Effects of exercise‐induced intracellular acidosis on the phosphocreatine recovery kinetics: a 31P MRS study in three muscle groups in humans

Little is known about the metabolic differences that exist among different muscle groups within the same subjects. Therefore, we used ³¹P‐magnetic resonance spectroscopy (³¹P‐MRS) to investigate muscle oxidative capacity and the potential effects of pH on PCr recovery kinetics between muscles of different phenotypes (quadriceps (Q), finger (FF) and plantar flexors (PF)) in the same cohort of 16 untrained adults. The estimated muscle oxidative capacity was lower in Q (29 ± 12 mM min^‐1, CV_{inter‐subject} = 42%) as compared with PF (46 ± 20 mM min^‐1, CV_{inter‐subject} = 44%) and tended to be higher in FF (43 ± 35 mM min^‐1, CV_{inter‐subject} = 80%). The coefficient of variation (CV) of oxidative capacity between muscles within the group was 59 ± 24%. PCr recovery time constant was correlated with end‐exercise pH in Q (p < 0.01), FF (p < 0.05) and PF (p <0.05) as well as proton efflux rate in FF (p < 0.01), PF (p < 0.01) and Q (p = 0.12). We also observed a steeper slope of the relationship between end‐exercise acidosis and PCr recovery kinetics in FF compared with either PF or Q muscles. Overall, this study supports the concept of skeletal muscle heterogeneity by revealing a comparable inter‐ and intra‐individual variability in oxidative capacity across three skeletal muscles in untrained individuals. These findings also indicate that the sensitivity of mitochondrial respiration to the inhibition associated with cytosolic acidosis is greater in the finger flexor muscles compared with locomotor muscles, which might be related to differences in permeability in the mitochondrial membrane and, to some extent, to proton efflux rates. Copyright © 2013 John Wiley & Sons, Ltd.     

Diffusion‐weighted MRI with intravoxel incoherent motion modeling for assessment of muscle perfusion in the thigh during post‐exercise hyperemia in younger and older adults

Aging is associated with impaired endothelium‐dependent vasodilation that leads to muscle perfusion impairment and contributes to organ dysfunction. Impaired muscle perfusion may result in inadequate delivery of oxygen and nutrients during and after muscle contraction, leading to muscle damage. The ability to study the relationship between perfusion and muscle damage has been limited using traditional muscle perfusion measures, which are invasive and risky. To overcome this limitation, we optimized a diffusion‐weighted MRI sequence and validated an intravoxel incoherent motion (IVIM) analysis based on Monte Carlo simulation to study muscle perfusion impairment with aging during post‐exercise hyperemia. Simulation results demonstrated that the bias of IVIM‐derived perfusion fraction (f_p ) and diffusion of water molecules in extra‐vascular tissue (D ) ranged from ?3.3% to 14% and from ?16.5% to 0.002%, respectively, in the optimized experimental condition. The dispersion in f_p and D ranged from 3.2% to 9.5% and from 0.9% to 1.1%, respectively. The mid‐thigh of the left leg of four younger (21–30 year old) and four older (60–90 year old) healthy females was studied using the optimized protocol at baseline and at seven time increments occurring every 3.25 min following in‐magnet dynamic knee extension exercise performed using a MR‐compatible ergometer with a workload of 0.4 bar for 2.5 min. After exercise, both f_p and D significantly increased in the rectus femoris (active muscle during exercise) but not in adductor magnus (inactive muscle), reflecting the fact that the local increase in perfusion with both groups showed a maximum value in the second post‐exercise time‐point. A significantly greater increase in perfusion from the baseline (p < 0.05) was observed in the younger group (37 ± 12.05%) compared with the older group (17.57 ± 15.92%) at the first post‐exercise measurement. This work establishes a reliable non‐invasive method that can be used to study the effects of aging on dynamic changes in muscle perfusion as they relate to important measures of physical function.     

