Strenuous resistance exercise effects on magnetic resonance diffusion parameters and muscle-tendon function in human skeletal muscle

Changes in inflammatory edema and fat fraction of thigh muscles following a half-marathon in recreational marathon runners

It is known that microtrauma exists in the thigh muscles after long-distance running such as the half-marathon. Moreover, training characteristics of long-distance runners may influence the specificity of the distribution of muscle fiber types in the thigh and affect muscle responses to lipid metabolism. However, the specific changes in microtrauma and intramuscular lipid in thigh muscles after a half-marathon are unknown. A cohort of 20 healthy recreational marathon runners was recruited to complete a half-marathon. MRI T2 mapping and 6-echo q-Dixon sequences were employed at baseline (P1), 2-3 h after running (P2), and 1 day after running (P3). Inflammatory markers (the T2 values) and intramuscular fat fraction (the proton density fat fraction, PDFF) were measured in thigh muscles to detect microtrauma and intramuscular lipid changes, respectively. One-way analysis of variance showed significant time effects for T2 values and PDFF. Post hoc analysis of the 14 datasets collected at three time points revealed significantly higher T2 values in all thigh muscles after running (all p < 0.05). Significant differences in T2 values persisted for all thigh muscles at P3 compared to P1 (all p < 0.05). The PDFF of the vastus lateralis and vastus medialis was significantly decreased at P2 compared to P1 (p < 0.05). No significant differences in PDFF were observed for the thigh muscles at P3 compared to P1. The manifestations of inflammation edema and intramuscular lipid investigated through MRI may offer valuable insights for recreational marathon runners regarding the lower limb movement characteristics during half-marathon running.

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.

Low back pain in adolescent rowers: Association to muscle changes detected by magnetic resonance imaging

Adult elite rowers are at risk of developing low back pain (LBP). However, LBP data on adolescent elite rowers is currently insufficient. Therefore, the aim of this study was to assess LBP prevalence, LBP intensity and training characteristics in male adolescent elite rowers and a healthy control group. Twenty rowers (mean age 15.8 ± 1.2 years) and a non-athletic control group matched by age and gender (n = 13) were prospectively enrolled and underwent LBP assessment with a validated questionnaire and magnetic resonance imaging (MRI) of the lumbar spine muscles, which included a T2-mapping sequence. From the quantitative image data, T2 relaxation times were calculated. The prevalence of LBP in the last 24 hours and 3 months in the rowing group was 55.0% and 85.0%, respectively, compared to 23.1% and 30.8% in the control group (p < 0.001). Rowers had significantly longer T2 relaxation times of the paraspinal muscles compared to controls (p ≤ 0.041). LBP intensity was associated with longer T2 relaxation times (p < 0.001). Adolescent rowers had a higher prevalence of LBP compared to an age-matched control group. The observed increase in T2 relaxation might be explained by muscle soreness due to strenuous exercise, which is correlated with short-term pain intensity.

Sodium and quantitative hydrogen parameter changes in muscle tissue after eccentric exercise and in delayed-onset muscle soreness assessed with magnetic resonance imaging

The objective of the current study was to assess sodium (²³ Na) and quantitative proton (¹ H) parameter changes in muscle tissue with magnetic resonance imaging (MRI) after eccentric exercise and in delayed-onset muscle soreness (DOMS). Fourteen participants (mean age: 25 ± 4 years) underwent ²³ Na/¹ H MRI of the calf muscle on a 3-T MRI system before exercise (t0), directly after eccentric exercise (t1), and 48 h postintervention (t2). In addition to tissue sodium concentration (TSC), intracellular-weighted sodium (ICwS) signal was acquired using a three-dimensional density-adapted radial projection readout with an additional inversion recovery preparation module. Phantoms containing saline solution served as references to quantify sodium concentrations. The ¹ H MRI protocol consisted of a T₁ -weighted turbo spin echo sequence, a T₂ -weighted turbo inversion recovery, as well as water T₂ mapping and water T₁ mapping. Additionally, blood serum creatine kinase (CK) levels were assessed at baseline and 48 h after exercise. The TSC and ICwS of exercised muscles increased significantly from t0 to t1 and decreased significantly from t1 to t2. In the soleus muscle (SM), ICwS decreased below baseline values at t2. In the tibialis anterior muscle (TA), TSC and ICwS remained at baseline levels at each measurement point. However, high-CK participants (i.e., participants with a more than 10-fold CK increase, n = 3) displayed different behavior, with 2- to 4-fold increases in TSC values in the medial gastrocnemius muscle (MGM) at t2. ¹ H water T₁ relaxation times increased significantly after 48 h in the MGM and SM. ¹ H water T₂ relaxation times and muscle volume increased in the MGM at t2. Sodium MRI parameters and water relaxation times peaked at different points. Whereas water relaxation times were highest at t2, sodium MRI parameters had already returned to baseline values (or even below baseline values, for low-CK participants) by this point. The observed changes in ion concentrations and water relaxation time parameters could enable a better understanding of the physiological processes during DOMS and muscle regeneration. In the future, this might help to optimize training and to reduce associated sports injuries.

