Bruce M. Damon

Validation of diffusion tensor MRI-based muscle fiber tracking.

Diffusion‐tensor (DT) MRI fiber tracking may potentially be used for in vivo structural analysis. The purpose of this study was to assess quantitatively the ability of a DT‐MRI fiber‐tracking algorithm to measure the fiber orientation (pennation) in skeletal muscle in vivo. In five adult Sprague‐Dawley rats, the pennation angle (θ) was measured in the rat lateral gastrocnemius with DT‐MRI (θDT‐MRI) and by direct anatomical inspection (DAI) (θDAI). The mean θDT‐MRI was not significantly different from the mean θDAI. In addition, the two methods were highly correlated (r = 0.89) and the regression of θDT‐MRI on θDAI resulted in a slope not significantly different from 1 and an intercept not significantly different from zero. These data indicate that DT‐MRI‐based fiber tracking as implemented here is a valid tool for in vivo structural analysis of small‐animal skeletal muscle. Magn Reson Med 48:97–104, 2002.

Effects of image noise in muscle diffusion tensor (DT)-MRI assessed using numerical simulations.

Diffusion tensor (DT)‐MRI studies of skeletal muscle provide information about muscle architecture, microstructure, and damage. However, the effects of noise, the diffusion weighting (b)‐value, and partial volume artifacts on the estimation of the diffusion tensor (D) are unknown. This study investigated these issues using Monte Carlo simulations of 3 × 9 voxel regions of interest (ROIs) containing muscle, adipose tissue, and intermediate degrees of muscle volume fractions (fM). A total of 1000 simulations were performed for each of eight b‐values and 11 SNR levels. The dependencies of the eigenvalues (λ1–3), mean diffusivity (λ), and fractional anisotropy (FA), and the angular deviation of the first eigenvector from its true value (α) were observed. For moderate b‐values (b = 435–725 s/mm2) and fM = 1, an accuracy of 5% was obtained for λ1–3, λ, and FA with an SNR of 25. An accuracy of 1% was obtained for λ1–3, λ, and FA with fM = 1 and SNR = 50. For regions with fM = 8/9, 5% accuracy was obtained with SNR = 40. For α, SNRs of ≥25 and ≥45 were required for ±4.5° uncertainty with fM = 1 and fM = 0.5, respectively; SNR ≥ 60 was required for ±9° uncertainty in single muscle voxels. These findings may influence the design and interpretation of DT‐MRI studies of muscle microstructure, damage, and architecture. Magn Reson Med 60:934–944, 2008.

Quantitative assessment of DTI-based muscle fiber tracking and optimal tracking parameters

Diffusion tensor imaging‐based fiber tracking in skeletal muscle has been used to reconstruct and quantify muscle architecture. In addition, the consistent pattern of muscle fiber geometry enables a quantitative assessment of the fiber tracking. This work describes a method to determine the accuracy of individual muscle fiber tracts based on the location at which the fibers terminate, the fiber path, and similarity to the neighboring fibers. In addition, the effect of different stop criteria settings on this quantitative assessment was investigated. Fiber tracking was performed on the tibialis anterior muscle of nine healthy subjects. Complete fiber tracts covered 89.4 ± 9.6% and 75.0 ± 15.2% of the aponeurosis area in the superficial and deep compartments, respectively. Applications of the method include the exclusion of erroneous fiber‐tracking results, quantitative assessment of data set quality, and the assessment of fiber‐tracking stop criteria. Magn Reson Med 61:467–472, 2009.

Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion

This study compared magnetic resonance imaging (MRI) and surface electromyography (EMG) to evaluate the effect of knee angle upon plantar flexion activity in the triceps surae muscles [medial & lateral gastrocnemius (MG, LG) and the soleus (SOL)]. Two weight & height matched groups performed identical protocols, twelve (6M, 6F) in the MRI group, twelve (8M, 4F) in the EMG group. Subjects plantar flexed dynamically for 2 min at 25% of 1-repetition maximum voluntary contraction (1-RM). Exercise was performed with the knee extended (0 degrees flexion), flexed (90 degrees ), and partially flexed (45 degrees ). In the MRI group spin-echo images were acquired before and immediately following each exercise session. T(2) times, calculated at rest and after exercise by fitting the echoes to a monoexponential decay pattern with a least-squares algorithm, were compared with EMG data. In the EMG group a bipolar electrode was used to collect samples were from the MG, LG, SOL, and anterior tibialis (TA) during exercise at each knee angle, MRI also examined the peroneus (PER). At 0 degrees flexion MRI demonstrated a significant post-exercise T(2) increase in the MG (p < or = 0.001), LG (p < or = 0.001), and PER (p < or = 0.01), with no T(2) change in the SOL or TA. At 90 degrees flexion there was a significant T(2) increase in the SOL (p < or = 0.001) with no significant T(2) change in the MG, LG, PER, or TA. At 45 degrees T(2) increased significantly in the SOL (p < or = 0.001) and LG (p < or = 0.05), but not the MG, PER, or TA. EMG produced similar results with the exception that there was significant activity in the TA during the relaxation cycle of the 90 degrees protocol. We conclude that: 1) Soleus activity is measurable by MRI; and 2) MRI and EMG produce similar results from different physiological sources, and are therefore complementary tools for evaluating muscle activity.