Age‐ and sex‐related differences in muscle phosphocreatine and oxygenation kinetics during high‐intensity exercise in adolescents and adults

The aim of this investigation was to examine the adaptation of the muscle phosphates (e.g. phosphocreatine (PCr) and ADP) implicated in regulating oxidative phosphorylation, and oxygenation at the onset of high intensity exercise in children and adults. The hypotheses were threefold: primary PCr kinetics would be faster in children than adults; the amplitude of the PCr slow component would be attenuated in children; and the amplitude of the deoxyhaemoglobin/myoglobin (HHb) slow component would be reduced in children. Eleven children (5 girls, 6 boys, 13 ± 1 years) and 11 adults (5 women, 6 men, 24 ± 4 years) completed two to four constant work rate exercise tests within a 1.5 T MR scanner. Quadriceps muscle energetics during high intensity exercise were monitored using ³¹P‐MRS. Muscle oxygenation was monitored using near‐infrared spectroscopy. The time constant for the PCr response was not significantly different in boys (31 ± 10 s), girls (31 ± 10 s), men (44 ± 20 s) or women (29 ± 14 s, main effects: age, p = 0.37, sex, p = 0.25). The amplitude of the PCr slow component relative to end‐exercise PCr was not significantly different between children (23 ± 23%) and adults (17 ± 13%, p = 0.47). End‐exercise [PCr] was significantly lower, and [ADP] higher, in females (18 ± 4 mM and 53 ± 16 µM) than males (23 ± 4 mM, p = 0.02 and 37 ± 11 µM, p = 0.02), but did not differ with age ([PCr]: p = 0.96, [ADP]: p = 0.72). The mean response time for muscle tissue deoxygenation was significantly faster in children (22 ± 4 s) than adults (27 ± 7 s, p = 0.01). The results of this study show that the control of oxidative metabolism at the onset of high intensity exercise is adult‐like in 13‐year‐old children, but that matching of oxygen delivery to extraction is more precise in adults. Copyright © 2010 John Wiley & Sons, Ltd.     

Time‐dependent diffusion MRI as a probe of microstructural changes in a mouse model of Duchenne muscular dystrophy

Dystrophic muscles show a high variability of fibre sizes and altered sarcolemmal integrity, which are typically assessed by histology. Time‐dependent diffusion MRI is sensitive to tissue microstructure and its investigation through age‐related changes in dystrophic and healthy muscles may help the understanding of the onset and progression of Duchenne muscular dystrophy (DMD). We investigated the capability of time‐dependent diffusion MRI to quantify age and disease‐related changes in hind‐limb muscle microstructure between dystrophic (mdx) and wild‐type (WT) mice of three age groups (7.5, 22 and 44 weeks). Diffusion time‐dependent apparent diffusion coefficients (ADCs) of the gastrocnemius and tibialis anterior muscles were determined versus age and diffusion‐gradient orientation at six diffusion times (Δ; range: 25–350 ms). Mean muscle ADCs were compared between groups and ages, and correlated with T₂, using Student's t test, one‐way analysis of variance and Pearson correlation, respectively. Muscle fibre sizes and sarcolemmal integrity were evaluated by histology and compared with diffusion measurements. Hind‐limb muscle ADC showed characteristic restricted diffusion behaviour in both mdx and WT animals with decreasing ADC values at longer Δ. Significant differences in ADC were observed at long Δ values (≥ 250 ms; p < 0.05, comparison between groups; p < 0.01, comparison between ages) with ADC increased by 5–15% in dystrophic muscles, indicative of reduced diffusion restriction. No significant correlation was found between T₂ and ADC. Additionally, muscle fibre size distributions showed higher variability and lower mean fibre size in mdx than WT animals (p < 0.001). The extensive Evans Blue Dye uptake shown in dystrophic muscles revealed substantial sarcolemmal damage, suggesting diffusion measurements as more consistent with altered permeability rather than changes in muscle fibre sizes. This study shows the potential of diffusion MRI to non‐invasively discriminate between dystrophic and healthy muscles with enhanced sensitivity when using long Δ.     