Time-course of physical properties of the psoas major muscle after exercise as assessed by MR elastography

This study aimed to analyze the time-course of the physical properties of the psoas major muscle (PM) before and after exercise using magnetic resonance elastography (MRE). Muscle stiffness is one of the important properties associated with muscle function. However, there was no research on the stiffness of the PM after exercise. In this study, we investigated time-course changes of the shear modulus of the PM after exercise. Furthermore, T2 values and apparent diffusion coefficient (ADC), as the additional information associated with muscular physical properties, were also measured simultaneously. Healthy young male volunteers were recruited in this study (n = 9) and they were required to perform a hand-to-knee isometric and unilateral exercise (left side). At each time-point before and after exercise, a set of 3 types of MR scans to measure multiple physical properties of the PM [shear modulus (MRE), T2 values, and ADC] were repeatedly taken. On day 1, a single set MR scan was taken before exercise (pre-exercise MR scan), and 6 sets MR scans were taken (5.5 to 38.0 min after exercise). After about 10-min rest (46.0 to 56.0 min after exercise), 4 sets MR scans were taken (57.5 to 77.0 min after exercise). About 10-min rest was taken again (85.0-95.0 min after exercise), 4 sets MR scans were taken (96.5 to 116.0 min after exercise). On days 2 and 7, a single set MR scan (MRE, T2 value, and ADC) was taken on each experimental day. The data were analyzed as relative changes (%) of the given parameters to the pre-exercise values. The results indicated significant decreases in PM shear modulus up to about 30 min after exercise. Then, it gradually increased and showed significant increases at about 100 min after exercise compared to that before exercise. T2 values and ADC showed significant increases up to about 65 min after exercise compared to those before exercise, and then returned to the pre-exercise values. On days 2 and 7, all values showed no significant changes compared to the pre-exercise values. This study is the first to report the time-course of the physical properties of the PM after exercise.

The behaviour of T2* and T2 relaxation time in extrinsic foot muscles under continuous exercise: A prospective analysis during extended running

Objectives

Previous studies on T2* and T2 relaxation time of the muscles have shown that exercise leads to an initial increase, presumably representing different intramuscular physiological processes such as increase in intracellular volume or blood oxygenation level dependent effects with a subsequent decrease after cessation of exercise. Their behaviour during prolonged exercise is still unknown but could provide important information for example about the pathophysiology of overuse injuries. The aim of this study was to evaluate the temporal course of T2* and T2 relaxation time in extrinsic foot muscles during prolonged exercise and determine the optimal mapping technique.

Methods

Ten participants had to run a total of 75 minutes at their individual highest possible running speed, with interleaved MR scans at baseline and after 2.5, 5, 10, 15, 45 and 75 minutes. The examined extrinsic foot muscles were manually segmented, and relaxation time were analysed regarding its respective time course.

Results

T2* and T2 relaxation time showed an initial increase, followed by a plateau phase between 2.5 and 15 minutes and a subsequent decrease. For the T2* relaxation time, this pattern was also apparent, but less pronounced, with more muscles not reaching significance (p<0.05) when comparing different time points.

Conclusions

T2* and T2 relaxation time showed a similar course with an initial rapid increase, a plateau phase and a subsequent decrease under prolonged exercise. Moderate but long-term muscular activity appears to have a weaker effect on T2* relaxation time than on T2 relaxation time.

Increased muscle fiber fractional anisotropy value using diffusion tensor imaging after compression without fiber injury

BACKGROUND

Recent studies have indicated that injuries such as muscle tears modify the microstructural integrity of muscle, leading to substantial alterations in measured diffusion parameters. Therefore, the fractional anisotropy (FA) value decreases. However, we hypothesized that soft tissue, such as muscle tissue, undergoes reversible changes under conditions of compression without fiber injury.

PURPOSE

To evaluate the FA change due to compression in muscle tissue without fiber injury.

MATERIAL AND METHODS

Diffusion tensor imaging (DTI) was performed on both feet of 10 healthy volunteers (mean age = 35.0 ± 10.39 years; age range = 23-52 years) using a 3.0-T magnetic resonance imaging (MRI) scanner with an eight-channel phased array knee coil. An MRI-compatible sphygmomanometer was applied to the individuals' lower legs and individuals were placed in a compressed state. Then, rest intervals of 5 min were set in re-rest state after compression. The FA value, apparent diffusion coefficient (ADC), and eigenvalues (λ1, λ2, λ3) of the gastrocnemius and soleus muscle were measured at each state.

RESULTS

The mean FA values increased in all muscles in a compressed state, while the mean λ3 decreased. In all muscles, significant differences were found between the rest and compressed states in terms of mean FA and λ3 (P < 0.0001).

CONCLUSION

We confirmed the reversibility of the DTI metrics, which suggests that there was no muscle injury during this study. In cases of compression without fiber injury, the FA value increases, because fibers are strongly aligned in the longitudinal direction.

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.

