Diffusion tensor MRI to assess damage in healthy and dystrophic skeletal muscle after lengthening contractions

Significant age-related differences between lower leg muscles of older and younger female subjects detected by ultrashort echo time magnetization transfer modeling

Magnetization transfer (MT) magnetic resonance imaging (MRI) can be used to estimate the fraction of water and macromolecular proton pools in tissues. MT modeling paired with ultrashort echo time acquisition (UTE-MT modeling) has been proposed to improve the evaluation of the myotendinous junction and fibrosis in muscle tissues, which the latter increases with aging. This study aimed to determine if the UTE-MT modeling technique is sensitive to age-related changes in the skeletal muscles of the lower leg. Institutional review board approval was obtained, and all recruited subjects provided written informed consent. The legs of 31 healthy younger (28.1 ± 6.1 years old, BMI = 22.3 ± 3.5) and 20 older (74.7 ± 5.5 years old, BMI = 26.7 ± 5.9) female subjects were imaged using UTE sequences on a 3 T MRI scanner. MT ratio (MTR), macromolecular fraction (MMF), macromolecular T2 (T2-MM), and water T2 (T2-W) were calculated using UTE-MT modeling for the anterior tibialis (ATM), posterior tibialis (PTM), soleus (SM), and combined lateral muscles. Results were compared between groups using the Wilcoxon rank sum test. Three independent observers selected regions of interest (ROIs) and processed UTE-MRI images separately, and the intraclass correlation coefficient (ICC) was calculated for a reproducibility study. Significantly lower mean MTR and MMF values were present in the older compared with the younger group in all studied lower leg muscles. T2-MM showed significantly lower values in the older group only for PTM and SM muscles. In contrast, T2-W showed significantly higher values in the older group. The age-related differences were more pronounced for MMF (-17 to -19%) and T2-W (+20 to 47%) measurements in all muscle groups compared with other investigated MR measures. ICCs were higher than 0.93, indicating excellent consistency between the ROI selection and MRI measurements of independent readers. As demonstrated by significant differences between younger and older groups, this research emphasizes the potential of UTE-MT MRI techniques in evaluating age-related skeletal muscle changes.

The impact of diffusion tensor imaging tractography settings on muscle fascicle architecture and diffusion parameter estimates: Tract length, completion, and curvature are most sensitive to tractography settings

Diffusion-tensor (DT)-MRI tractography provides information about properties relevant to muscle health and function, including estimates of architectural properties such as fascicle length, pennation angle, and curvature and diffusion properties such as mean diffusivity (MD) and fractional anisotropy (FA). Tractography settings, including integration algorithms, thresholds for early tract termination, and tract smoothing approaches, impact the accuracy of the muscle property estimates. However, muscle DT-MRI tractography is performed using a variety of these settings, complicating comparisons between different studies. The effects of different tractography settings on muscle architecture estimates have not been fully explored, and optimized settings for muscle tractography have not yet been determined. We examined the influence of integration algorithm and termination check settings combined with a range of step sizes, termination criteria, and smoothing polynomial orders on tract characteristics, completion/reason for termination, and goodness of fit between fiber tracts and smoothing polynomials using 3-T DT-MR images of the lower leg muscles of seven healthy adults. We found that tract length and completion were highly sensitive to strict FA and intersegment angle thresholds (25%-69% reduction in complete fiber tracts from lowest to highest minimum FA threshold and 11%-36% reduction from highest to lowest intersegment angle threshold). Higher order polynomials (third and fourth order vs. second order) better fit the muscle fiber trajectories, but curvature estimates were highly sensitive to smoothing polynomial order (3.9-6.6 m-1 increase for second- vs. fourth-order fitting polynomials). Step size impacted curvature estimates, albeit to a lesser degree. Integration algorithm had little impact, and mean pennation angle, and tract-based FA and MD, were relatively insensitive to all parameters. The results demonstrate which muscle diffusion measures and architectural estimates are most sensitive to varying tractography settings and support the need for consistent reporting of tractography details to aid interpretation and comparison of results between studies.

Diffusion tensor imaging reveals subclinical alterations in muscles of patients with Becker muscular dystrophy

OBJECTIVES

Becker muscular dystrophy (BMD) is a relatively less investigated neuromuscular disease, partially overlapping the phenotype of Duchenne dystrophy (DMD). Physiopathological and anatomical patterns are still not comprehensively known, despite recent effort in the search of early biomarkers. Aim of this study was to selectively compare normal appearing muscles of BMD with healthy controls.

METHODS

Among a pool of 40 BMD patients and 20 healthy controls, Sartorius and gracilis muscles were selected on the basis of a blinded clinical quantitative/qualitative evaluation, if classified as normal (0 or 1 on Mercuri scale) and subsequently segmented on diffusion tensor MRI scans with a tractographic approach. Diffusion derived parameters were extracted.

RESULTS

Non-parametric testing revealed significant differences between normal and normal appearing BMD derived parameters in both muscles, the difference being more evident in sartorius. Bonferroni-corrected P-values (<.05) of Mann-Whitney test could discriminate between BMD and controls for standard deviation of all diffusion parameters (mean diffusivity, fractional anisotropy, axial and radial diffusivity) in both sartorius and gracilis, while in sartorius the significant difference was found also in the average values of the same parameters (with exception of RD).

CONCLUSIONS

This method could identify microstructural alterations in BMD normal appearing sartorius and gracilis.

ADVANCES IN KNOWLEDGE

Diffusion based MRI could be able to identify possible early or subclinical microstructural alterations in dystrophic patients with BMD.

Changes in lumbar muscle diffusion tensor indices with age

Objective

To investigate differences in diffusion tensor imaging (DTI) parameters and proton density fat fraction (PDFF) in the spinal muscles of younger and older adult males.

Methods

Twelve younger (19-30 years) and 12 older (61-81years) healthy, physically active male participants underwent T1W, T2W, Dixon and DTI of the lumbar spine. The eigenvalues (λ1, λ2, and λ3), fractional anisotropy (FA), and mean diffusivity (MD) from the DTI together with the PDFF were determined in the multifidus, medial and lateral erector spinae (ESmed, ESlat), and quadratus lumborum (QL) muscles. A two-way ANOVA was used to investigate differences with age and muscle and t-tests for differences in individual muscles with age.

Results

The ANOVA gave significant differences with age for all DTI parameters and the PDFF (P < .01) and with muscle (P < .01) for all DTI parameters except for λ1 and for the PDFF. The mean of the eigenvalues and MD were lower and the FA higher in the older age group with differences reaching statistical significance for all DTI measures for ESlat and QL (P < .01) but only in ESmed for λ3 and MD (P < .05).

Conclusions

Differences in DTI parameters of muscle with age result from changes in both in the intra- and extra-cellular space and cannot be uniquely explained in terms of fibre length and diameter.

Advances in knowledge

Previous studies looking at age have used small groups with uneven age spacing. Our study uses two well defined and separated age groups.

Advanced muscle imaging in adolescent elite rowers utilizing diffusion tensor imaging: Association of imaging findings with stroke typology

PURPOSE

Muscular overuse injuries are a common health issue in elite athletes. Changes in the muscular microenvironment can be depicted by Diffusion Tensor Imaging (DTI). We hypothesize that the biomechanics of different stroke typologies plays a role in muscle injury and tested our hypothesis by magnetic resonance imaging (MRI) examination of the lumbar spine muscles of adolescent rowers utilizing DTI.

METHODS AND MATERIALS

Twenty-two male elite rowers (12 sweep, 10 scull rowers) with a mean age of 15.8 ± 1.2 years underwent 3-Tesla MRI of the lumbar spine 6 hours after cessation of training. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were calculated for the erector spinae and multifidus muscle. Student's t-test was used to test differences of DTI parameters between sweep and scull rowers and a Pearson correlation was utilized to correlate the parameters to training volume.