Repeatability of DTI-based skeletal muscle fiber tracking

Diffusion tensor imaging (DTI)‐based muscle fiber tracking enables the measurement of muscle architectural parameters, such as pennation angle (θ) and fiber tract length (Lft), throughout the entire muscle. Little is known, however, about the repeatability of either the muscle architectural measures or the underlying diffusion measures. Therefore, the goal of this study was to investigate the repeatability of DTI fiber tracking‐based measurements and θ and Lft. Four DTI acquisitions were performed on two days that allowed for between acquisition, within day, and between day analyses. The eigenvalues and fractional anisotropy were calculated at the maximum cross‐sectional area of, and fiber tracking was performed in, the tibialis anterior muscle of nine healthy subjects. The between acquisitions condition had the highest repeatability for the DTI indices and the architectural parameters. The overall inter class correlation coefficients (ICCs) were greater than 0.6 for both θ and Lft and the repeatability coefficients were θ < 10.2° and Lft < 50 mm. In conclusion, under the experimental and data analysis conditions used, the repeatability of the diffusion measures is very good and repeatability of the architectural measurements is acceptable. Therefore, this study demonstrates the feasibility for longitudinal studies of alterations in muscle architecture using DTI‐based fiber tracking, under similar noise conditions and with similar diffusion characteristics. Copyright

Intracellular acidification and volume increases explain R2 decreases in exercising muscle

Exercise‐induced decreases in the 1H transverse relaxation rate (R2) of muscle have been well documented, but the mechanism remains unclear. In this study, the hypothesis was tested that R2 decreases could be explained by pH decreases and apparent intracellular volume (Vi′) increases. 31P and 1H spectroscopy, biexponential R2 analysis, and imaging were performed prior to and following fatiguing exercise in iodoacetate‐treated (IAA, to inhibit glycolysis), NaCN‐treated (to inhibit oxidative phosphorylation), and untreated frog gastrocnemii. In all exercised muscles, the apparent intracellular R2 (R2i′) and pH decreased, while intracellular osmolytes and Vi′ increased. These effects were larger in NaCN‐treated and untreated muscles than in IAA‐treated muscles. Multiple regression analysis showed that pH and Vi′ changes explain 70% of the R2i′ variance. Separate experiments in unexercised muscles demonstrated causal relationships between pH and R2i′ and between Vi′ and R2i′. These data indicate that the R2 change of exercise is primarily an intracellular phenomenon caused by the accumulation of the end‐products of anaerobic metabolism. In the NaCN‐treated and untreated muscles, the R2i′ change increased as field strength increased, suggesting a role for pH‐modulated chemical exchange. Magn Reson Med 47:14–23, 2002.

DTI-based muscle fiber tracking of the quadriceps mechanism in lateral patellar dislocation

To determine the feasibility of using diffusion tensor MRI (DT‐MRI) ‐based muscle fiber tracking to create biomechanical models of the quadriceps mechanism in healthy subjects and those with chronic lateral patellar dislocation (LPD).

Absolute and relative contributions of BOLD effects to the muscle functional MRI signal intensity time course: effect of exercise intensity.

The time course of exercise‐induced T2‐weighted signal intensity (SI) changes contains an initial rise, early dip, and secondary rise. The purposes of this study were to test the hypothesis that the secondary rise occurs earlier during more intense contractions, and to determine the contribution of BOLD contrast to the SI changes. Eight subjects performed 90‐s isometric dorsiflexion contractions at 30% and 60% of maximum voluntary contraction (MVC) while T2‐weighted (TR/TE = 4000 ms/35 ms) images were acquired and total hemoglobin ([THb]) and oxy‐Hb saturation (%HbO2) were measured. At 30% MVC, [THb] remained constant and %HbO2 decreased from 66.3% (standard error [SEM] = 2.6%) to 32.4% (SEM = 6.4%). At t = 88 s, SI increased by ∼8% and was greater than at t = 8 and 56s. At 60% MVC, [THb] remained constant and %HbO2 decreased from 70.2% (SEM = 2.3%) to 40.4% (SEM = 5.4%). SI increased by ∼17% and at t = 56 and 88 s was greater than at t = 8 and 20 s. The absolute contribution of calculated BOLD effects was −1% at 30% and 60% MVC. The relative contribution was greater at 30% than at 60% MVC (up to −26% and −10%, respectively). We conclude that the secondary rise occurs earlier at 60% MVC and that the relative contribution of BOLD effects is greater during less intense contractions. Magn Reson Med 58:335–345, 2007.

Dual gradient-echo MRI of post-contraction changes in skeletal muscle blood volume and oxygenation.

Analysis of post‐contraction MRI signal intensity (SI) transients may allow noninvasive studies of microvascular reactivity and blood oxygenation recovery. The purpose of this study was to determine the physiological basis for post‐contraction changes in short‐echo (6 ms) and long‐echo (46 ms) gradient‐echo (GRE) MRI signals (S6 and S46, respectively). Six healthy subjects were studied with the use of dual GRE MRI and near‐infrared spectroscopy (NIRS). S6, S46, total hemoglobin concentration ([THb]), and oxyhemoglobin saturation (%HbO2) were measured before, during, and after 2 and 8 s dorsiflexion maximal voluntary contractions, and 5 min of proximal arterial occlusion. The changes in S6 and [THb] after the 2‐s contractions were similar to those following 8‐s contractions, but changes in %HbO2 and S46 were greater following 8‐s contractions than after the 2‐s contractions. [THb] and S6 did not change during and following 5 min of arterial occlusion, but %HbO2 and S46 were both significantly depressed at similar occlusion durations. Also, distance measures indicated similarity between S6 and [THb] and between S46 and %HbO2. We conclude that following brief human skeletal muscle contractions, changes in S6 primarily reflect changes in blood volume and changes in S46 primarily reflect changes in blood oxygenation. Magn Reson Med 57:670–679, 2007.

Quantitative effects of inclusion of fat on muscle diffusion tensor MRI measurements.

To determine the minimum water percentage in a muscle region of interest that would allow diffusion tensor (DT−) MRI data to reflect the diffusion properties of pure muscle accurately.