A Longitudinal Study of T2 Mapping Combined With Diffusion Tensor Imaging to Quantitatively Evaluate Tissue Repair of Rat Skeletal Muscle After Frostbite

Purpose: T2 mapping and diffusion tensor imaging (DTI) enable the detection of changes in the skeletal muscle microenvironment. We assessed T2 relaxation times, DTI metrics, performed histological characterization of frostbite-induced skeletal muscle injury and repair, and provided diagnostic imaging biomarkers.

Design and Methods: Thirty-six Sprague Dawley rats (200 ± 10 g) were obtained. Thirty rats were used for establishing a skeletal muscle frostbite model, and six were untreated controls. Functional MR sequences were performed on rats on days 0, 3, 5, 10, and 14 (n = 6 per time point). Rats were then sacrificed to obtain the quadriceps muscles. Tensor eigenvalues (λ1, λ2, and λ3), mean diffusivity (MD), fractional anisotropy (FA), and T2 values were compared between the frostbite model and control rats. ImageJ was used to measure the extracellular area fraction (EAF), muscle fiber cross-sectional area (fCSA), and skeletal muscle tumor necrosis factor α (TNF-α), and Myod1 expression. The correlation between the histological and imaging parameters of the frostbitten skeletal muscle was evaluated. Kolmogorov–Smirnoff test, Leven’s test, one-way ANOVA, and Spearman coefficient were used for analysis.

Results: T2 relaxation time of frostbitten skeletal muscle was higher at all time points (p < 0.01). T2 relaxation time correlated with EAF, and TNF-α and Myod1 expression (r = 0.42, p < 0.05; r = 0.86, p < 0.01; r = 0.84, p < 0.01). The average tensor metrics (MD, λ1, λ2, and λ3) of skeletal muscle at 3 and 5 days of frostbite increased (p < 0.05), and fCSA correlated with λ1, λ2, and λ3, and MD (r = 0.65, p < 0.01; r = 0.48, p < 0.01; r = 0.52, p < 0.01; r = 0.62, p < 0.01).

Conclusion: T2 mapping and DTI imaging detect frostbite-induced skeletal muscle injury early. This combined approach can quantitatively assess skeletal muscle repair and regeneration within 2 weeks of frostbite. Imaging biomarkers for the diagnosis of frostbite were suggested.

Physiological responses of human skeletal muscle to acute blood flow restricted exercise assessed by multimodal MRI

Important physiological quantities for investigating muscle hypertrophy include blood oxygenation, cell swelling, and changes in blood flow. The purpose of this study was to compare the acute changes of these parameters in human skeletal muscle induced by low-load (20% 1-RM) blood flow-restricted (BFR-20) knee extensor exercise compared with free-flow work-matched (FF-20_WM) and free-flow 50% 1-RM (FF-50) knee extensor exercise using multimodal magnetic resonance imaging (MRI). Subjects (n = 11) completed acute exercise sessions for each exercise mode in an MRI scanner, where interleaved measures of muscle R₂ (indicator of edema), [Formula: see text] (indicator of deoxyhemoglobin), macrovascular blood flow, and diffusion were performed before, between sets, and after the final set for each exercise protocol. BFR-20 exercise resulted in larger acute decreases in R₂ and greater increases in cross-sectional area than FF-20_WM and FF-50 (P < 0.01). Blood oxygenation decreased between sets during BFR-20, as indicated by a 13.6% increase in [Formula: see text] values (P < 0.01)), whereas they remained unchanged for FF-20_WM and decreased during FF-50 exercise. Quadriceps blood flow between sets was highest for the heavier load (FF-50), averaging 305 mL/min, and lowest for BFR-20 at 123 ± 73 mL/min until post-exercise cuff release, where blood flow rates in BFR-20 exceeded both FF protocols (P < 0.01). Acute changes in diffusion rates were similar for all exercise protocols. This study was able to differentiate the acute exercise response of selected physiological factors associated with skeletal muscle hypertrophy. Marked differences in these parameters were found to exist between BFR and FF exercise conditions, which contribute to explain the anabolic potential of low-load blood flow restricted muscle exercise.NEW & NOTEWORTHY Acute changes in blood flow, diffusion, blood oxygenation, cross-sectional area, and the "T₂ shift" are evaluated in human skeletal muscle in response to blood flow-restricted (BFR) and conventional free-flow knee extensor exercise performed in an MRI scanner. The acute physiological response to exercise was dependent on the magnitude of load and the application of BFR. Physiological variables changed markedly and established a steady state rapidly after the first of four exercise sets.

Quantitative MRI Reveals Microstructural Changes in the Upper Leg Muscles After Running a Marathon

Background

The majority of sports‐related injuries involve skeletal muscle. Unlike acute trauma, which is often caused by a single traumatic event leading to acute symptoms, exercise‐induced microtrauma may remain subclinical and difficult to detect. Therefore, novel methods to detect and localize subclinical exercise‐induced muscle microtrauma are desirable.

Purpose

To assess acute and delayed microstructural changes in upper leg muscles with multiparametric quantitative MRI after running a marathon.

Study Type

Longitudinal; 1‐week prior, 24–48 hours postmarathon and 2‐week follow‐up

Population

Eleven men participants (age: 47–68 years).