RESULTS

ADC values in the erector spinae and multifidus muscle were significantly higher (p = 0.039) and FA values significantly lower (p < 0.001) in sweep rowers compared to scull rowers. There was no significant association between DTI parameters and training volume (r ≤ -0.459, p ≥ 0.074).

CONCLUSIONS

Our DTI results show that lumbar spine muscle diffusivity is higher in sweep rowers than in scull rowers. Altered muscle diffusivity is suggestive of microscopic tissue disruption and might be attributable to biomechanical differences between stroke typologies.

Clinical applications of skeletal muscle diffusion tensor imaging

Diffusion tensor imaging (DTI) may allow the determination of new threshold values, based on water anisotropy, to differentiate between healthy muscle and various pathological processes. Additionally, it may quantify treatment monitoring or training effects. Most current studies have evaluated the potential of DTI of skeletal muscle to assess sports-related injuries or therapy, and training monitoring. Another critical area of application of this technique is the characterization and monitoring of primary and secondary myopathies. In this manuscript, we review the application of DTI in the evaluation of skeletal muscle in these and other novel clinical scenarios, with emphasis on the use of quantitative imaging-derived biomarkers. Finally, the main limitations of the introduction of DTI in the clinical setting and potential areas of future use are discussed.

DTI assessment of the maturing growth plate of the knee in adolescents and young adults

PURPOSE

To assess the growth plates of the knee in a healthy population of young adults and adolescents using DTI, and to correlate the findings with chronological age and skeletal maturation.

METHODS

A prospective, cross-sectional study to assess the tibial and femoral growth plates with DTI in 155 healthy volunteers aged between 14.0 and 21 years old. Echo-planar DTI with 15 directions and b value of 0 and 600 s/mm² was performed on a 3 T whole-body scanner.

RESULTS

A relationship was observed between chronological age and most DTI metrics (fractional anisotropy, mean diffusivity, and radial diffusivity), tract length and volume. (No significant relationship could be seen for axonal diffusivity and tract length.) Subdivision according to skeletal maturation showed the greatest tract lengths and volumes seen in stage 4b and not 4a. The intra-observer agreement was significant (P = 0.01) for all the measured variables, but agreement varied (femur 0.53 - 0.98; tibia 0.58 - 0.98). Spearman's correlation showed a significant correlation for age (P = 0.05; P = 0.01) as well as for the fractional anisotropy value within all variables in both femur and tibia. Tract number and volume had a similar correlation with most variables, especially the DTI metrics, and would seem to be interchangeable.

CONCLUSION

The current study indicates that DTI metrics could be a tool to assess the skeletal maturation process of the growth plate and its activity. Tractography seems promising to assess the activity of the growth plate in a younger population but must be used with caution in the more mature growth plate.

To Explore the Changes and Differences of Microstructure of Vocal Fold in Vocal Fold Paralysis and Cricoarytenoid Joint Dislocation by Diffusion Tensor Imaging

OBJECTIVE

The diffusion characteristics of water molecules were measured in the vocal folds of canines exhibiting unilateral vocal fold paralysis and unilateral cricoarytenoid joint dislocation. These characteristics were used in conjunction with a histological examination of the microstructural changes of vocal fold muscle fibers to explore the feasibility of diffusion tensor imaging (DTI) in distinguishing unilateral vocal fold paralysis and unilateral cricoarytenoid joint dislocation as well as evaluating microstructural changes.

METHODS

Ten beagles were randomly divided into three groups: four in the unilateral vocal fold paralysis group, four in the unilateral cricoarytenoid joint dislocation group, and two in the normal group. Unilateral recurrent laryngeal nerve resection was performed in the vocal fold paralysis group. Unilateral cricoarytenoid joint dislocation surgery was performed in the dislocation group. No intervention was performed in the normal group. Four months postintervention, the larynges were excised and put into a magnetic resonance imaging (MRI) system (9.4T BioSpec MRI, Bruker, German) for scanning, followed by an analysis of diffusion parameters among the different groups for statistical significance. After MRI scanning, the vocal folds were cut into sections, stained with hematoxylin and eosin, and scanned digitally. The mean cross-sectional area of muscle fibers, and the mean diameter of muscle fibers in the vocal folds were calculated by target detection and extraction technology. Mean values of each measurement were used to compare the differences among the three groups. Pearson correlation analysis was performed on the DTI parameters and the results from histological section extraction.

RESULTS

The paralysis group had significantly higher Fractional Anisotropy (FA) compared to the dislocation group and normal group (P = 0.004). The paralysis group also had a significantly lower Tensor Trace value compared to the dislocation group and normal group (P = 0.000). The average cross-sectional area of vocal fold muscle fibers in the paralysis group was significantly smaller than the dislocation group and normal group (P = 0.000). Pearson correlation analysis yielded values of, r = -0.785, P = 0.01 between the average cross-sectional area of vocal muscle fibers and FA, and values of r = 0.881, P = 0.00 between Tensor Trace and the average cross-sectional area of vocal muscle.

CONCLUSION

FA and Tensor Trace can be used as effective parameters to reflect the changes of microstructure in vocal fold paralysis and cricoarytenoid joint dislocation. DTI is an objective and quantitative method to effectively evaluate unilateral vocal fold paralysis and unilateral cricoarytenoid joint dislocation, also capable of noninvasively evaluating vocal fold muscle fiber microstructure.

Robust Assessment of Macromolecular Fraction (MMF) in Muscle with Differing Fat Fraction Using Ultrashort Echo Time (UTE) Magnetization Transfer Modeling with Measured T1

Magnetic resonance imaging (MRI) is widely regarded as the most comprehensive imaging modality to assess skeletal muscle quality and quantity. Magnetization transfer (MT) imaging can be used to estimate the fraction of water and macromolecular proton pools, with the latter including the myofibrillar proteins and collagen, which are related to the muscle quality and its ability to generate force. MT modeling combined with ultrashort echo time (UTE-MT modeling) may improve the evaluation of the myotendinous junction and regions with fibrotic tissues in the skeletal muscles, which possess short T2 values and higher bound-water concentration. The fat present in muscle has always been a source of concern in macromolecular fraction (MMF) calculation. This study aimed to investigate the impact of fat fraction (FF) on the estimated MMF in bovine skeletal muscle phantoms embedded in pure fat. MMF was calculated for several regions of interest (ROIs) with differing FFs using UTE-MT modeling with and without T1 measurement and B1 correction. Calculated MMF using measured T1 showed a robust trend, particularly with a negligible error (<3%) for FF < 20%. Around 5% MMF reduction occurred for FF > 30%. However, MMF estimation using a constant T1 was robust only for regions with FF < 10%. The MTR and T1 values were also robust for only FF < 10%. This study highlights the potential of the UTE-MT modeling with accurate T1 measurement for robust muscle assessment while remaining insensitive to fat infiltration up to moderate levels.

Diffusion tensor imaging indices of acute muscle damage are augmented after exercise in peripheral arterial disease

PURPOSE

Although it is known that peripheral arterial disease (PAD) is associated with chronic myopathies, the acute muscular responses to exercise in this population are less clear. This study used diffusion tensor imaging (DTI) to compare acute exercise-related muscle damage between PAD patients and healthy controls.

METHODS

Eight PAD patients and seven healthy controls performed graded plantar flexion in the bore of a 3T MRI scanner. Exercise began at 2 kg and increased by 2 kg every 2 min until failure, or completion of 10 min of exercise. DTI images were acquired from the lower leg pre- and post-exercise, and were analyzed for mean diffusivity, fractional anisotropy (FA), and eigenvalues 1-3 (λ1-3) of the medial gastrocnemius (MG) and tibialis anterior (TA).

