Search

Purpose: Implement STEAM‐DTI to model time‐dependent diffusion eigenvalues using the random permeable barrier model (RPBM) to study age‐related differences in the medial gastrocnemius (MG) muscle. Validate diffusion model‐extracted fiber diameter for histological assessment. Methods: Diffusion imaging at different diffusion times (Δ) was performed on seven young and six senior participants. Time‐dependent diffusion eigenvalues (λ2(t), λ3(t), and D⊥(t); average of λ2(t) and λ3(t)) were fit to the RPBM to extract tissue microstructure parameters. Biopsy of the MG tissue for histological assessment was performed on a subset of participants (four young, six senior). Results: λ3(t) was significantly higher in the senior cohort for the range of diffusion times. RPBM fits to λ2(t) yielded fiber diameters in agreement to those from histology for both cohorts. The senior cohort had lower values of volume fraction of membranes, ζ, in fits to λ2(t), λ3(t), and D⊥(t) (significant for fit to λ3(t)). Fits of fiber diameter from RPBM to that from histology had the highest correlation for the fit to λ2(t). Conclusion: The age‐related patterns in λ2(t) and λ3(t) could tentatively be explained from RPBM fits; these patterns may potentially arise from a decrease in fiber asymmetry and an increase in permeability with age. [ABSTRACT FROM AUTHOR]

2D Janus transition metal dichalcogenides (TMDs) have attracted attention due to their emergent properties arising from broken mirror symmetry and self‐driven polarization fields. While it has been proposed that their vdW superlattices hold the key to achieving superior properties in piezoelectricity and photovoltaic, available synthesis has ultimately limited their realization. Here, the first packed vdW nanoscrolls made from Janus TMDs through a simple one‐drop solution technique are reported. The results, including ab initio simulations, show that the Bohr radius difference between the top sulfur and the bottom selenium atoms within Janus MSeS${\rm{M}}_{{\rm{Se}}}^{\rm{S}}$ (M = Mo, W) results in a permanent compressive surface strain that acts as a nanoscroll formation catalyst after small liquid interaction. Unlike classical 2D layers, the surface strain in Janus TMDs can be engineered from compressive to tensile by placing larger Bohr radius atoms on top (MSSe)${\rm{M}}_{\rm{S}}^{{\rm{Se}}})\ $to yield inverted C scrolls. Detailed microscopy studies offer the first insights into their morphology and readily formed Moiré lattices. In contrast, spectroscopy and FETs studies establish their excitonic and device properties and highlight significant differences compared to 2D flat Janus TMDs. These results introduce the first polar Janus TMD nanoscrolls and introduce inherent strain‐driven scrolling dynamics as a catalyst to create superlattices. [ABSTRACT FROM AUTHOR]

Recent studies have established Van der Waals (vdW) layered and 2D rare‐earth tritellurides (RTe3) as superconductors and near room‐temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium‐based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe3 sheets oxidize to form thin TeOx layers that are primarily confined to the surface, edges, and grain boundaries. Time‐resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe3 compared to the least stable HoTe3. Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O2 molecules, while humidity (H2O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe3 layers owing to the presence of underlying pristine/nonoxidized RTe3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in‐depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics. [ABSTRACT FROM AUTHOR]

This article discusses the use of MRI to assess knee cartilage and its role in the management of osteoarthritis (OA). Traditional MRI approaches have limitations in detecting subtle changes in cartilage degeneration, so quantitative cartilage MRI techniques have been developed to assess compositional changes in the cartilage. The article also explores the effects of loading on cartilage deformation and the potential for using MR imaging to study the short-term effects of loading. The authors present a new approach that uses prospective motion correction to capture cartilage deformation during the entire loading and recovery cycle. While this technique shows promise, further studies are needed to establish its clinical utility in the management of OA. [Extracted from the article]