Comparison of different MRI sequences in lesion detection and early response evaluation of diffuse large B‐cell lymphoma – a whole‐body MRI and diffusion‐weighted imaging study

To compare different MRI sequences for the detection of lesions and the evaluation of response to chemotherapy in patients with diffuse large B‐cell lymphoma (DLBCL), 18 patients with histology‐confirmed DLBCL underwent 3‐T MRI scanning prior to and 1 week after chemotherapy. The MRI sequences included T₁‐weighted pre‐ and post‐contrast, T₂‐weighted with and without fat suppression, and a single‐shot echo‐planar diffusion‐weighted imaging (DWI) with two b values (0 and 800 s/mm²). Conventional MRI sequence comparisons were performed using the contrast ratio between tumor and normal vertebral body instead of signal intensity. The apparent diffusion coefficient (ADC) of the tumor was measured directly on the parametric ADC map. The tumor volume was used as a reference for the evaluation of chemotherapy response. The mean tumor volume was 374 mL at baseline, and decreased by 65% 1 week after chemotherapy (p < 0.01). The T₂‐weighted image with fat suppression showed a significantly higher contrast ratio compared with images from all other conventional MRI sequences, both before and after treatment (p < 0.01, respectively). The contrast ratio of the T₂‐weighted image with fat suppression decreased significantly (p < 0.01), and that of the T₁‐weighted pre‐contrast image increased significantly (p < 0.01), after treatment. However, there was no correlation between the change in contrast ratio and tumor volume. The mean ADC value was 0.68 × 10^–3 mm²/s at baseline; it increased by 89% after chemotherapy (p < 0.001), and the change in ADC value correlated with the change in tumor volume (r = 0.66, p < 0.01). The baseline ADC value also correlated inversely with the percentage change in ADC after treatment (r = ?0.62, p < 0.01). In conclusion, this study indicates that T₂‐weighted imaging with fat suppression is the best conventional sequence for the detection of lesions and evaluation of the efficacy of chemotherapy in DLBCL. DWI with ADC mapping is an imaging modality with both diagnostic and prognostic value that could complement conventional MRI. Copyright © 2013 John Wiley & Sons, Ltd.     

Triggered intravoxel incoherent motion MRI for the assessment of calf muscle perfusion during isometric intermittent exercise

The main aim of this paper was to propose triggered intravoxel incoherent motion (IVIM) imaging sequences for the evaluation of perfusion changes in calf muscles before, during and after isometric intermittent exercise. Twelve healthy volunteers were involved in the study. The subjects were asked to perform intermittent isometric plantar flexions inside the MRI bore. MRI of the calf muscles was performed on a 3.0 T scanner and diffusion‐weighted (DW) images were obtained using eight different b values (0 to 500 s/mm²). Acquisitions were performed at rest, during exercise and in the subsequent recovery phase. A motion‐triggered echo‐planar imaging DW sequence was implemented to avoid movement artifacts. Image quality was evaluated using the average edge strength (AES) as a quantitative metric to assess the motion artifact effect. IVIM parameters (diffusion D, perfusion fraction f and pseudo‐diffusion D*) were estimated using a segmented fitting approach and evaluated in gastrocnemius and soleus muscles. No differences were observed in quality of IVIM images between resting state and triggered exercise, whereas the non‐triggered images acquired during exercise had a significantly lower value of AES (reduction of more than 20%). The isometric intermittent plantar‐flexion exercise induced an increase of all IVIM parameters (D by 10%; f by 90%; D* by 124%; fD* by 260%), in agreement with the increased muscle perfusion occurring during exercise. Finally, IVIM parameters reverted to the resting values within 3 min during the recovery phase. In conclusion, the IVIM approach, if properly adapted using motion‐triggered sequences, seems to be a promising method to investigate muscle perfusion during isometric exercise.     

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 T_d 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 D₀, 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. D₀ 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. Copyright © 2014 John Wiley & Sons, Ltd.     

Interrelation of 31P‐MRS metabolism measurements in resting and exercised quadriceps muscle of overweight‐to‐obese sedentary individuals