RESULTS

Results indicated a significant leg by time interaction for mean diffusivity, explained by a significantly greater increase in diffusivity of the MG in the most affected legs of PAD patients (11.1 × 10-4 ± 0.5 × 10-4 mm2/s vs. 12.7 × 10-4 ± 1.2 × 10-4 mm2/s at pre and post, respectively, P = 0.02) compared to healthy control subjects (10.8 × 10-4 ± 0.3 × 10-4 mm2/s vs. 11.2 × 10-4 ± 0.5 × 10-4 mm2/s at pre and post, respectively, P = 1.0). No significant differences were observed for the TA, or λ1-3 (all P ≥ 0.06). Moreover, no reciprocal changes were observed for FA in either group (all P ≥ 0.29).

CONCLUSION

These data suggest that calf muscle diffusivity increases more in PAD patients compared to controls after exercise. These findings are consistent with the notion that acute exercise results in increased muscle damage in PAD.

Exercise-induced muscle damage: multi-parametric MRI quantitative assessment

BACKGROUND

To explore the value of magnetic resonance quantitative analysis using diffusion tensor imaging, T2 mapping, and intravoxel incoherent motion in the evaluation of eccentric exercise-induced muscle damage and to compare the effects of various eccentric exercise modes at different time points in rats.

METHODS

A total of 174 Sprague-Dawley male rats were randomly divided into five groups: control, once-only exercise, continuous exercise, intermittent exercise, and once-fatigue exercise groups. Each experimental group was divided into seven time-subgroups: 0.5 h, 24 h, 48 h, 72 h, 96 h, 120 h and 168 h after exercise. The quadriceps femoris muscles were then scanned using magnetic resonance imaging. The apparent diffusion coefficient and fractional anisotropy values of diffusion tensor imaging, T2 values of T2 mapping, D and D* values of intravoxel incoherent motion and optical density values of desmin were measured. Associations among different eccentric exercise programmes, magnetic resonance imaging findings, and histopathological results were evaluated. Dunnett's test, two-way repeated measures analysis of variance, and Pearson correlation analysis were used for statistical analysis.

RESULTS

Diffusion tensor imaging showed that the number of muscle fibre bundles decreased to varying degrees with different time points and eccentric exercises. Apparent diffusion coefficient values of the exercise groups showed a trend that first increased and then decreased, the opposite of fractional anisotropy. The specimens in all eccentric exercise programmes showed high signal T2 values after exercise, the highest among which was in the once-fatigue exercise group. D and D* in the experimental groups were significantly higher than those in the control group at 0.5-48 h after exercise. The apparent diffusion coefficient, fractional anisotropy, T2, D and D* values correlated with the optical density values of desmin.

CONCLUSIONS

Diffusion tensor imaging, T2 mapping, and intravoxel incoherent motion technology accurately reflect the degree of skeletal muscle damage and recovery associated with eccentric exercise. The degree of muscle damage was the lowest in the continuous exercise group and the highest in the once-fatigue exercise group, which may provide more information and guidance for the formulation of physical and athletic training programmes.

Using MRI to quantify skeletal muscle pathology in Duchenne muscular dystrophy: A systematic mapping review

There is a great demand for accurate non‐invasive measures to better define the natural history of disease progression or treatment outcome in Duchenne muscular dystrophy (DMD) and to facilitate the inclusion of a large range of participants in DMD clinical trials. This review aims to investigate which MRI sequences and analysis methods have been used and to identify future needs. Medline, Embase, Scopus, Web of Science, Inspec, and Compendex databases were searched up to 2 November 2019, using keywords “magnetic resonance imaging” and “Duchenne muscular dystrophy.” The review showed the trend of using T1w and T2w MRI images for semi‐qualitative inspection of structural alterations of DMD muscle using a diversity of grading scales, with increasing use of T2map, Dixon, and MR spectroscopy (MRS). High‐field (>3T) MRI dominated the studies with animal models. The quantitative MRI techniques have allowed a more precise estimation of local or generalized disease severity. Longitudinal studies assessing the effect of an intervention have also become more prominent, in both clinical and animal model subjects. Quality assessment of the included longitudinal studies was performed using the Newcastle‐Ottawa Quality Assessment Scale adapted to comprise bias in selection, comparability, exposure, and outcome. Additional large clinical trials are needed to consolidate research using MRI as a biomarker in DMD and to validate findings against established gold standards. This future work should use a multiparametric and quantitative MRI acquisition protocol, assess the repeatability of measurements, and correlate findings to histologic parameters.

Differences between muscle from osteoporotic and osteoarthritic subjects: in vitro study by diffusion-tensor MRI and histological findings

BACKGROUND

Osteoarthritis and osteoporosis are strongly coupled with alterations of muscles quality and fats metabolism. However, there are no studies for investigating possible differences between osteoporotic and osteoarthritic muscles. Understanding muscle-bone and muscle-cartilage interactions would be of high clinical value.

AIM

Investigate potential microstructural and physiological differences between osteoporotic and osteoarthritic muscles by diffusion Nuclear Magnetic Resonance (NMR) imaging (diffusion MRI) and histological findings.

METHODS

Vastus-lateralis muscles excised from osteoporotic (n = 26, T Score <  - 2.5, Kellgren-Lawrence ≤ 2) and osteoarthritic (n = 26, T Score >  - 2.5, Kellgren--Lawrence 3 and 4) age-matched women were investigated by NMR relaxometry, diffusion-tensor imaging (DTI) at 9.4 T, and histological techniques. Intramyocellular (IMCL) and extramyocellular (EMCL) lipid were quantified. The percentage and mean diameters of fibers I and II were evaluated. Relationship between mean diffusivity (MD), fractional anisotropy (FA), the DTI eigenvalues (λ₁, λ₂, λ₃), histological findings in muscles and clinical data (Kellgren-Lawrence and T score, age, menopausal age, body mass index) were studied. Pairwise comparisons between groups were made using one-way analysis of variance and correlation between variables was assessed with linear correlation analysis (Pearson's r coefficient).

RESULTS

Osteoporotic muscles showed higher MD, λ_1, λ₂, λ₃ compared to osteoarthritis ones. This is explainable with a significant higher density of IMCL droplets found inside the osteoarthritic muscles and a large amount of fibrotic tissue and IMCL infiltration between fibers, i.e. in endomysium and perimysium that lead to a more hindered diffusion. Furthermore, histological analysis suggests mitochondrial degeneration as the origin of the greatest amount of IMCL droplets in osteoarthritic muscles.

CONCLUSION

This work highlights differences between muscles of osteoporotic and osteoarthritic subjects that can be quantified by NMR DTI investigations.

Muscle phenotype of a rat model of Duchenne muscular dystrophy

INTRODUCTION

Our aim was to assess key muscle imaging and contractility parameters in the Duchenne muscular dystrophy (DMD) rat model (Dmd-KO rat), which have not yet been characterized sufficiently.

METHODS

We performed in-vivo magnetic resonance imaging (MRI) for thigh and leg muscles, and performed hematoxylin and eosin (H&E) staining and in-vivo muscle contractility testing in specific hindlimb muscles.

RESULTS

MRI prior to testing muscle contractility revealed multiple, unevenly distributed focal hyperintensities in the Dmd-KO rat quadriceps and tibialis anterior muscles. H&E staining showed corresponding areas of inflammation and ongoing regeneration. In-vivo contractile testing showed maximal force generated by Dmd-KO muscles was significantly lower, and susceptibility to injury was ~ two-fold greater in the Dmd-KO rats compared to wild-type (WT) rats.

DISCUSSION

Together, the MRI findings, histological findings, and the low strength and high susceptibility to injury in muscles support use of the Dmd-KO rat as an animal model of DMD.

Paraspinal Muscle DTI Metrics Predict Muscle Strength

Background

The paraspinal muscles play an important role in the onset and progression of lower back pain. It would be of clinical interest to identify imaging biomarkers of the paraspinal musculature that are related to muscle function and strength. Diffusion tensor imaging (DTI) enables the microstructural examination of muscle tissue and its pathological changes.