The article discusses the use of diffusion tensor imaging (DTI) to characterize the architecture of the supraspinatus and infraspinatus muscles in the shoulder. The authors propose a new method that captures myocyte orientation using anatomical points as landmarks, allowing for the quantification of spatial patterns of myocyte angulation. The study shows that trained shoulders have different spatial patterns of myocyte orientation compared to untrained shoulders, suggesting that muscle fibers remodel in response to exercise. While the new method is computationally less intense and requires less user interaction compared to fiber quantification, further research is needed to determine its clinical utility and its ability to detect structural changes in the muscle. Both fiber tracking/quantification and pixel-level myocyte orientation are promising approaches to studying skeletal muscle architecture, with each method providing unique information. [Extracted from the article]

Passive materials in human skeletal muscle tissues play an important role in force output of skeletal muscles. This paper introduces a multiscale modeling framework to investigate how age‐associated variations on microscale passive muscle components, including microstructural geometry (e.g., connective tissue thickness) and material properties (e.g., anisotropy), influence the force output and deformations of the continuum skeletal muscle. We first define a representative volume element (RVE) for the microstructure of muscle and determine the homogenized macroscale mechanical properties of the RVE from the separate mechanical properties of the individual components of the RVE, including muscle fibers and connective tissue with its associated collagen fibers. The homogenized properties of the RVE are then used to define the elements of the continuum muscle model to evaluate the force output and deformations of the whole muscle. Conversely, the regional deformations of the continuum model are fed back to the RVE model to determine the responses of the individual microscale components. Simulations of muscle isometric contractions at a range of muscle lengths are performed to investigate the effects of muscle architectural changes (e.g., pennation angles) due to aging on force output and muscle deformation. The correlations between the pennation angle, the shear deformation in the microscale connective tissue (an indicator for the lateral force transmission), the angle difference between the fiber direction and principal strain direction and the resulting shear deformation at the continuum scale, as well as the force output of the skeletal muscle are also discussed. [ABSTRACT FROM AUTHOR]

Purpose: This study implements a compressed sensing (CS) 3‐directional velocity encoded phase contrast (VE‐PC) imaging for studying skeletal muscle kinematics within 40 s. Methods: Independent variable density random sampling in the phase encoding direction for each temporal frame was implemented for various combinations of CS‐factors and views per segment. CS reconstruction was performed for the combined multicoil, temporal datasets using temporal Fourier transform followed by temporal principal component analysis sparsifying transformations. The method was tested on a flow phantom and in vivo, on velocity and strain rate of the medial gastrocnemius muscle of 11 subjects performing isometric contractions. Results: For the flow phantom, velocity from 8 undersampled sequences matched very well with the flowmeter values over a range of velocities spanning in vivo muscle velocities. Bland‐Altman plots of the peak strain rate eigenvalues comparing 7 undersampled sequences was in good agreement with the reference (full k‐space) scan. CS‐factor of 4 combined with views per segment of 4 (scan times reduced by 4) yielded images with no visual artifacts allowing and yielded velocities and strain rate maps in the lower leg muscle in 40 s. Conclusion: This study shows that a reduction in scan time of velocity encoded phase contrast imaging up to a factor of 4 is possible using the proposed CS reconstruction. [ABSTRACT FROM AUTHOR]

Background: Diffusion tensor imaging (DTI) assesses underlying tissue microstructure, and has been applied to studying skeletal muscle. Unloading of the lower leg causes decreases in muscle force, mass, and muscle protein synthesis as well as changes in muscle architecture.Purpose: To monitor the change in DTI indices in the medial gastrocnemius (MG) after 4-week unilateral limb suspension (ULLS) and to explore the feasibility of extracting tissue microstructural parameters based on a two-compartment diffusion model.Study Type: Prospective cohort study.Subjects: Seven moderately active subjects (29.1 ± 5.7 years).Field Strength/sequence: 3T, single-shot fat-suppressed echo planar spin echo sequence.Assessment: Suspension-related changes in the DTI indices (eigenvalues: λ1 , λ2 , λ3 , fractional anisotropy; coefficient of planarity) were