Phosphorus magnetic resonance spectroscopy (³¹P‐MRS) enables the non‐invasive evaluation of muscle metabolism. Resting Pi‐to‐ATP flux can be assessed through magnetization transfer (MT) techniques, and maximal oxidative flux (Q_max) can be calculated by monitoring of phosphocreatine (PCr) recovery after exercise. In this study, the muscle metabolism parameters of 13 overweight‐to‐obese sedentary individuals were measured with both MT and dynamic PCr recovery measurements, and the interrelation between these measurements was investigated. In the dynamic experiments, knee extensions were performed at a workload of 30% of maximal voluntary capacity, and the consecutive PCr recovery was measured in a quadriceps muscle with a time resolution of 2 s with non‐localized ³¹P‐MRS at 3 T. Resting skeletal muscle metabolism was assessed through MT measurements of the same muscle group at 7 T. Significant linear correlations between the Q_max and the MT parameters k_ATP (r = 0.77, P = 0.002) and F_ATP (r = 0.62, P = 0.023) were found in the study population. This would imply that the MT technique can possibly be used as an alternative method to assess muscle metabolism when necessary (e.g. in individuals after stroke or in uncooperative patients). Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic intravoxel incoherent motion imaging of skeletal muscle at rest and after exercise

The purpose of this work was to demonstrate the feasibility of intravoxel incoherent motion imaging (IVIM) for non‐invasive quantification of perfusion and diffusion effects in skeletal muscle at rest and following exercise. After IRB approval, eight healthy volunteers underwent diffusion‐weighted MRI of the forearm at 3 T and eight different b values between 0 and 500 s/mm² with a temporal resolution of 57 s per dataset. Dynamic images were acquired before and after a standardized handgrip exercise. Diffusion (D) and pseudodiffusion (D*) coefficients as well as the perfusion fraction (F_P) were measured in regions of interest in the flexor digitorum superficialis and profundus (FDS/FDP), brachioradialis, and extensor carpi radialis longus and brevis muscles by using a multi‐step bi‐exponential analysis in MATLAB. Parametrical maps were calculated voxel‐wise. Differences in D, D*, and F_P between muscle groups and between time points were calculated using a repeated measures analysis of variance with post hoc Bonferroni tests. Mean values and standard deviations at rest were the following: D*, 28.5 ± 11.4 × 10^?3 mm²/s; F_P, 0.03 ± 0.01; D, 1.45 ± 0.09 × 10^?3 mm²/s. Changes of IVIM parameters were clearly visible on the parametrical maps. In the FDS/FDP, D* increased by 289 ± 236% (p < 0.029), F_P by 138 ± 58% (p < 0.01), and D by 17 ± 9% (p < 0.01). A significant increase of IVIM parameters could also be detected in the brachioradialis muscle, which however was significantly lower than in the FDS/FDP. After 20 min, all parameters were still significantly elevated in the FDS/FDP but not in the brachioradialis muscle compared with the resting state. The IVIM approach allows simultaneous quantification of muscle perfusion and diffusion effects at rest and following exercise. It may thus provide a useful alternative to other non‐invasive methods such as arterial spin labeling. Possible fields of interest for this technique include perfusion‐related muscle diseases, such as peripheral arterial occlusive disease. Copyright © 2014 John Wiley & Sons, Ltd.     

Measuring perfusion and bioenergetics simultaneously in mouse skeletal muscle: a multiparametric functional‐NMR approach

A totally noninvasive set‐up was developed for comprehensive NMR evaluation of mouse skeletal muscle function in vivo. Dynamic pulsed arterial spin labeling‐NMRI perfusion and blood oxygenation level‐dependent (BOLD) signal measurements were interleaved with ³¹P NMRS to measure both vascular response and oxidative capacities during stimulated exercise and subsequent recovery. Force output was recorded with a dedicated ergometer. Twelve exercise bouts were performed. The perfusion, BOLD signal, pH and force–time integral were obtained from mouse legs for each exercise. All reached a steady state after the second exercise, justifying the pointwise summation of the last 10 exercises to compensate for the limited ³¹P signal. In this way, a high temporal resolution of 2.5 s was achieved to provide a time constant for phosphocreatine (PCr) recovery (τ_PCr). The higher signal‐to‐noise ratio improved the precision of τ_PCr measurement [coefficient of variation (CV) = 16.5% vs CV = 49.2% for a single exercise at a resolution of 30 s]. Inter‐animal summation confirmed that τ_PCr was stable at steady state, but shorter (89.3 ± 8.6 s) than after the first exercise (148 s, p < 0.05). This novel experimental approach provides an assessment of muscle vascular response simultaneously to energetic function in vivo. Its pertinence was illustrated by observing the establishment of a metabolic steady state. This comprehensive tool offers new perspectives for the study of muscle pathology in mice models. Copyright © 2010 John Wiley & Sons, Ltd.     