Purpose

To investigate associations of DTI parameters of the lumbar paraspinal muscles with isometric strength measurements in healthy volunteers.

Study Type

Prospective.

Subjects

Twenty‐one healthy subjects (12 male, 9 female; age = 30.1 ± 5.6 years; body mass index [BMI] = 27.5 ± 2.6 kg/m²) were recruited.

Field Strength/Sequence

3 T/single‐shot echo planar imaging (ss‐EPI) DTI in 24 directions; six‐echo 3D spoiled gradient echo sequence for chemical shift encoding‐based water–fat separation.

Assessment

Paraspinal muscles at the lumbar spine were examined. Erector spinae muscles were segmented bilaterally; cross‐sectional area (CSA), proton density fat fraction (PDFF), and DTI parameters were calculated. Muscle flexion and extension maximum isometric torque values [Nm] at the back were measured with an isokinetic dynamometer and the ratio of extension to flexion strength (E/F) calculated.

Statistical Tests

Pearson correlation coefficients; multivariate regression models.

Results

Significant positive correlations were found between the ratio of extension to flexion (E/F) strength and mean diffusivity (MD) (P = 0.019), RD (P = 0.02) and the eigenvalues (λ1: P = 0.026, λ2: P = 0.033, λ3: P = 0.014). In multivariate regression models λ3 of the erector spinae muscle λ3 and gender remained statistically significant predictors of E/F (R²_adj = 0.42, P = 0.003).

Data Conclusion

DTI allowed the identification of muscle microstructure differences related to back muscle function that were not reflected by CSA and PDFF. DTI may potentially track subtle changes of back muscle tissue composition.

Level of Evidence: 3

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2019;50:816–823.

Muscle Architecture Assessment: Strengths, Shortcomings and New Frontiers of in Vivo Imaging Techniques

Skeletal muscle structural assembly (and its remodeling in response to loading-unloading states) can be investigated macroscopically by assessing muscle architecture, described as fascicle geometric disposition within the muscle. Over recent decades, various medical imaging techniques have been developed to facilitate the in vivo assessment of muscle architecture. However, the main advantages and limitations of these methodologies have been fragmentally discussed. In the present article, the main techniques used for the evaluation of muscle architecture are presented: conventional B-mode ultrasonography, extended-field-of-view ultrasound, 3-D ultrasound and magnetic resonance imaging-based diffusion tensor imaging. By critically discussing potentials and shortcomings of each methodology, we aim to provide readers with an overview of both established and new techniques for the in vivo assessment of muscle architecture. This review may serve as decision guidance facilitating selection of the appropriate technique to be applied in biomedical research or clinical routine.

Diet-Induced Obesity Affects Muscle Regeneration After Murine Blunt Muscle Trauma-A Broad Spectrum Analysis

Injury to skeletal muscle affects millions of people worldwide. The underlying regenerative process however, is a very complex mechanism, time-wise highly coordinated, and subdivided in an initial inflammatory, a regenerative and a remodeling phase. Muscle regeneration can be impaired by several factors, among them diet-induced obesity (DIO). In order to evaluate if obesity negatively affects healing processes after trauma, we utilized a blunt injury approach to damage the extensor iliotibialis anticus muscle on the left hind limb of obese and normal weight C57BL/6J without showing any significant differences in force input between normal weight and obese mice. Magnetic resonance imaging (MRI) of the injury and regeneration process revealed edema formation and hemorrhage exudate in muscle tissue of normal weight and obese mice. In addition, morphological analysis of physiological changes revealed tissue necrosis, immune cell infiltration, extracellular matrix (ECM) remodeling, and fibrosis formation in the damaged muscle tissue. Regeneration was delayed in muscles of obese mice, with a higher incidence of fibrosis formation due to hampered expression levels of genes involved in ECM organization. Furthermore, a detailed molecular fingerprint in different stages of muscle regeneration underlined a delay or even lack of a regenerative response to injury in obese mice. A time-lapse heatmap determined 81 differentially expressed genes (DEG) with at least three hits in our model at all-time points, suggesting key candidates with a high impact on muscle regeneration. Pathway analysis of the DEG revealed five pathways with a high confidence level: myeloid leukocyte migration, regulation of tumor necrosis factor production, CD4-positive, alpha-beta T cell differentiation, ECM organization, and toll-like receptor (TLR) signaling. Moreover, changes in complement-, Wnt-, and satellite cell-related genes were found to be impaired in obese animals after trauma. Furthermore, histological satellite cell evaluation showed lower satellite cell numbers in the obese model upon injury. Ankrd1, C3ar1, Ccl8, Mpeg1, and Myog expression levels were also verified by qPCR. In summary, increased fibrosis formation, the reduction of Pax7⁺ satellite cells as well as specific changes in gene expression and signaling pathways could explain the delay of tissue regeneration in obese mice post trauma.

Normalized STEAM-based diffusion tensor imaging provides a robust assessment of muscle tears in football players: preliminary results of a new approach to evaluate muscle injuries

Objectives

To assess acute muscle tears in professional football players by diffusion tensor imaging (DTI) and evaluate the impact of normalization of data.

Methods

Eight football players with acute lower limb muscle tears were examined. DTI metrics of the injured muscle and corresponding healthy contralateral muscle and of ROIs drawn in muscle tears (ROI_tear) in the corresponding healthy contralateral muscle (ROI_{hc_t}) in a healthy area ipsilateral to the injury (ROI_hi) and in a corresponding contralateral area (ROI_{hc_i}) were compared. The same comparison was performed for ratios of the injured (ROI_tear/ROI_hi) and contralateral sides (ROI_{hc_t}/ROI_{hc_i}). ANOVA, Bonferroni-corrected post-hoc and Student’s t-tests were used.

Results

Analyses of the entire muscle did not show any differences (p>0.05 each) except for axial diffusivity (AD; p=0.048). ROI_tear showed higher mean diffusivity (MD) and AD than ROI_{hc_t} (p<0.05). Fractional anisotropy (FA) was lower in ROI_tear than in ROI_hi and ROI_{hc_t} (p<0.05). Radial diffusivity (RD) was higher in ROI_tear than in any other ROI (p<0.05). Ratios revealed higher MD and RD and lower FA and reduced number and length of fibre tracts on the injured side (p<0.05 each).

Conclusions

DTI allowed a robust assessment of muscle tears in athletes especially after normalization to healthy muscle tissue.

Key Points

• STEAM-based DTI allows the investigation of muscle tears affecting professional football players.

• Fractional anisotropy and mean diffusivity differ between injured and healthy muscle areas.

• Only normalized data show differences of fibre tracking metrics in muscle tears.

• The normalization of DTI-metrics enables a more robust characterization of muscle tears.

Diffusion Properties and 3D Architecture of Human Lower Leg Muscles Assessed with Ultra-High-Field-Strength Diffusion-Tensor MR Imaging and Tractography: Reproducibility and Sensitivity to Sex Difference and Intramuscular Variability

Purpose To demonstrate the reproducibility of the diffusion properties and three-dimensional structural organization measurements of the lower leg muscles by using diffusion-tensor imaging (DTI) assessed with ultra-high-field-strength (7.0-T) magnetic resonance (MR) imaging and tractography of skeletal muscle fibers. On the basis of robust statistical mapping analyses, this study also aimed at determining the sensitivity of the measurements to sex difference and intramuscular variability. Materials and Methods All examinations were performed with ethical review board approval; written informed consent was obtained from all volunteers. Reproducibility of diffusion tensor indexes assessment including eigenvalues, mean diffusivity, and fractional anisotropy (FA) as well as muscle volume and architecture (ie, fiber length and pennation angle) were characterized in lower leg muscles (n = 8). Intramuscular variability and sex differences were characterized in young healthy men and women (n = 10 in each group). Student t test, statistical parametric mapping, correlation coefficients (Spearman rho and Pearson product-moment) and coefficient of variation (CV) were used for statistical data analysis. Results High reproducibility of measurements (mean CV ± standard deviation, 4.6% ± 3.8) was determined in diffusion properties and architectural parameters. Significant sex differences were detected in FA (4.2% in women for the entire lower leg; P = .001) and muscle volume (21.7% in men for the entire lower leg; P = .008), whereas architecture parameters were almost identical across sex. Additional differences were found independently of sex in diffusion properties and architecture along several muscles of the lower leg. Conclusion The high-spatial-resolution DTI assessed with 7.0-T MR imaging allows a reproducible assessment of structural organization of superficial and deep muscles, giving indirect information on muscle function. ^©RSNA, 2018 Online supplemental material is available for this article.