statistically analyzed. Changes in model-derived tissue parameters (muscle fiber circularity and diameter, intracellular volume fraction, and residence time) after suspension are qualitatively discussed.Statistical Tests: Changes in the DTI indices of the MG between pre- and postsuspension were assessed using repeated-measures two-way analysis of variance (ANOVA).Results: All the eigenvalues (λ1 : P = 0.025, λ2 : P = 0.035, λ3 : P = 0.049) as well as anisotropic diffusion coefficient (P = 0.029) were significantly smaller post-ULLS. Diffusion modeling revealed that fibers were more circular (circularity index increased from 0.55 to 0.95) with a smaller diameter (diameter decreased from 82-60 μm) postsuspension.Data Conclusion: We have shown that DTI indices change with disuse and modeling can relate these voxel level changes to changes in the tissue microarchitecture.Level Of Evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018. [ABSTRACT FROM AUTHOR]

Background: Strain rate (SR) is a measure of the rate of regional deformation that can be computed by analyzing velocity‐encoded phase‐contrast 2D images. Recent studies have explored the changes in normal components of the strain tensor in aging muscle, while shear strain may also provide valuable information. Purpose: To compute the shear SR from velocity‐encoded MRI of the lower leg and to study the correlation of SR parameters measured in the medial gastrocnemius (MG) to muscle force in a cohort of young and senior subjects. Study Type: Prospective cohort study. Subjects: Six young (26.1 ± 2.3 years) and six senior (76.7 ± 8.3 years) healthy females; two other subjects were scanned on three separate occasions for repeatability studies. Field Strength/Sequence: 1.5T using a single oblique sagittal slice with velocity‐encoding in three directions (velocity‐encoded phase contrast gradient echo sequence). Assessment: Age‐related and regional differences in the SR eigenvalues (SRfiber, SRin‐plane), normal SRs (SRff, SRcc), and shear SRs (SRfc, SRfc_max) were statistically analyzed. Statistical Tests: Difference between young and senior cohorts were assessed using two‐way analysis of variance (ANOVAs). The coefficient of variation and repeatability coefficient were calculated from repeat studies. Univariate and stepwise multivariable linear regression was performed to identify predictors of force. Results: During isometric plantarflexion contraction, SRs in the principal basis (SRfiber, SRin‐plane) and maximum shear SR (SRfc_max) was significantly lower in the senior cohort (P < 0.05). On multiple variable regression, maximum shear SR (SRfc_max) and normal SR in the fiber cross‐section (SRcc) were significantly associated with force (R = 0.681, F = 14.034, P < 0.001). Data Conclusion: This study establishes that computation of shear strain is feasible and is a significant predictor of force variability with age. Level of Evidence: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:1351–1357. [ABSTRACT FROM AUTHOR]

Background: Urinary incontinence is a major clinical problem arising primarily from age-related degenerative changes to the sphincter muscles. However, the precise anatomy of the normal male sphincter muscles has yet to be established. Diffusion tensor imaging (DTI) may offer a unique insight into muscle microstructure and fiber architecture.Purpose: To explore the anatomy of the urethral sphincter muscles pertinent to urinary continence function using DT-MRI.Study Type: Prospective cohort study.Subjects: Eleven normal male subjects (mean age: 25.4 years); two subjects were scanned in three separate sessions to assess reproducibility.Field Strength/sequence: 3T; using a diffusion-weighted spin echo planar sequence.Assessment: DT parameters including fractional anisotropy (FA), primary (λ1 ), secondary (λ2 ), and tertiary (λ3 ) eigenvalues, Apparent diffusion coefficient and radial diffusivity were analyzed statistically, while tracked muscle fibers were assessed visually.Statistical Tests: Regional differences (sphincters and longitudinal muscle of the urethra) in the DTI indices were assessed by one-way analysis of variance. A Tukey post-hoc test was used to identify significant differences between muscle regions.Results: Two sphincter muscles, one proximal near the base of the bladder, corresponding to the lisso-sphincter, and the other distal to the end of the prostate corresponding to the rhabdo-sphincter, surrounding a central urethral muscle fiber bundle, were clearly identified. FA was higher and λ3 lower in the proximal sphincter muscle compared to the central urethral muscle and the distal sphincter (P < 0.05). The average coefficient of variation ranged from 5-12% for the DTI indices.Data Conclusion: Since DTI values are known to reflect underlying tissue microarchitecture, significant differences in DTI indices identified here between the muscles of the urethral complex may potentially arise from differences in tissue microarchitecture that may in turn be related to the specific function of the sphincter and other muscles.Level Of Evidence: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;48:1002-1011. [ABSTRACT FROM AUTHOR]

