simultaneously using magnetic resonance elastography

To date, non-invasive methods to detect kidney malignancies and mild tumors remain a challenge. The purpose of this study was to establish the proper imaging protocol to determine kidney stiffness and its spatial distribution within the various kidney compartments such as the renal sinus, medulla, and cortex. Here, we have used magnetic resonance elastography (MRE) along with coronal oblique acquisition to simultaneously measure kidney stiffness in comparison with other tissues including the liver, spleen, and psoas.