Impaired contractile function of the supraspinatus in the acute period following a rotator cuff tear

Background

Rotator cuff (RTC) tears are a common clinical problem resulting in adverse changes to the muscle, but there is limited information comparing histopathology to contractile function. This study assessed supraspinatus force and susceptibility to injury in the rat model of RTC tear, and compared these functional changes to histopathology of the muscle.

Methods

Unilateral RTC tears were induced in male rats via tenotomy of the supraspinatus and infraspinatus. Maximal tetanic force and susceptibility to injury of the supraspinatus muscle were measured in vivo at day 2 and day 15 after tenotomy. Supraspinatus muscles were weighed and harvested for histologic analysis of the neuromuscular junction (NMJ), intramuscular lipid, and collagen.

Results

Tenotomy resulted in eventual atrophy and weakness. Despite no loss in muscle mass at day 2 there was a 30% reduction in contractile force, and a decrease in NMJ continuity and size. Reduced force persisted at day 15, a time point when muscle atrophy was evident but NMJ morphology was restored. At day 15, torn muscles had decreased collagen-packing density and were also more susceptible to contraction-induced injury.

Conclusion

Muscle size and histopathology are not direct indicators of overall RTC contractile health. Changes in NMJ morphology and collagen organization were associated with changes in contractile function and thus may play a role in response to injury. Although our findings are limited to the acute phase after a RTC tear, the most salient finding is that RTC tenotomy results in increased susceptibility to injury of the supraspinatus.

Diffusion tensor imaging in the musculoskeletal and peripheral nerve systems: from experimental to clinical applications

Magnetic resonance imaging (MRI) is a well-established imaging modality which is used in all districts of the musculoskeletal and peripheral nerve systems. More recently, initial studies have applied multiparametric MRI to evaluate quantitatively different aspects of musculoskeletal and peripheral nerve diseases, thus providing not only images but also numbers and clinical data. Besides ¹H and ³¹P magnetic resonance spectroscopy, diffusion-weighted imaging (DWI) and blood oxygenation level-dependent imaging, diffusion tensor imaging (DTI) is a relatively new MRI-based technique relying on principles of DWI, which has traditionally been used mainly for evaluating the central nervous system to track fibre course. In the musculoskeletal and peripheral nerve systems, DTI has been mostly used in experimental settings, with still few indications in clinical practice. In this review, we describe the potential use of DTI to evaluate different musculoskeletal and peripheral nerve conditions, emphasising the translational aspects of this technique from the experimental to the clinical setting.

Magnetic Resonance Monitoring of Disease Progression in mdx Mice on Different Genetic Backgrounds

Genetic modifiers alter disease progression in both preclinical models and subjects with Duchenne muscular dystrophy (DMD). Using multiparametric magnetic resonance (MR) techniques, we compared the skeletal and cardiac muscles of two different dystrophic mouse models of DMD, which are on different genetic backgrounds, the C57BL/10ScSn-Dmdmdx (B10-mdx) and D2.B10-Dmdmdx (D2-mdx). The proton transverse relaxation constant (T₂) using both MR imaging and spectroscopy revealed significant age-related differences in dystrophic skeletal and cardiac muscles as compared with their age-matched controls. D2-mdx muscles demonstrated an earlier and accelerated decrease in muscle T₂ compared with age-matched B10-mdx muscles. Diffusion-weighted MR imaging indicated differences in the underlying muscle structure between the mouse strains. The fractional anisotropy, mean diffusion, and radial diffusion of water varied significantly between the two dystrophic strains. Muscle structural differences were confirmed by histological analyses of the gastrocnemius, revealing a decreased muscle fiber size and increased fibrosis in skeletal muscle fibers of D2-mdx mice compared with B10-mdx and control. Cardiac involvement was also detected in D2-mdx myocardium based on both decreased function and myocardial T₂. These data indicate that MR parameters may be used as sensitive biomarkers to detect fibrotic tissue deposition and fiber atrophy in dystrophic strains.

Non-invasive assessment of muscle injury in healthy and dystrophic animals with electrical impedance myography

INTRODUCTION

Dystrophic muscle is particularly susceptible to eccentric contraction-induced injury. We tested the hypothesis that electrical impedance myography (EIM) can detect injury induced by maximal-force lengthening contractions.

METHODS

We induced injury in the quadriceps of wild-type (WT) and dystrophic (mdx) mice with eccentric contractions using an established model.

RESULTS

mdx quadriceps had significantly greater losses in peak twitch and tetany compared with losses in WT quadriceps. Injured muscle showed a significant increase in EIM characteristic frequency in both WT (177 ± 7.7%) and mdx (167 ± 7.8%) quadriceps. EIM also revealed decreased extracellular resistance for both WT and mdx quadriceps after injury.

DISCUSSION

Our results show overall agreement between muscle function and EIM measurements of injured muscle, indicating that EIM is a viable tool to assess injury in dystrophic muscle. Muscle Nerve 56: E85-E94, 2017.

Quantitative Magnetic Resonance Imaging of Skeletal Muscle Disease

Quantitative magnetic resonance imaging (qMRI) describes the development and use of MRI to quantify physical, chemical, and/or biological properties of living systems. Neuromuscular diseases often exhibit a temporally varying, spatially heterogeneous, and multi-faceted pathology. The goal of this protocol is to characterize this pathology using qMRI methods. The MRI acquisition protocol begins with localizer images (used to locate the position of the body and tissue of interest within the MRI system), quality control measurements of relevant magnetic field distributions, and structural imaging for general anatomical characterization. The qMRI portion of the protocol includes measurements of the longitudinal and transverse relaxation time constants (T1 and T2, respectively). Also acquired are diffusion-tensor MRI data, in which water diffusivity is measured and used to infer pathological processes such as edema. Quantitative magnetization transfer imaging is used to characterize the relative tissue content of macromolecular and free water protons. Lastly, fat-water MRI methods are used to characterize fibro-adipose tissue replacement of muscle. In addition to describing the data acquisition and analysis procedures, this paper also discusses the potential problems associated with these methods, the analysis and interpretation of the data, MRI safety, and strategies for artifact reduction and protocol optimization.

Fractional anisotropy and diffusivity changes in thyroid-associated orbitopathy

INTRODUCTION

To investigate the extraocular muscle (EOM) changes in thyroid-associated orbitopathy (TAO) on DTI and the correlations between DTI parameters and clinical features.

METHODS

Twenty TAO patients and 20 age- and sex-matched controls provided informed consent and were enrolled. Ten-directional DTI was acquired in orbit. Fractional anisotropy (FA), mean, axial, and radial diffusivities were obtained at medial and lateral EOMs in both orbits. EOM thickness was measured in patients using axial CT images. FA and diffusivities were compared between patients and controls. The relationships between DTI values and muscle thickness and exophthalmos were evaluated. DTI values compared between patients in active and inactive phases by clinical activity score of TAO. DTI values were also compared between acute and chronic stages by the duration of disease.