Field Strength/Sequence

Spin‐echo echo planar imaging (SE‐EPI) with diffusion weighting, multispin echo, Dixon, and fat‐suppressed turbo spin‐echo (TSE) sequences at 3T. MR datasets and creatine kinase (CK) concentrations were obtained at three timepoints.

Assessment

Diffusion parameters, perfusion fractions, and quantitative (q)T₂ values were determined for hamstring and quadriceps muscles, TSE images were scored for acute injury. The vastus medialis and biceps femoris long head muscles were divided and analyzed in five segments to assess local damage.

Statistical Tests

Differences between timepoints in MR parameters were assessed with a multilevel linear mixed model and in CK concentrations with a Friedman test. Mean diffusivity (MD) and qT₂ for whole muscle and muscle segments were compared using a multivariate analysis of covariance (MANCOVA).

Results

CK concentrations were elevated (1194 U/L [166–3906], P < 0.001) at 24–48 hours postmarathon and returned to premarathon values (323 U/L [56–2216]) at 2‐week follow‐up. Most of the MRI diffusion indices in muscles without acute injury changed at 24–48 hours postmarathon and returned to premarathon values at follow‐up (MD, RD, and λ3; P < 0.006). qT₂ values (P = 0.003) and perfusion fractions (P = 0.003) were higher at baseline compared to follow‐up. Local assessments of MD and qT₂ revealed more pronounced changes than whole muscle assessment (2–3‐fold; P < 0.01).

Data Conclusion

Marathon running‐induced microtrauma was detected with MRI in individual whole upper leg muscles and even more pronounced on local segments.

Level of Evidence

2

Technical Efficacy Stage

3 J. Magn. Reson. Imaging 2020;52:407–417.

Determining Subject-Specific Lower-Limb Muscle Architecture Data for Musculoskeletal Models Using Diffusion Tensor Imaging

Accurate individualized muscle architecture data are crucial for generating subject-specific musculoskeletal models to investigate movement and dynamic muscle function. Diffusion tensor imaging (DTI) magnetic resonance (MR) imaging has emerged as a promising method of gathering muscle architecture data in vivo; however, its accuracy in estimating parameters such as muscle fiber lengths for creating subject-specific musculoskeletal models has not been tested. Here, we provide a validation of the method of using anatomical magnetic resonance imaging (MRI) and DTI to gather muscle architecture data in vivo by directly comparing those data obtained from MR scans of three human cadaveric lower limbs to those from dissections. DTI was used to measure fiber lengths and pennation angles, while the anatomical images were used to estimate muscle mass, which were used to calculate physiological cross-sectional area (PCSA). The same data were then obtained through dissections, where it was found that on average muscle masses and fiber lengths matched well between the two methods (4% and 1% differences, respectively), while PCSA values had slightly larger differences (6%). Overall, these results suggest that DTI is a promising technique to gather in vivo muscle architecture data, but further refinement and complementary imaging techniques may be needed to realize these goals.

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.

Diffusion Tensor Imaging of the Calf Muscles in Subjects With and Without Diabetes Mellitus

BACKGROUND

Diffusion tensor imaging (DTI) has been used to characterize calf skeletal muscle architecture.

PURPOSE

To assess the diffusional properties of the calf muscles of subjects with and without diabetes, at rest and during isometric plantarflexion exercise.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-six subjects in two groups: 13 healthy and 13 subjects with type 2 diabetes (DM); each group consisted of seven females and six males.

FIELD STRENGTH/SEQUENCE

3T/2D single-shot spin echo planar imaging.

ASSESSMENT

Fractional anisotropy (FA), mean diffusivity (MD), diffusion eigenvalues, and fiber tracking indices were obtained from the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) muscles of the calf at rest and during isometric plantarflexion exercise.

STATISTICAL TESTS

We used a combination of nonparametric (Wilcoxon) and parametric (t-test) statistical assessments.

RESULTS

The medial gastrocnemius muscle had more indices with significant differences between the two groups (six indices with P < 0.05) than did the lateral gastrocnemius (three indices with P < 0.05) and soleus muscles (only one index with P < 0.05). While the healthy group showed elevated MD values from rest to exercise (MG = 5.83%, LG = 13.45%, and SOL = 11.68%), the diabetic MD showed higher increases (MG = 19.74%, LG = 29.31%, and SOL = 20.84%) that were different between groups (MG: P = 0.009, LG: P = 0.037, and SOL: P = 0.049).

DATA CONCLUSION

Our results indicate considerable diffusional changes between healthy subjects and subjects with diabetes at rest and during isometric plantarflexion exercise in the calf muscles. The medial gastrocnemius muscle displayed the most diffusion sensitivity to diabetes-related microstructural changes.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:1285-1295.

Human calf muscles changes after strength training as revealed by diffusion tensor imaging

BACKGROUND

Diffusion tensor imaging (DTI) is a non-invasive MR technique widely employed to study muscle anatomy. DTI parameters have been used to investigate microstructural changes dependent on demographic factors or transient condition such as exercise. The present study is aimed at investigating the diffusion parameters changes of the human calf muscles after a 3-months strength training protocol.