Purpose This study explores changes in strain rate (SR) (rate of regional deformation) parameters extracted from velocity-encoded MRI and their relationship to muscle force loss following 4-week unilateral lower limb suspension in healthy humans. Methods Two-dimensional SR maps were derived from three directional velocity-encoded MR phase-contrast images of the medial gastrocnemius in seven subjects. Atrophy-related and regional differences in the SR eigenvalues, angle between the SR and muscle fiber (SR-fiber angle), and strain rates in the fiber basis were statistically analyzed using analysis of variance and linear regression. Results During isometric contraction, SR in the fiber cross section (SRin-plane) was significantly lower, and the SR-fiber angle was significantly higher postsuspension ( P < 0.05). On multiple variable regression analysis, the volume of medial gastrocnemius, SRin-plane, and SR-fiber angle were significantly associated with force and changes in the, and the SR eigenvalues and shear SR were significantly associated with change in force with disuse. Conclusions Changes in SR-fiber angle, SRin-plane, and shear SR as well as their ability to predict force and force changes may reflect the role of remodeling of the extracellular matrix in disuse atrophy and its functional consequence in reducing lateral transmission of force. Magn Reson Med 79:912-922, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose To develop and evaluate an automated algorithm to segment intramuscular adipose (IMAT) and connective (IMCT) tissue from musculoskeletal MRI images acquired with a dual echo Ultrashort TE (UTE) sequence. Theory and Methods The dual echo images and calculated structure tensor images are the inputs to the multichannel fuzzy cluster mean (MCFCM) algorithm. Modifications to the basic multichannel fuzzy cluster mean include an adaptive spatial term and bias shading correction. The algorithm was tested on digital phantoms simulating IMAT/IMCT tissue under varying conditions of image noise and bias and on ten subjects with varying amounts of IMAT/IMCT. Results The MCFCM including the adaptive spatial term and bias shading correction performed better than the original MCFCM and adaptive spatial MCFCM algorithms. IMAT/IMCT was segmented from the unsmoothed simulated phantom data with a mean Dice coefficient of 0.933 ±0.001 when contrast-to-noise (CNR) was 140 and bias was varied between 30% and 65%. The algorithm yielded accurate in vivo segmentations of IMAT/IMCT with a mean Dice coefficient of 0.977 ±0.066. Conclusion The proposed algorithm is completely automated and yielded accurate segmentation of intramuscular adipose and connective tissue in the digital phantom and in human calf data. Magn Reson Med 77:870-883, 2017. © 2016 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]

Purpose The strain rate (SR) tensor measures the principal directions and magnitude of the instantaneous deformation; this study aims to track age-related changes in the 2D SR tensor in the medial gastrocnemius during passive joint rotation and active isometric contraction. Methods SR tensors were derived from velocity encoded magnetic resonance phase-contrast images in nine young (28 years) and eight senior (78 years) women. Strain rates along and in the cross-section of the fiber were calculated from the SR tensor and used to derive the out-plane SR. Age-related and regional differences in the SR eigenvalues, orientation, and the angle between the SR and muscle fiber (SR-fiber angle) were statistically analyzed. Results SR along the fiber was significantly different between the cohorts during isometric contraction with higher values in the young ( P < 0.05). The SR-fiber angle was larger in the young for both motion types but this difference was not statistically significant. Significant regional differences in the SR indices was seen in passive joint rotation ( P < 0.05) for both cohorts. Conclusion SR mapping reflects age-related and regional differences during active and passive motion respectively; this may arise from differences in contractility (active motion) and elastic properties (active and passive motion). Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc. Magn Reson Med 73:1852-1863, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Purpose To investigate age related changes in diffusion tensor indices and fiber architecture of the medial and lateral gastrocnemius (MG and LG) muscles using diffusion tensor imaging (DTI). Materials and Methods The lower leg of five young and five senior subjects was scanned at 3 Tesla and DTI indices extracted using three methods: region of interest, histogram, and tract based. Tracked fibers were automatically edited to ensure physiologically relevant tracks. Pennation angles were measured with respect to the deep and superficial aponeuroses of both muscles. Results The three methods provided internally consistent measures of the DTI indices (correlation coefficient in the range of 0.90-0.99). The primary, secondary, and tertiary eigenvalues in the MG and LG increased significantly in the senior cohort ( P < 0.05), while the small increase in fractional anisotropy with age was not significant (MG/LG: P = 0.39/0.85; 95% confidence interval: [−0.059/−0.056, 0.116/0.064]). Fiber lengths of MG fibers originating distally were significantly decreased in seniors ( P < 0.05) while pennation angles decreased with age in the MG and LG but this was not significant. Conclusion Fiber atrophy and increased fibrosis have opposing effects on the diffusion indices resulting in a complicated dependence with aging. Fiber architectural changes could play a role in determining aging muscle function. J. Magn. Reson. Imaging 2015;41:941-953. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Purpose: To design a computer-controlled, magnetic resonance (MR)-compatible foot pedal device that allows in vivo mapping of changes in morphology and in strain of different musculoskeletal components of the lower leg under passive, isometric, concentric, and eccentric contractions. Materials and Methods: A programmable servomotor in the control room pumped hydraulic fluid to rotate a foot-pedal inside the magnet. To validate the performance of the device, six subjects were imaged with gated velocity-encoded phase-contrast (VE-PC) imaging to investigate the dynamics of muscle and aponeurotic structures. Results: Artifact-free VE-PC imaging clearly delineated different muscle compartments by differences in distribution of mechanical strains. High repeatability of contraction cycles allowed establishing that fascicles lengthened 6.1% more during passive compared with eccentric contractions. Aponeurosis separation during passive (range between three locations: −2.6≈1.3 mm) and active (range: −2.4≈1.6 mm) contractions were similar but significantly different from concentric (range: −0.9≈3.3 mm), with proximal and distal regions showing mostly negative values for the first two modes, but positive for the last. Conclusion: The device was sufficiently robust and artifact-free to accurately assess, using VE-PC imaging, physiologically important structure and dynamics of the musculotendon complex. J. Magn. Reson. Imaging 2012;36:405-410. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The present study examined spatial changes in the muscle activation pattern with different contraction intensity using magnetic resonance imaging (MRI). Transverse relaxation time (T2)-weighted MRI was used to acquire 22 axial slices, which covered the entire medial gastrocnemius muscle, before and immediately after seven sets of 10 plantarflexions with loads that were approximately 20 and 60% of their one-repetition maximum, respectively. Reconstructions of the activated regions from MR images revealed the following: (1) the muscle activation determined by MRI correlated significantly ( P < 0.05) with the results of electromyography; (2) the activated area distribution increased with increasing force levels; and (3) differential activation between 20 and 60% load was distributed in a spatially non-uniform manner within the muscle, and some regions indicated reduced activity. These results indicate that at submaximal contraction intensity, activation does not necessarily increase in the whole muscle with increase in contraction intensity. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the reproducibility of diffusion tensor imaging (DTI) -derived indices and fiber architecture of calf muscles at 1.5 Tesla (T), to establish an imaging based method to confirm ankle position, and to compare fiber architecture at different ankle positions. Materials and Methods: Six subjects were imaged at 1.5T with the foot in neutral and plantarflexed positions. DTI indices were calculated in four muscle compartments (medial and lateral gastrocnemius [MG, LG], superficial and deep anterior tibialis [AT-S, AT-D]). Two subjects were scanned on 3 days to calculate the coefficient of variability (CV) and the repeatability coefficient (RC). Results: DTI indices were close to the values obtained in earlier 3T and 1.5T studies. Fractional anisotropy decreased significantly in the MG and increased significantly in the AT-S and AT-D compartments while fiber orientation with respect to the magnet Z-axis increased significantly in the MG and decreased significantly in the AT-S compartment with plantarflexion. The CV and RC for the DTI indices and fiber orientations were comparable to 3T studies. Fiber lengths and orientation angles in the MG matched corresponding measures from ultrasound studies. Conclusion: DTI at 1.5T provides reproducible measures of diffusion indices and fiber architecture of calf muscle at different muscle lengths. J. Magn. Reson. Imaging 2011;. © 2011 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Purpose To devise a method using velocity encoded phase contrast MRI and MR-compatible dynamometry, for in vivo estimation of elastic properties of the human Achilles tendon and to assess within-session and day-to-day repeatability of this technique. Materials and Methods Achilles tendon force and calcaneus-movement-adjusted displacement were measured during a submaximal isometric plantarflexion in 4 healthy subjects, four repeated trials each. The measured force-length (F-L) relationship was least-squares fitted to a cubic polynomial. Typical error was calculated for tendon displacement at multiple force levels, stiffness from the 'linear region,' and transition point from the displacement point separating the linear and nonlinear parts of the curve. Results Displacements of the tendon were determined up to a maximum force of 500N, with mean stiffness of 234 ± 53 N/mm, mean transition point of 2.70 ± 0.23 mm and maximum tendon displacement of 3.38 mm. Variability of tendon displacement was not dependent on the force level. Overall typical errors were 0.09 mm and 0.16 mm for within-session and between days, respectively. Typical error of transition point was 0.05 mm and 0.14 mm. Stiffness had typical errors of 47.24 N/mm and 51.95 N/mm. The tendon cross-sectional area and calcaneus displacement were found to be very significant factors in minimizing the individual differences in F-L curves. Conclusion The method yielded F-L relationships, stiffness, and transition point values that showed good within and day-to-day repeatability. The technique compared well with the more conventional one using ultrasonography. Its reliability indicates potential for measuring tendon structural changes after an injury, disease, and altered loading. J. Magn. Reson. Imaging 2008;28:1039-1045. © 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Current methods of clinical assessment of muscle coordination and function after stroke do not provide information on deep muscles. The objective of this study was to examine how stroke affects both superficial and deep muscles' coordination and whether muscle function improves after rehabilitation. Muscle function, coordination, and activity of quadriceps femoris (QF) and hamstrings were evaluated in 10 stroke patients with mild hemiparesis and in 6 controls using velocity-encoded cine phase-contrast magnetic resonance imaging (VE-PC MRI), surface electromyography (sEMG), and maximal voluntary isometric contraction torque (MVC). At baseline, the peak muscle velocity of the rectus femoris (RF) and the ratio between the peak velocities of the RF and vasti were lower in the affected limb (AL) of stroke patients than in controls. Co-contraction of agonists and antagonists was higher in the AL than in controls. Muscle activity measured by sEMG showed similar behavior. After rehabilitation, the activity ratio of hamstrings and adductors to QF decreased slightly toward normal so there were no significant differences between the AL and controls. Impaired biarticular RF muscle function in stroke patients is the limiting factor during knee extension-flexion movements. After rehabilitation, improved functional performance was partly explained by the fact that the activities of the RF and vasti became more synchronized. VE-PC MRI can provide quantitative in vivo measurements of both superficial and deep muscles, and the information acquired after stroke can be utilized to render therapy more efficient and individually tailored. Muscle Nerve, 2008 [ABSTRACT FROM AUTHOR]