Demonstration of asymmetric muscle perfusion of the back after exercise in patients with adolescent idiopathic scoliosis using intravoxel incoherent motion (IVIM) MRI

The purpose of this work was to quantify muscular perfusion patterns of back muscles after exercise in patients with adolescent idiopathic scoliosis (AIS) using intravoxel incoherent motion (IVIM) MR perfusion imaging. The paraspinal muscles of eight patients with AIS (Cobb angle 35 ± 10°, range [25‐47°]) and nine healthy volunteers were scanned with a 1.5 T MRI, at rest and after performing a symmetric back muscle exercise on a Roman chair. An IVIM sequence with 16 b‐values from 0 to 900 s/mm² was acquired, and the IVIM bi‐exponential signal equation model was fitted in two steps. Perfusion asymmetries were evaluated using the blood flow related IVIM fD* parameter in regions of interest placed within the paraspinal muscles. Statistical significance was assessed using a Student t‐test. The observed perfusion pattern after performing a Roman chair muscle exercise differed consistently in patients with AIS compared with healthy normal volunteers, and consisted of an asymmetrical increase in IVIM fD* [10^?3 mm²/s] above the lumbar convexity from 6.5 ± 5.8 to 28.8 ± 26.8 (p < 0.005), with no increase in the concavity (decrease from 6.5 ± 10.0 to 3.2 ± 1.5 (p = 0.19)), compared with a bilateral symmetric increase in the healthy volunteers (right, increase from 3.3 ± 2.1 to 10.1 ± 4.6 (p < 0.05); left, 6.7 ± 10.7 to 13.3 ± 7.0 (p < 0.05)). In conclusion, patients with AIS exhibit significant asymmetric muscle perfusion over the convexity of the scoliotic curvature after Roman chair exercise.     

Dynamic multivoxel‐localized 31P MRS during plantar flexion exercise with variable knee angle

Exercise studies investigating the metabolic response of calf muscles using ³¹P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time‐resolved multivoxel ³¹P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(～40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end‐exercise PCr depletions in gastrocnemius medialis (R²=0.8) and soleus (R²=0.53). PCr recovery times and end‐exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel ³¹P MRS during muscle exercise and recovery, and the findings should be used in future study design.     

Relationship between diffusion parameters derived from intravoxel incoherent motion MRI and perfusion measured by dynamic contrast‐enhanced MRI of soft tissue tumors

Our aim was to evaluate the link between diffusion parameters measured by intravoxel incoherent motion (IVIM) diffusion‐weighted imaging (DWI) and the perfusion metrics obtained with dynamic contrast‐enhanced (DCE) MRI in soft tissue tumors (STTs). Twenty‐eight patients affected by histopathologically confirmed STT were included in a prospective study. All patients underwent both DCE MRI and IVIM DWI. The perfusion fraction f, diffusion coefficient D and perfusion‐related diffusion coefficient D* were estimated using a bi‐exponential function to fit the DWI data. DCE MRI was acquired with a temporal resolution of 3–5 s. Maps of the initial area under the gadolinium concentration curve (IAUGC), time to peak (TTP) and maximum slope of increase (MSI) were derived using commercial software. The relationships between the DCE MRI and IVIM DWI measurements were assessed by Spearman's test. To exclude false positive results under multiple testing, the false discovery rate (FDR) procedure was applied. The Mann–Whitney test was used to evaluate the differences between all variables in patients with non‐myxoid and myxoid STT. No significant relationship was found between IVIM parameters and any DCE MRI parameters. Higher f and D*f values were found in non‐myxoid tumors compared with myxoid tumors (p = 0.004 and p = 0.003, respectively). MSI was significantly higher in non‐myxoid tumors than in myxoid tumors (p = 0.029). From the visual assessments of single clinical cases, both f and D*f maps were in satisfactory agreement with DCE maps in the extreme cases of an avascular mass and a highly vascularized mass, whereas, for tumors with slight vascularity or with a highly heterogeneous perfusion pattern, this association was not straightforward. Although IVIM DWI was demonstrated to be feasible in STT, our data did not support evident relationships between perfusion‐related IVIM parameters and perfusion measured by DCE MRI. Copyright © 2015 John Wiley & Sons, Ltd.     