METHODS

Ten young men were trained for improving size and strength of the medial (GCM), lateral gastrocnemius (GCL) and soleus (SL) three times a week, with at least 24 hours between training sessions, for a period of three months. Diffusion weighted magnetic resonance images were acquired at the beginning of the training period (TPRE) and at three months (TPOST) using a 3T scanner. The fractional anisotropy (FA), mean diffusivity (MD) and tensor eigenvalues (λ1, λ2, λ3) were derived from the diffusion weighted imaging data.

RESULTS

We found a significant increase in λ1, λ2, λ3 and MD values and muscle volumes between TPRE and TPOST in all the examined muscles both for the left and right side. No significant differences were highlighted for FA.

CONCLUSIONS

DTI enables the investigation of muscle microstructure, allowing for the assessment of diffusion parameters variation of the muscle tissue in response to training thus being a useful tool to investigate physiological and pathological changes in skeletal muscle microstructure which could be employed to test the outcomes and the effectiveness of a given training protocol.

The vestibulomyogenic balance response is elevated following high-intensity lengthening contractions of the lower limb

The purpose was to investigate whether exercise-induced muscle weakness of the plantar and dorsiflexors through high-intensity lengthening contractions increases the vestibulomyogenic balance response. Nine males (∼25 years) participated in three experimental testing days to evaluate the vestibular control of standing balance and neuromuscular function of the plantar and dorsiflexors pre- and post (30 min, and 1 and 7 days) high-intensity lengthening plantar and dorsiflexions. To evaluate the vestibular-evoked balance response, participants stood quietly on a force plate while exposed to continuous, random electrical vestibular stimulation (EVS) for two 90-s trials. Relationships between EVS-antero-posterior (AP) forces and EVS-medial gastrocnemius electromyography (EMG) were estimated in the frequency domain (i.e., coherence). Weakness of the right plantar and dorsiflexors were assessed using maximal voluntary contraction (MVC) torque. The lengthening contractions induced a 13 and 24% reduction in plantar and dorsiflexor MVC torque, respectively (p < 0.05) of the exercised leg, which did not recover by 1 day post. The EVS-EMG coherence increased over a range of frequencies up to 7 days post compared to pre-lengthening contractions. Conversely, EVS-AP forces coherence exhibited limited changes. The greater EVS-EMG coherence post exercise-induced muscle weakness may be a compensatory mechanism to maintain the whole-body vestibular-evoked balance response when muscle strength is reduced.

Spatially resolved kinetics of skeletal muscle exercise response and recovery with multiple echo diffusion tensor imaging (MEDITI): a feasibility study

OBJECTIVES

We describe measurement of skeletal muscle kinetics with multiple echo diffusion tensor imaging (MEDITI). This approach allows characterization of the microstructural dynamics in healthy and pathologic muscle.

MATERIALS AND METHODS

In a Siemens 3-T Skyra scanner, MEDITI was used to collect dynamic DTI with a combination of rapid diffusion encoding, radial imaging, and compressed sensing reconstruction in a multi-compartment agarose gel rotation phantom and within in vivo calf muscle. An MR-compatible ergometer (Ergospect Trispect) was employed to enable in-scanner plantar flexion exercise. In a HIPAA-compliant study with written informed consent, post-exercise recovery of DTI metrics was quantified in eight volunteers. Exercise response of DTI metrics was compared with that of T2-weighted imaging and characterized by a gamma variate model.

RESULTS

Phantom results show quantification of diffusivities in each compartment over its full dynamic rotation. In vivo calf imaging results indicate larger radial than axial exercise response and recovery in the plantar flexion-challenged gastrocnemius medialis (fractional response: nT2w = 0.385 ± 0.244, nMD = 0.163 ± 0.130, nλ₁ = 0.110 ± 0.093, nλ_rad = 0.303 ± 0.185). Diffusion and T2-weighted response magnitudes were correlated (e.g., r = 0.792, p = 0.019 for nMD vs. nT2w).

CONCLUSION

We have demonstrated the feasibility of MEDITI for capturing spatially resolved diffusion tensor data in dynamic systems including post-exercise skeletal muscle recovery following in-scanner plantar flexion.

T2 , T1ρ and TRAFF4 detect early regenerative changes in mouse ischemic skeletal muscle