RESULTS

In medial EOM, FA was significantly lower in patients (p < 0.001) and negatively correlated with muscle thickness (r = -0.604, p < 0.001). Radial diffusivity was significantly higher in patients (p = 0.010) and correlated with muscle thickness (r = 0.349, p = 0.027). In lateral EOM, DTI values did not differ between patients and controls. In the acute stage, the diffusivities of the medial rectus EOM were increased compared with the chronic stage. DTI values of the medial and lateral rectus EOM did not differ significantly between active and inactive phases.

CONCLUSION

DTI can be used to diagnose TAO with FA and radial diffusivity change in EOM. Diffusivities can be used to differentiate acute and chronic stage of TAO. However, DTI values showed limitation in reflecting TAO activity according to the CAS.

Loss of electrical anisotropy is an unrecognized feature of dystrophic muscle that may serve as a convenient index of disease status

OBJECTIVE

We sought to understand the alteration in the anisotropic, or direction dependent, character of muscle as measured by electrical impedance myography (EIM) in subjects with Duchenne muscular dystrophy (DMD) and its potential to serve as a biomarker of disease status.

METHODS

Thirty-six boys with DMD and 27 healthy controls were measured with EIM, with electrical current applied both parallel and perpendicular to the major muscle fiber direction. In addition, muscle extracted from 10 mdx and 10 wild-type mice were measured analogously.

RESULTS

Normalized reactance anisotropy, a direction-dependent measure of membrane charge storage capability, was significantly lower in the four muscles of DMD subjects as compared to controls (p<0.01). Normalized reactance anisotropy also decreased with increasing age in DMD subjects (r=-0.36, p=0.031), but not in healthy boys. Analogous changes were observed in mdx mouse gastrocnemius as compared to wild type (p=0.019).

CONCLUSION

These results support that loss of electrical anisotropy is a previously unrecognized feature of dystrophic muscle.

SIGNIFICANCE

Anisotropic alterations may offer novel indices to assist in neuromuscular disease diagnosis and to serve as easy-to-obtain biomarkers in clinical therapeutic trials.

A method to test contractility of the supraspinatus muscle in mouse, rat, and rabbit

The rotator cuff (RTC) muscles not only generate movement but also provide important shoulder joint stability. RTC tears, particularly in the supraspinatus muscle, are a common clinical problem. Despite some biological healing after RTC repair, persistent problems include poor functional outcomes with high retear rates after surgical repair. Animal models allow further exploration of the sequela of RTC injury such as fibrosis, inflammation, and fatty infiltration, but there are few options regarding contractility for mouse, rat, and rabbit. Histological findings can provide a "direct measure" of damage, but the most comprehensive measure of the overall health of the muscle is contractile force. However, information regarding normal supraspinatus size and contractile function is scarce. Animal models provide the means to compare muscle histology, imaging, and contractility within individual muscles in various models of injury and disease, but to date, most testing of animal contractile force has been limited primarily to hindlimb muscles. Here, we describe an in vivo method to assess contractility of the supraspinatus muscle and describe differences in methods and representative outcomes for mouse, rat, and rabbit.

Techniques and applications of skeletal muscle diffusion tensor imaging: A review

Diffusion tensor imaging (DTI) is increasingly applied to study skeletal muscle physiology, anatomy, and pathology. The reason for this growing interest is that DTI offers unique, noninvasive, and potentially diagnostically relevant imaging readouts of skeletal muscle structure that are difficult or impossible to obtain otherwise. DTI has been shown to be feasible within most skeletal muscles. DTI parameters are highly sensitive to patient-specific properties such as age, body mass index (BMI), and gender, but also to more transient factors such as exercise, rest, pressure, temperature, and relative joint position. However, when designing a DTI study one should not only be aware of sensitivity to the above-mentioned factors but also the fact that the DTI parameters are dependent on several acquisition parameters such as echo time, b-value, and diffusion mixing time. The purpose of this review is to provide an overview of DTI studies covering the technical, demographic, and clinical aspects of DTI in skeletal muscles. First we will focus on the critical aspects of the acquisition protocol. Second, we will cover the reported normal variance in skeletal muscle diffusion parameters, and finally we provide an overview of clinical studies and reported parameter changes due to several (patho-)physiological conditions.

Assessment of aged mdx mice by electrical impedance myography and magnetic resonance imaging

INTRODUCTION

Similar to magnetic resonance imaging (MRI), electrical impedance myography (EIM) is dependent on the presence and location of water in muscle to assess neuromuscular diseases. We compared the 2 technologies in mdx mice to better understand their relationship.

METHODS

EIM and MRI, using T2 relaxation and diffusion-weighted imaging (DWI), were performed on the gastrocnemius of 10 mdx and 10 wild-type mice. Muscle function and tissue composition measurements were compared with the EIM and MRI data.

RESULTS

EIM reactance and T2 relaxation mapping can discriminate healthy from diseased mice (P <  .001 for both), but DWI could not. Both T2 relaxation and EIM reactance also correlated closely with muscle function/composition and with each other.

CONCLUSION

Given the low cost of EIM and the simplicity of application, it may be a valuable alternative to muscle MRI in Duchenne muscular dystrophy, where simple cumulative indices of muscle health are being sought.

Dynamic diffusion-tensor measurements in muscle tissue using the single-line multiple-echo diffusion-tensor acquisition technique at 3T

When diffusion biomarkers display transient changes, i.e. in muscle following exercise, traditional diffusion-tensor imaging (DTI) methods lack the temporal resolution to resolve the dynamics. This article presents an MRI method for dynamic diffusion-tensor acquisitions on a clinical 3T scanner. This method, the Single-Line Multiple-Echo Diffusion-Tensor Acquisition Technique (SL-MEDITATE), achieves a high temporal resolution (4 s) by rapid diffusion encoding through the acquisition of multiple echoes with unique diffusion sensitization and limiting the readout to a single line volume. The method is demonstrated in a rotating anisotropic phantom, a flow phantom with adjustable flow speed and in vivo skeletal calf muscle of healthy volunteers following a plantar flexion exercise. The rotating and flow-varying phantom experiments show that SL-MEDITATE correctly identifies the rotation of the first diffusion eigenvector and the changes in diffusion-tensor parameter magnitudes, respectively. Immediately following exercise, the in vivo mean diffusivity (MD) time courses show, before the well-known increase, an initial decrease that is not typically observed in traditional DTI. In conclusion, SL-MEDITATE can be used to capture transient changes in tissue anisotropy in a single line. Future progress might allow for dynamic DTI when combined with appropriate k-space trajectories and compressed sensing reconstruction.

Return to sport after muscle injury

Skeletal muscle injuries are among the most common sports-related injuries that result in time lost from practice and competition. The cellular response to muscle injury can often result in changes made to the muscle fibers as well as the surrounding extracellular matrix during repair. This can negatively affect the force and range of the injured muscle even after the patient's return to play. Diagnosis of skeletal muscle injury involves both history and physical examinations; imaging modalities including ultrasound and magnetic resonance imaging (MRI) can also be used to assess the extent of injury. Current research is investigating potential methods, including clinical factors and MRI, by which to predict a patient's return to sports. Overall, function of acutely injured muscles seems to improve with time. Current treatment methods for skeletal muscle injuries include injections of steroids, anesthetics, and platelet-rich plasma (PRP). Other proposed methods involve inhibitors of key players in fibrotic pathways, such as transforming growth factor (TGF)-ß and angiotensin II, as well as muscle-derived stem cells.

Physical rehabilitation improves muscle function following volumetric muscle loss injury

Background

Given the clinical practice of prescribing physical rehabilitation for the treatment of VML injuries, the present study examined the functional and histomorphological adaptations in the volumetric muscle loss (VML) injured muscle to physical rehabilitation.

Methods

Tibialis anterior muscle VML injury was created in Lewis rats (n = 32), and were randomly assigned to either sedentary (SED) or physical rehabilitation (RUN) group. After 1 week, RUN rats were given unlimited access to voluntary running wheels either 1 or 7 weeks (2 or 8 weeks post-injury). At 2 weeks post-injury, TA muscles were harvested for molecular analyses. At 8 weeks post-injury, the rats underwent in vivo function testing. The explanted tissue was analyzed using histological and immunofluorescence procedures.