Response of HT29 colorectal xenograft model to cediranib assessed with 18F‐fluoromisonidazole positron emission tomography,dynamic contrast‐enhanced and diffusion‐weighted MRI

Cediranib is a small‐molecule pan‐vascular endothelial growth factor receptor inhibitor. The tumor response to short‐term cediranib treatment was studied using dynamic contrast‐enhanced and diffusion‐weighted MRI at 7 T, as well as ¹⁸F‐fluoromisonidazole positron emission tomography and histological markers. Rats bearing subcutaneous HT29 human colorectal tumors were imaged at baseline; they then received three doses of cediranib (3 mg/kg per dose daily) or vehicle (dosed daily), with follow‐up imaging performed 2 h after the final cediranib or vehicle dose. Tumors were excised and evaluated for the perfusion marker Hoechst 33342, the endothelial cell marker CD31, smooth muscle actin, intercapillary distance and tumor necrosis. Dynamic contrast‐enhanced MRI‐derived parameters decreased significantly in cediranib‐treated tumors relative to pretreatment values [the muscle‐normalized initial area under the gadolinium concentration curve decreased by 48% (p = 0.002), the enhancing fraction by 43% (p = 0.003) and K^trans by 57% (p = 0.003)], but remained unchanged in controls. No change between the pre‐ and post‐treatment tumor apparent diffusion coefficients in either the cediranib‐ or vehicle‐treated group was observed over the course of this study. The ¹⁸F‐fluoromisonidazole mean standardized uptake value decreased by 33% (p = 0.008) in the cediranib group, but showed no significant change in the control group. Histological analysis showed that the number of CD31‐positive vessels (59 per mm²), the fraction of smooth muscle actin‐positive vessels (80–87%) and the intercapillary distance (0.17 mm) were similar in cediranib‐ and vehicle‐treated groups. The fraction of perfused blood vessels in cediranib‐treated tumors (81 ± 7%) was lower than that in vehicle controls (91 ± 3%, p = 0.02). The necrotic fraction was slightly higher in cediranib‐treated rats (34 ± 12%) than in controls (26 ± 10%, p = 0.23). These findings suggest that short‐term treatment with cediranib causes a decrease in tumor perfusion/permeability across the tumor cross‐section, but changes in vascular morphology, vessel density or tumor cellularity are not manifested at this early time point. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Feasibility and repeatability of localized 31P‐MRS four‐angle saturation transfer (FAST) of the human gastrocnemius muscle using a surface coil at 7 T

Phosphorus (³¹P) MRS, combined with saturation transfer (ST), provides non‐invasive insight into muscle energy metabolism. However, even at 7 T, the standard ST method with T₁^app measured by inversion recovery takes about 10 min, making it impractical for dynamic examinations. An alternative method, i.e. four‐angle saturation transfer (FAST), can shorten the examination time. The aim of this study was to test the feasibility, repeatability, and possible time resolution of the localized FAST technique measurement on an ultra‐high‐field MR system, to accelerate the measurement of both P_i‐to‐ATP and PCr‐to‐ATP reaction rates in the human gastrocnemius muscle and to test the feasibility of using the FAST method for dynamic measurements. We measured the exchange rates and metabolic fluxes in the gastrocnemius muscle of eight healthy subjects at 7 T with the depth‐resolved surface coil MRS (DRESS)‐localized FAST method. For comparison, a standard ST localized method was also used. The measurement time for the localized FAST experiment was 3.5 min compared with the 10 min for the standard localized ST experiment. In addition, in five healthy volunteers, P_i‐to‐ATP and PCr‐to‐ATP metabolic fluxes were measured in the gastrocnemius muscle at rest and during plantar flexion by the DRESS‐localized FAST method. The repeatability of PCr‐to‐ATP and P_i‐to‐ATP exchange rate constants, determined by the slab‐selective localized FAST method at 7 T, is high, as the coefficients of variation remained below 20%, and the results of the exchange rates measured with the FAST method are comparable to those measured with standard ST. During physical activity, the PCr‐to‐ATP metabolic flux decreased (from F_CK = 8.21 ± 1.15 mM s^?1 to F_CK = 3.86 ± 1.38 mM s^?1) and the P_i‐to‐ATP flux increased (from F_ATP = 0.43 ± 0.14 mM s^?1 to F_ATP = 0.74 ± 0.13 mM s^?1). In conclusion, we could demonstrate that measurements in the gastrocnemius muscle are feasible at rest and are short enough to be used during exercise with the DRESS‐localized FAST method at 7 T. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