The identification of areas with regenerative potential in ischemic tissues would allow the targeting of treatments supporting tissue recovery. The regeneration process involves the activation of several cellular and molecular responses which could be detected using magnetic resonance imaging (MRI). However, to date, magnetic resonance (MR) relaxation parameters have received little attention in the diagnosis and follow-up of limb ischemia. The purpose of this study was to evaluate the feasibility of different MRI relaxation and diffusion tensor imaging parameters in the detection of areas showing early signs of regeneration in ischemic mouse skeletal muscles. T₂ and T_1ρ relaxation time constants, together with T_RAFFn , T₁ and diffusion tensor imaging, were evaluated to differentiate areas of regeneration in a mouse hind limb ischemia model before and 0, 1, 4, 7, 14 and 30 days after ischemia. All the measured relaxation times were longer in the areas of early regeneration compared with normal muscle tissue. The relaxation times increased after ischemia in the ischemic muscles, reaching a maximum at 4-7 days after occlusion, coinciding with the appearance of early signs of regeneration. Fractional anisotropy decreased significantly (p < 0.05) on days 1-4, whereas mean diffusivity, λ₁ and λ₂ decreased later, starting at day 7 after ischemia compared with the pre-operational time point. The percentages of areas with different tissue morphologies were determined based on histological analysis of the ischemic muscle cross-sections, and correlations between the percentages obtained and different relaxation times were calculated. The highest correlation between relaxation times and histology was achieved with T₂ , T_1ρ and T_RAFF4 (R² = 0.96, R² = 0.92 and R² = 0.84, respectively, p < 0.01)_. Early regenerative changes were visible using T₂ , T_1ρ and T_RAFF4 MR relaxation time constants in skeletal muscle after ischemia. These markers could potentially be used for the identification of targets for therapies supporting muscle regeneration after ischemic injury.

Localization of damage in the human leg muscles induced by downhill running

We investigated localization of damage within the knee extensors (KEs) and plantar flexors (PFs) induced by downhill running (DR) by using transverse relaxation time (T₂)-weighted magnetic resonance imaging (MRI). Fourteen young adults performed 45-min DR (-15% slope) at their maximal tolerable velocity. At pre- and 24, 48, and 72 h post-exercise, T₂-MRI was scanned and T₂ values for each muscle composing KEs and PFs at proximal, middle, and distal sites were calculated. Maximal isometric torque and rate of torque development (RTD: 0-30, 0-50, 0-100, 0-200 ms) were also measured. Maximal torque significantly decreased in KEs (14-17%) and PFs (6-8%) at 24-48 h post-exercise, with greater reductions for KEs. RTD in all phases, except for 0-200 ms in PFs, significantly decreased in KEs (11-42%) and PFs (13-23%) at least at one time point post-exercise. T₂ significantly increased at several sites (3-5%) in both muscle groups at 24 and/or 48 h post-exercise. Among the T₂-increased sites, the peak effect size (Cohen's d) regarding T₂ change was pronounced at proximal (1.05) and middle (1.64) vastus intermedius compared to the other sites (0.72-0.77). These results suggest that DR induces damage in both KEs and PFs, and especially affects proximal-middle sites of the vastus intermedius.

In vivo measurement of membrane permeability and myofiber size in human muscle using time-dependent diffusion tensor imaging and the random permeable barrier model

The time dependence of the diffusion coefficient is a hallmark of tissue complexity at the micrometer level. Here we demonstrate how biophysical modeling, combined with a specifically tailored diffusion MRI acquisition performing diffusion tensor imaging (DTI) for varying diffusion times, can be used to determine fiber size and membrane permeability of muscle fibers in vivo. We describe the random permeable barrier model (RPBM) and its assumptions, as well as the details of stimulated echo DTI acquisition, signal processing steps, and potential pitfalls. We illustrate the RPBM method on a few pilot examples involving human subjects (previously published as well as new), such as revealing myofiber size derived from RPBM increase after training in a calf muscle, and size decrease with atrophy in shoulder rotator cuff muscle. Finally, we comment on the potential clinical relevance of our results. Copyright © 2016 John Wiley & Sons, Ltd.

Dynamic DTI (dDTI) shows differing temporal activation patterns in post-exercise skeletal muscles

OBJECT

To assess post-exercise recovery of human calf muscles using dynamic diffusion tensor imaging (dDTI).

MATERIALS AND METHODS

DTI data (6 directions, b = 0 and 400 s/mm²) were acquired every 35 s from seven healthy men using a 3T MRI, prior to (4 volumes) and immediately following exercise (13 volumes, ~7.5 min). Exercise consisted of 5-min in-bore repetitive dorsiflexion-eversion foot motion with 0.78 kg resistance. Diffusion tensors calculated at each time point produced maps of mean diffusivity (MD), fractional anisotropy (FA), radial diffusivity (RD), and signal at b = 0 s/mm² (S₀). Region-of-interest (ROI) analysis was performed on five calf muscles: tibialis anterior (ATIB), extensor digitorum longus (EDL) peroneus longus (PER), soleus (SOL), and lateral gastrocnemius (LG).

RESULTS

Active muscles (ATIB, EDL, PER) showed significantly elevated initial MD post-exercise, while predicted inactive muscles (SOL, LG) did not (p < 0.0001). The EDL showed a greater initial increase in MD (1.90 × 10^-4mm²/s) than ATIB (1.03 × 10^-4mm²/s) or PER (8.79 × 10^-5 mm²/s) (p = 7.40 × 10^-4), and remained significantly elevated across more time points than ATIB or PER. Significant increases were observed in post-exercise EDL S₀ relative to other muscles across the majority of time points (p < 0.01 to p < 0.001).

CONCLUSIONS

dDTI can be used to differentiate exercise-induced changes between muscles. These differences are suggested to be related to differences in fiber composition.

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.

Effect of exercise-induced muscle damage on muscle hardness evaluated by ultrasound real-time tissue elastography

Purpose

To assess the effect of exercise-induced muscle damage on muscle hardness and evaluate the relationship between muscle hardness and muscle damage indicators.