Results

The primary findings of the study are that physical rehabilitation in the form of voluntary wheel running promotes ~ 17% improvement in maximal isometric torque, and a ~ 13% increase in weight of the injured muscle, but it did so without significant morphological adaptations (e.g., no hypertrophy and hyperplasia). Wheel running up-regulated metabolic genes (SIRT-1, PGC-1α) only in the uninjured muscles, and a greater deposition of fibrous tissue in the defect area of the injured muscle preceded by an up-regulation of pro-fibrotic genes (Collagen I, TGF-β1). Therefore, it is plausible that the wheel running related functional improvements were due to improved force transmission and not muscle regeneration.

Conclusions

This is the first study to demonstrate improvement in functional performance of non-repaired VML injured muscle with physical rehabilitation in the form of voluntary wheel running. This study provides information for the first time on the basic changes in the VML injured muscle with physical rehabilitation, which may aid in the development of appropriate physical rehabilitation regimen(s).

A stepwise procedure to test contractility and susceptibility to injury for the rodent quadriceps muscle

In patients with muscle injury or muscle disease, assessment of muscle damage is typically limited to clinical signs, such as tenderness, strength, range of motion, and more recently, imaging studies. Biological markers can also be used in measuring muscle injury, such as increased creatine kinase levels in the blood, but these are not always correlated with loss in muscle function (i.e. loss of force production). This is even true of histological findings from animals, which provide a "direct measure" of damage, but do not account for loss of function. The most comprehensive measure of the overall health of the muscle is contractile force. To date, animal models testing contractile force have been limited to the muscle groups moving the ankle. Here we describe an in vivo animal model for the quadriceps, with abilities to measure torque, produce a reliable muscle injury, and follow muscle recovery within the same animal over time. We also describe a second model used for direct measurement of force from an isolated quadriceps muscle in situ.

Multi-parametric MRI characterization of healthy human thigh muscles at 3.0 T - relaxation, magnetization transfer, fat/water, and diffusion tensor imaging

Muscle diseases commonly have clinical presentations of inflammation, fat infiltration, fibrosis, and atrophy. However, the results of existing laboratory tests and clinical presentations are not well correlated. Advanced quantitative MRI techniques may allow the assessment of myo-pathological changes in a sensitive and objective manner. To progress towards this goal, an array of quantitative MRI protocols was implemented for human thigh muscles; their reproducibility was assessed; and the statistical relationships among parameters were determined. These quantitative methods included fat/water imaging, multiple spin-echo T2 imaging (with and without fat signal suppression, FS), selective inversion recovery for T1 and quantitative magnetization transfer (qMT) imaging (with and without FS), and diffusion tensor imaging. Data were acquired at 3.0 T from nine healthy subjects. To assess the repeatability of each method, the subjects were re-imaged an average of 35 days later. Pre-testing lifestyle restrictions were applied to standardize physiological conditions across scans. Strong between-day intra-class correlations were observed in all quantitative indices except for the macromolecular-to-free water pool size ratio (PSR) with FS, a metric derived from qMT data. Two-way analysis of variance revealed no significant between-day differences in the mean values for any parameter estimate. The repeatability was further assessed with Bland-Altman plots, and low repeatability coefficients were obtained for all parameters. Among-muscle differences in the quantitative MRI indices and inter-class correlations among the parameters were identified. There were inverse relationships between fractional anisotropy (FA) and the second eigenvalue, the third eigenvalue, and the standard deviation of the first eigenvector. The FA was positively related to the PSR, while the other diffusion indices were inversely related to the PSR. These findings support the use of these T1 , T2 , fat/water, and DTI protocols for characterizing skeletal muscle using MRI. Moreover, the data support the existence of a common biophysical mechanism, water content, as a source of variation in these parameters.

In vivo anisotropic mechanical properties of dystrophic skeletal muscles measured by anisotropic MR elastographic imaging: the mdx mouse model of muscular dystrophy

PURPOSE

To evaluate the utility of mechanical anisotropy (shear storage modulus parallel to fiber/shear storage modulus perpendicular to fiber) measured by combined magnetic resonance (MR) elastography and diffusion-tensor imaging ( DTI diffusion-tensor imaging ) technique (anisotropic MR elastography) to distinguish between healthy and necrotic muscle with different degrees of muscle necrosis in the mdx mouse model of muscular dystrophy.

MATERIALS AND METHODS

The experimental protocol was approved by the regional animal ethics committee. Twenty-one mdx and 21 wild-type ( WT wild type ) mice were used in our study. Animals were divided into exercised and sedentary groups. Anisotropic MR elastography was used to obtain mechanical anisotropic shear moduli for the lateral gastrocnemius and plantaris muscles in a 7-T MR imager, from which the mechanical anisotropic ratio was calculated. The animals were imaged before and after 10 weeks of a horizontal treadmill running protocol. Spearman rank correlations were used to compare MR elastographic data with muscle necrotic area percentage from histologic analysis. Mechanical anisotropy in WT wild type and mdx mice muscle were compared by using t test and one-way analysis of variance, and receiver operating characteristic curves were constructed by using statistical software.

RESULTS

Anisotropic MR elastography was able to be used to distinguish between the muscles of mdx and WT wild type mice, with an area under the receiver operating characteristic curve of 0.8. Strong negative correlation (rs = -0.701; P < .001) between the mechanical anisotropic ratio and the percentage of muscle necrotic area was found. By comparing mice with no or mild (0%-5% mean necrotic area) and severe (>5% mean necrotic area) muscle necrosis, an area under the receiver operating characteristic curve of 0.964 was achieved. Diffusion parameters alone were unable to distinguish between the WT wild type and mdx mice at any time point.

CONCLUSION

The mechanical anisotropic ratio of the shear storage moduli measured by an anisotropic MR elastographic technique can distinguish between healthy muscle and dystrophic muscle.

Multi-parametric MRI characterization of inflammation in murine skeletal muscle

Myopathies often display a common set of complex pathologies that include muscle weakness, inflammation, compromised membrane integrity, fat deposition, and fibrosis. Multi-parametric, quantitative, non-invasive imaging approaches may be able to resolve these individual pathological components. The goal of this study was to use multi-parametric MRI to investigate inflammation as an isolated pathological feature. Proton relaxation, diffusion tensor imaging (DTI), quantitative magnetization transfer (qMT-MRI), and dynamic contrast enhanced (DCE-MRI) parameters were calculated from data acquired in a single imaging session conducted 6-8 hours following the injection of λ-carrageenan, a local inflammatory agent. T2 increased in the inflamed muscle and transitioned to bi-exponential behavior. In diffusion measurements, all three eigenvalues and the apparent diffusion coefficient increased, but λ3 had the largest relative change. Analysis of the qMT data revealed that the T1 of the free pool and the observed T1 both increased in the inflamed tissue, while the ratio of exchanging spins in the solid pool to those in the free water pool (the pool size ratio) significantly decreased. DCE-MRI data also supported observations of an increase in extracellular volume. These findings enriched the understanding of the relation between multiple quantitative MRI parameters and an isolated inflammatory pathology, and may potentially be employed for other single or complex myopathy models.