Purpose To study spatial and temporal distribution of wall shear stress (WSS) of intracranial aneurysm models using magnetic resonance phase contrast (MR-PC) imaging, and to compare results to those obtained in our previous study using laser Doppler velocimetry (LDV). Materials and Methods High-resolution MR-PC imaging was performed on aneurysm models created from 3D in vivo data of a basilar artery (BA) tip and a middle cerebral artery (MCA) aneurysm. WSS was measured by calculating the rate of shearing strain using image postprocessing techniques. WSS was characterized using several parameters, including average WSS and oscillatory shear index (OSI). Results Both aneurysms showed the highest WSS at an ostium. A bleb region in the BA tip and a dome region in the MCA aneurysm were consistently exposed to high WSS within a small local area with high spatial variation and little temporal change in comparison to other aneurysmal regions. High correlation was noted for the spatial distribution at the peak systole between the MR-PC and the LDV studies, with the correlation coefficient value of 77% to 81%. Conclusion MR-PC imaging showed good agreement with the reference LDV study, confirming its potential to study hemodynamics of physiologically realistic in vitro intracranial aneurysm models. Characteristic patterns of WSS, such as high spatial variation and absence of temporal variation, combined with locally increased high WSS as determined by MR-PC imaging, may elucidate the effect of WSS on aneurysmal development and rupture. J. Magn. Reson. Imaging 2007;25:1120-1130. © 2007 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Purpose To demonstrate the feasibility of in vivo calf muscle fiber tracking in human subjects. Materials and Methods An EPI-based diffusion tensor imaging (DTI) sequence with six-direction diffusion gradient sensitization was implemented, and DT images were acquired at 3 Tesla on five subjects using an extremity coil. The mean diffusivity, fractional anisotropy (FA), and fiber angle (with respect to the magnet z-axis) were measured in different muscles, and fibers were tracked from several regions of interest (ROIs). Results The fiber orientations in the current DTI studies agree well with those determined in previous spectroscopic studies. The orientation angles ranged from 13.4° in the lateral gastrocnemius to 48.5° in the medial soleus. The diffusion ellipsoid in muscle tissue is anisotropic and approximates a prolate model, as shown by color maps of the anisotropy. Fibers were tracked from the different muscle regions, and the unipennate and bipennate structure of muscle fibers was visualized. Conclusion The study clearly shows that in vivo fiber tracking of muscle fibers is feasible and could potentially be applied to study muscle structure function relationships. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Standard phase-contrast flow quantification (PC-FQ) using radiofrequency (RF) spoiled steady-state (SS) incoherent gradient-echo sequences have a relatively low signal-to-noise ratio (SNR). Unspoiled SS coherent (SSC) gradient-echo sequences have a higher intrinsic SNR and are T2/ T1 weighted so that blood has a relatively large signal compared to other tissues. An SSC sequence that was modified to allow in-plane velocity encoding is presented. Velocity encoding was achieved by inverting the readout gradients. This offers the benefit that there is no resultant increase in repetition time (TR), which avoids increased sensitivity to off-resonance artifacts when conventional velocity-encoding methods using separate velocity-encoding gradients are extended to SSC sequences. The results of standard PC-FQ and the new method from in vitro experiments of constant and sinusoidal flow, and in vivo imaging of the carotid artery were compared. Vector field maps and paths obtained from particle-tracking calculations based on the velocity-encoded images were used to visualize the velocity data. The technique has the potential to increase the precision of PC-FQ measurements. Magn Reson Med 54:138-145, 2005. © 2005 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Purpose To develop and compare phase-contrast (PC) and spin-tag (ST) MR imaging techniques for accurate quantification of velocity and displacement distribution in the muscle tendon complex of the lower leg during isometric contractions under in vivo conditions, in healthy subjects and subjects with atrophy. Materials and Methods Techniques were developed to acquire PC and ST dynamic images, gated to the force exerted by a subject during isometric contraction. Algorithms were optimized for correction of phase shading errors. Flow velocity quantification was validated in phantoms and ex vivo rabbit muscles. Trajectories of pixels calculated from PC images were compared with those in ST images. Velocity distributions were determined in healthy muscles, those atrophied by four weeks of suspension, and during physiotherapy-induced