Methods

Seven men (mean 25.3 years; 172.7 cm; 66.8 kg) performed the single-leg ankle plantar flexion exercise involving both concentric and eccentric contractions (10 sets of 40 repetitions). The hardness of the medial gastrocnemius (MG) was evaluated using ultrasound real-time tissue elastography before, from day 1 to 4, and day 7 after exercise. The strain ratio between the MG and a reference material was calculated. Simultaneously, we evaluated the magnetic resonance T2 value (an index of edema) of the triceps surae, the ankle dorsiflexion range of motion (ROM), and calf muscle soreness. Serum creatine kinase activity was assessed before, 2 and 4 h, and from day 1 to 4 after exercise.

Results

The MG showed lower strain ratio, indicating increased muscle hardness, on day 4 post-exercise (P < 0.01) and higher T2 values on days 1–7 post-exercise (P < 0.01) relative to each pre-exercise value. The ankle dorsiflexion ROM was lower on days 2–4 post-exercise (P < 0.01). The serum creatine kinase markedly increased on days 3 and 4 post-exercise (not significant). The degree of muscle soreness among the post-exercise time points was similar. The decreased strain ratio did not correlate with the increased T2, the decreased joint ROM or muscle soreness.

Conclusion

Muscle hardness increased after strenuous resistance exercise, but the change was not related with muscle edema, decreased joint ROM, or muscle soreness resulting from muscle damage.

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.

Assessment of Postoperative Tendon Quality in Patients With Achilles Tendon Rupture Using Diffusion Tensor Imaging and Tendon Fiber Tracking

Although pre- and postoperative imaging of Achilles tendon rupture (ATR) has been well documented, radiographic evaluations of postoperative intratendinous healing and microstructure are still lacking. Diffusion tensor imaging (DTI) is an innovative technique that offers a noninvasive method for describing the microstructure characteristics and organization of tissues. DTI was used in the present study for quantitative assessment of fiber continuity postoperatively in patients with acute ATR. The data from 16 patients with ATR from 2005 to 2012 were retrospectively analyzed. The microstructure of ART was evaluated using tendon fiber tracking, tendon continuity, fractional anisotropy, and apparent diffusion coefficient values by way of DTI. The distal and proximal portions were measured separately in both the ruptured and the healthy extremities of each patient. The mean patient age was 41.56 ± 8.49 (range 26 to 56) years. The median duration of follow-up was 21 (range 6 to 80) months. The tendon fractional anisotropy values of the ruptured Achilles tendon were significantly lower statistically than those of the normal side (p = .001). However, none of the differences between the 2 groups with respect to the distal and proximal apparent diffusion coefficient were statistically significant (p = .358 and p = .899, respectively). In addition, the fractional anisotropy and apparent diffusion coefficient measurements were not significantly different in the proximal and distal regions of the ruptured tendons compared with the healthy tendons. The present study used DTI and fiber tracking to demonstrate the radiologic properties of postoperative Achilles tendons with respect to trajectory and tendinous fiber continuity. Quantifying DTI and fiber tractography offers an innovative and effective tool that might be able to detect microstructural abnormalities not appreciable using conventional radiologic techniques.

Muscle changes detected with diffusion-tensor imaging after long-distance running

PURPOSE

To develop a protocol for diffusion-tensor imaging (DTI) of the complete upper legs and to demonstrate feasibility of detection of subclinical sports-related muscle changes in athletes after strenuous exercise, which remain undetected by using conventional T2-weighted magnetic resonance (MR) imaging with fat suppression.

MATERIALS AND METHODS

The research was approved by the institutional ethics committee review board, and the volunteers provided written consent before the study. Five male amateur long-distance runners underwent an MR examination (DTI, T1-weighted MR imaging, and T2-weighted MR imaging with fat suppression) of both upper legs 1 week before, 2 days after, and 3 weeks after they participated in a marathon. The tensor eigenvalues (λ1, λ2, and λ3), the mean diffusivity, and the fractional anisotropy (FA) were derived from the DTI data. Data per muscle from the three time-points were compared by using a two-way mixed-design analysis of variance with a Bonferroni posthoc test.

RESULTS

The DTI protocol allowed imaging of both complete upper legs with adequate signal-to-noise ratio and within a 20-minute imaging time. After the marathon, T2-weighted MR imaging revealed grade 1 muscle strains in nine of the 180 investigated muscles. The three eigenvalues, mean diffusivity, and FA were significantly increased (P < .05) in the biceps femoris muscle 2 days after running. Mean diffusivity and eigenvalues λ1 and λ2 were significantly (P < .05) increased in the semitendinosus and gracilis muscles 2 days after the marathon.

CONCLUSION

A feasible method for DTI measurements of the upper legs was developed that fully included frequently injured muscles, such as hamstrings, in one single imaging session. This study also revealed changes in DTI parameters that over time were not revealed by qualitative T2-weighted MR imaging with fat suppression.