7-Tesla magnetic resonance imaging precisely and noninvasively reflects inflammation and remodeling of the skeletal muscle in a mouse model of antisynthetase syndrome

Inflammatory myopathies comprise heterogeneous disorders. Their etiopathogenesis is poorly understood, because of the paucity of informative experimental models and of approaches for the noninvasive study of inflamed tissues. Magnetic resonance imaging (MRI) provides information about the state of the skeletal muscle that reflects various facets of inflammation and remodeling. This technique has been scarcely used in experimental models of inflammatory myopathies. We characterized the performance of MRI in a well-established mouse model of myositis and the antisynthetase syndrome, based on the immunization of wild-type mice with the amino-terminal fragment of histidyl-tRNA synthetase (HisRS). Over an eight-week period following myositis induction, MRI enabled precise identification of pathological events taking place in muscle tissue. Areas of edema and of active inflammation identified by histopathology paralleled muscle modifications detected noninvasively by MRI. Muscles changes were chronologically associated with the establishment of autoimmunity, as reflected by the development of anti-HisRS antibodies in the blood of immunized mice. MR imaging easily appreciated muscle damage and remodeling even if actual disruption of myofiber integrity (as assessed by serum concentrations of creatinine phosphokinase) was limited. Thus, MR imaging represents an informative and noninvasive analytical tool for studying in vivo immune-mediated muscle involvement.

Assessment of magnetic resonance techniques to measure muscle damage 24 h after eccentric exercise

The study examined which of a number of different magnetic resonance (MR) methods were sensitive to detecting muscle damage induced by eccentric exercise. Seventeen healthy, physically active participants, with muscle damage confirmed by non-MR methods were tested 24 h after performing eccentric exercise. Techniques investigated whether damage could be detected within the quadriceps muscle as a whole, and individually within the rectus femoris, vastus lateralis (VL), vastus medialis (VM), and vastus intermedius (VI). Relative to baseline values, significant changes were seen in leg and muscle cross-sectional areas and volumes and the resting inorganic phosphate concentration. Significant time effects over all muscles were also seen in the transverse relaxation time (T2) and apparent diffusion coefficient (ADC) values, with individually significant changes seen in the VL, VM, and VI for T2 and in the VI for ADC. A significant correlation was found between muscle volume and the average T2 change (r = 0.59) but not between T2 and ADC or Pi alterations. There were no significant time effects over all muscles for magnetization transfer contrast images, for baseline pH, phosphocreatine (PCr), phosphodiester, or ATP metabolite concentrations or the time constant describing the rate of PCr recovery following exercise.

DTI of human skeletal muscle: the effects of diffusion encoding parameters, signal-to-noise ratio and T2 on tensor indices and fiber tracts

In this study, we have performed simulations to address the effects of diffusion encoding parameters, signal-to-noise ratio (SNR) and T2 on skeletal muscle diffusion tensor indices and fiber tracts. Where appropriate, simulations were corroborated and validated by in vivo diffusion tensor imaging (DTI) of human skeletal muscle. Specifically, we have addressed: (i) the accuracy and precision of the diffusion parameters and eigenvectors at different SNR levels; (ii) the effects of the diffusion gradient direction encoding scheme; (iii) the optimal b value for diffusion tensor estimation; (iv) the effects of changes in skeletal muscle T2; and, finally, the influence of SNR on fiber tractography and derived (v) fiber lengths, (vi) pennation angles and (vii) fiber curvatures. We conclude that accurate DTI of skeletal muscle requires an SNR of at least 25, a b value of between 400 and 500 s/mm(2), and data acquired with at least 12 diffusion gradient directions homogeneously distributed on half a sphere. Furthermore, for DTI studies focusing on skeletal muscle injury or pathology, apparent changes in the diffusion parameters need to be interpreted with great care in view of the confounding effects of T2, particularly for moderate to low SNR values.

Investigation of muscle degeneration process in young rats with ischemia injury using MR diffusion tensor imaging

Skeletal muscle is vulnerable to ischemia injury after direct trauma or indirect causes. Magnetic resonance diffusion tensor imaging (DTI) has been demonstrated to be a powerful tool to investigate muscle structures. However, most previous DTI studies that investigated ischemia muscle were performed on mature species, results of which may not apply on other age populations, such as young groups. In this study, age-related variation of muscle regeneration course after ischemia injury was investigated for the first time. Ten young and ten mature SD rats were induced ischemia in hindlimbs, and the evolutions of the skeletal muscle regeneration were longitudinally studied using DTI before and till 21 days after surgery. Results showed that the young group was more sensitive to ischemia and recovered more rapidly than the mature one. This study confirms the age-related variation of muscle regeneration process, and may provide supplemental information for better understanding of muscle repair evolution.

Temporal changes in magnetic resonance imaging in the mdx mouse

Background

Duchenne muscular dystrophy (DMD) is characterized clinically by severe, progressive loss of skeletal muscle. The phenotype is much less severe in the mdx mouse model of DMD than that seen in patients with DMD. However, a “critical period” has been described for the mdx mouse, during which there is a peak in muscle weakness and degeneration/regeneration between the 2^nd and 5^th weeks of life. A number of studies have employed small animal magnetic resonance imaging (MRI) to examine skeletal muscle in various dystrophic models, but such studies represent a snapshot in time rather than a longitudinal view.

Results

The in vivo cross-sectional T₂-weighted image of the healthy (wild type, WT) muscles is homogeneously dark and this homogeneity does not change with time, as there is no disease. We, and others, have shown marked changes in MRI in dystrophic muscle, with multiple, unevenly distributed focal hyperintensities throughout the bulk of the muscles. Here we monitored an mdx mouse using MRI from 5 to 80 weeks of age. Temporal MRI scans show an increase in heterogeneity shortly after the critical period, at 9 and 13 weeks of age, with a decrease in heterogeneity thereafter. The 4.3-fold increase in percent heterogeneity at week 9 and 13 is consistent with the notion of an early critical period described for mdx mice.

Conclusions

Age is a significant variable in quantitative MR studies of the mdx mouse. The mdx mouse is typically studied during the critical period, at a time that most closely mimics the DMD pathology, but the preliminary findings here, albeit based on imaging only one mdx mouse over time, suggest that the changes in MRI can occur shortly after this period, when the muscles are still recovering.

Determination of three-dimensional muscle architectures: validation of the DTI-based fiber tractography method by manual digitization

In the last decade, diffusion tensor imaging (DTI) has been used increasingly to investigate three-dimensional (3D) muscle architectures. So far there is no study that has proved the validity of this method to determine fascicle lengths and pennation angles within a whole muscle. To verify the DTI method, fascicle lengths of m. soleus as well as their pennation angles have been measured using two different methods. First, the 3D muscle architecture was analyzed in vivo applying the DTI method with subsequent deterministic fiber tractography. In a second step, the muscle architecture of the same muscle was analyzed using a standard manual digitization system (MicroScribe MLX). Comparing both methods, we found differences for the median pennation angles (P < 0.001) but not for the median fascicle lengths (P = 0.216). Despite the statistical results, we conclude that the DTI method is appropriate to determine the global fiber orientation. The difference in median pennation angles determined with both methods is only about 1.2° (median pennation angle of MicroScribe: 9.7°; DTI: 8.5°) and probably has no practical relevance for muscle simulation studies. Determining fascicle lengths requires additional restriction and further development of the DTI method.

Eccentric exercise in aging and diseased skeletal muscle: good or bad?

Evidence is accumulating regarding the benefits of exercise in people who are more susceptible to injury, such as the elderly, or those with a neuromuscular disease, for example Duchenne muscular dystrophy (DMD). There appears to be a consensus that exercise can be safely performed in aging and diseased muscles, but the role of eccentric exercise is not as clear. Eccentric (lengthening) contractions have risks and benefits. Eccentric contractions are commonly performed on a daily basis, and high-force voluntary eccentric contractions are often employed in strength training paradigms with excellent results; however, high-force eccentric contractions are also linked to muscle damage. This minireview examines the benefits and safety issues of using eccentric exercise in at-risk populations. A common recommendation for all individuals is difficult to achieve, and guidelines are still being established. Some form of exercise is generally recommended with aging and even with diseased muscles, but the prescription (frequency, intensity, and duration) and type (resistance vs. aerobic) of exercise requires personal attention, as there is great diversity in the functional level and comorbidities in the elderly and those with neuromuscular disease.