recovery. Results The technique developed allowed acquisition of images retrospectively gated to the isometric contraction performed with the subject in the scanner. Significant phase shading errors in PC images (≈3 cm/second over the field of view) were reduced to the background noise level by the correction algorithm. Tissue trajectories calculated from PC images agreed very well with those from ST images both in human and excised animal tissues. Peak velocities in atrophied muscles were significantly lower compared to the preatrophy state but recovered to baseline values after six weeks of therapy. Conclusion We show the feasibility of monitoring muscle velocity and tissue displacement during voluntary contractions in humans under in vivo conditions using MR tissue motion mapping methods. The clinical feasibility of this technique in monitoring atrophied muscle is also demonstrated. J. Magn. Reson. Imaging 2004;20:1008-1019. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

This study demonstrates the feasibility of in vivo prostate diffusion tensor imaging (DTI) in human subjects. We implemented an EPI-based diffusion-weighted (DW) sequence with seven-direction diffusion gradient sensitization, and acquired DT images from six subjects using cardiac gating with a phased-array prostate surface coil operating in a linear mode. We calculated two indices to quantify diffusion anisotropy. The direction of the eigenvector corresponding to the leading eigenvalue was displayed by means of a color-coding scheme. The average diffusion values of the prostate peripheral zone (PZ) and central gland (CG) were 1.95 ± 0.08 × 10-3 mm2 s and 1.53 ± 0.34 × 10-3 mm2 s, respectively. The average fractional anisotropy (FA) values for the PZ and CG were 0.46 ± 0.04 and 0.40 ± 0.08, respectively. The diffusion ellipsoid in prostate tissue was anisotropic and approximated a prolate model, as shown in the color maps of the anisotropy. Consistent with the tissue architecture, the prostate fiber orientations were predominantly in the superior-inferior (SI) direction for both the PZ and CG. This study shows the feasibility of in vivo DTI and establishes normative DT values for six subjects. Magn Reson Med 52:530-537, 2004. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Human aging results in a progressive decline in the active force generation capability of skeletal muscle. While many factors related to the changes of morphological and structural properties in muscle fibers and the extracellular matrix (ECM) have been considered as possible reasons for causing age‐related force reduction, it is still not fully understood why the decrease in force generation under eccentric contraction (lengthening) is much less than that under concentric contraction (shortening). Biomechanically, it was observed that connective tissues (endomysium) stiffen as ages, and the volume ratio of connective tissues exhibits an age‐related increase. However, limited skeletal muscle models take into account the microstructural characteristics as well as the volume fraction of tissue material. This study aims to provide a numerical investigation in which the muscle fibers and the ECM are explicitly represented to allow quantitative assessment of the age‐related force reduction mechanism. To this end, a fiber‐level honeycomb‐like microstructure is constructed and modeled by a pixel‐based Reproducing Kernel Particle Method (RKPM), which allows modeling of smooth transition in biomaterial properties across material interfaces. The numerical investigation reveals that the increased stiffness of the passive materials of muscle tissue reduces the force generation capability under concentric contraction while maintains the force generation capability under eccentric contraction. The proposed RKPM microscopic model provides effective means for the cellular‐scale numerical investigation of skeletal muscle physiology. Novelty Statement: A cellular‐scale honeycomb‐like microstructural muscle model constructed from a histological cross‐sectional image of muscle is employed to study the causal relations between age‐associated microstructural changes and age‐related force loss using Reproducing Kernel Particle Method (RKPM).The employed RKPM offers an effective means for modeling biological materials based on pixel points in the medical images and allow modeling of smooth transition in the material properties across interfaces.The proposed microstructure‐informed muscle model enables quantitative evaluation on how cellular‐scale compositions contribute to muscle functionality and explain differences in age‐related force changes during concentric, isometric and eccentric contractions. [ABSTRACT FROM AUTHOR]