Time-dependent diffusion in skeletal muscle with the random permeable barrier model (RPBM): application to normal controls and chronic exertional compartment syndrome patients

The purpose of this work was to carry out diffusion tensor imaging (DTI) at multiple diffusion times Td in skeletal muscle in normal subjects and chronic exertional compartment syndrome (CECS) patients and analyze the data with the random permeable barrier model (RPBM) for biophysical specificity. Using an institutional review board approved HIPAA-compliant protocol, seven patients with clinical suspicion of CECS and eight healthy volunteers underwent DTI of the calf muscle in a Siemens MAGNETOM Verio 3 T scanner at rest and after treadmill exertion at four different T(d) values. Radial diffusion values λ(rad) were computed for each of seven different muscle compartments and analyzed with RPBM to produce estimates of free diffusivity D(0), fiber diameter a, and permeability κ. Fiber diameter estimates were compared with measurements from literature autopsy reference for several compartments. Response factors (post/pre-exercise ratios) were computed and compared between normal controls and CECS patients using a mixed-model two-way analysis of variance. All subjects and muscle compartments showed nearly time-independent diffusion along and strongly time-dependent diffusion transverse to the muscle fibers. RPBM estimates of fiber diameter correlated well with corresponding autopsy reference. D(0) showed significant (p < 0.05) increases with exercise for volunteers, and a increased significantly (p < 0.05) in volunteers. At the group level, response factors of all three parameters showed trends differentiating controls from CECS patients, with patients showing smaller diameter changes (p = 0.07), and larger permeability increases (p = 0.07) than controls. Time-dependent diffusion measurements combined with appropriate tissue modeling can provide enhanced microstructural specificity for in vivo tissue characterization. In CECS patients, our results suggest that high-pressure interfiber edema elevates free diffusion and restricts exercise-induced fiber dilation. Such specificity may be useful in differentiating CECS from other disorders or in predicting its response to either physical therapy or fasciotomy.

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.

Multiple-echo diffusion tensor acquisition technique (MEDITATE) on a 3T clinical scanner

This article describes the concepts and implementation of an MRI method, the multiple-echo diffusion tensor acquisition technique (MEDITATE), which is capable of acquiring apparent diffusion tensor maps in two scans on a 3T clinical scanner. In each MEDITATE scan, a set of RF pulses generates multiple echoes, the amplitudes of which are diffusion weighted in both magnitude and direction by a pattern of diffusion gradients. As a result, two scans acquired with different diffusion weighting strengths suffice for accurate estimation of diffusion tensor imaging (DTI) parameters. The MEDITATE variation presented here expands previous MEDITATE approaches to adapt to the clinical scanner platform, such as exploiting longitudinal magnetization storage to reduce T2 weighting. Fully segmented multi-shot Cartesian encoding is used for image encoding. MEDITATE was tested on isotropic (agar gel), anisotropic diffusion phantoms (asparagus) and in vivo skeletal muscle in healthy volunteers with cardiac gating. Comparisons of accuracy were performed with standard twice-refocused spin echo (TRSE) DTI in each case and good quantitative agreement was found between diffusion eigenvalues, mean diffusivity and fractional anisotropy derived from TRSE DTI and from the MEDITATE sequence. Orientation patterns were correctly reproduced in both isotropic and anisotropic phantoms, and approximately for in vivo imaging. This illustrates that the MEDITATE method of compressed diffusion encoding is feasible on the clinical scanner platform. With future development and employment of appropriate view-sharing image encoding, this technique may be used in clinical applications requiring time-sensitive acquisition of DTI parameters such as dynamical DTI in muscle.

Stimulated echo diffusion tensor imaging and SPAIR T2 -weighted imaging in chronic exertional compartment syndrome of the lower leg muscles

PURPOSE

To evaluate the performance of diffusion tensor imaging (DTI) in the evaluation of chronic exertional compartment syndrome (CECS) as compared to T2 -weighted (T2w) imaging.

MATERIALS AND METHODS

Using an Institutional Review Board (IRB)-approved, Health Insurance Portability and Accountability Act (HIPAA)-compliant protocol, spectral adiabatic inversion recovery (SPAIR) T2w imaging and stimulated echo DTI were applied to eight healthy volunteers and 14 suspected CECS patients before and after exertion. Longitudinal and transverse diffusion eigenvalues, mean diffusivity (MD), and fractional anisotropy (FA) were measured in seven calf muscle compartments, which in patients were classified by their response on T2w: normal (<20% change), and CECS (>20% change). Mixed model analysis of variance compared subject groups and compartments in terms of response factors (post/pre-exercise ratios) of DTI parameters.

RESULTS

All diffusivities significantly increased (P < 0.0001) and FA decreased (P = 0.0014) with exercise. Longitudinal diffusion responses were significantly smaller than transversal diffusion responses (P < 0.0001). Nineteen of 98 patient compartments were classified as CECS on T2w. MD increased by 3.8 ± 3.4% (volunteer), 7.4 ± 4.2% (normal), and 9.1 ± 7.0% (CECS) with exercise.

CONCLUSION

DTI shows promise as an ancillary imaging method in the diagnosis and understanding of the pathophysiology in CECS. Future studies may explore its utility in predicting response to treatment.