The interaction between apparent diffusion coefficients and transverse relaxation rates of human brain metabolites and water studied by diffusion‐weighted spectroscopy at 7 T

The dependence of apparent diffusion coefficients (ADCs) of molecules in biological tissues on an acquisition‐specific timescale is a powerful mechanism for studying tissue microstructure. Unlike water, metabolites are confined mainly to intracellular compartments, thus providing higher specificity to tissue microstructure. Compartment‐specific structural and chemical properties may also affect molecule transverse relaxation times (T₂). Here, we investigated the correlation between diffusion and relaxation for N‐acetylaspartate, creatine and choline compounds in human brain white matter in vivo at 7 T, and compared them with those of water under the same experimental conditions. Data were acquired in a volume of interest in parietal white matter at two different diffusion times, Δ = 44 and 246 ms, using a matrix of three echo times (T_E) and five diffusion weighting values (up to 4575 s/mm²). Significant differences in the dependence of the ADCs on T_E were found between water and metabolites, as well as among the different metabolites. A significant decrease in water ADC as a function of T_E was observed only at the longest diffusion time (p < 0.001), supporting the hypothesis that at least part of the restricted water pool can be associated with longer T₂, as suggested by previous studies in vitro. Metabolite data showed an increase of creatine (p < 0.05) and N‐acetylaspartate (p < 0.05) ADCs with T_E at Δ = 44 ms, and a decrease of creatine (p < 0.05) and N‐acetylaspartate (p = 0.1) ADCs with T_E at Δ = 246 ms. No dependence of choline ADC on T_E was observed. The metabolite results suggest that diffusion and relaxation properties are dictated not only by metabolite distribution in different cell types, but also by other mechanisms, such as interactions with membranes, exchange between “free” and “bound” states or interactions with microsusceptibility gradients. Copyright © 2014 John Wiley & Sons, Ltd.     

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

Muscle diseases commonly have clinical presentations of inflammation, fat infiltration, fibrosis, and atrophy. However, the results of existing laboratory tests and clinical presentations are not well correlated. Advanced quantitative MRI techniques may allow the assessment of myo‐pathological changes in a sensitive and objective manner. To progress towards this goal, an array of quantitative MRI protocols was implemented for human thigh muscles; their reproducibility was assessed; and the statistical relationships among parameters were determined. These quantitative methods included fat/water imaging, multiple spin‐echo T₂ imaging (with and without fat signal suppression, FS), selective inversion recovery for T₁ and quantitative magnetization transfer (qMT) imaging (with and without FS), and diffusion tensor imaging. Data were acquired at 3.0 T from nine healthy subjects. To assess the repeatability of each method, the subjects were re‐imaged an average of 35 days later. Pre‐testing lifestyle restrictions were applied to standardize physiological conditions across scans. Strong between‐day intra‐class correlations were observed in all quantitative indices except for the macromolecular‐to‐free water pool size ratio (PSR) with FS, a metric derived from qMT data. Two‐way analysis of variance revealed no significant between‐day differences in the mean values for any parameter estimate. The repeatability was further assessed with Bland–Altman plots, and low repeatability coefficients were obtained for all parameters. Among‐muscle differences in the quantitative MRI indices and inter‐class correlations among the parameters were identified. There were inverse relationships between fractional anisotropy (FA) and the second eigenvalue, the third eigenvalue, and the standard deviation of the first eigenvector. The FA was positively related to the PSR, while the other diffusion indices were inversely related to the PSR. These findings support the use of these T₁, T₂, fat/water, and DTI protocols for characterizing skeletal muscle using MRI. Moreover, the data support the existence of a common biophysical mechanism, water content, as a source of variation in these parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

Low‐volume muscle endurance training prevents decrease in muscle oxidative and endurance function during 21‐day forearm immobilization

Aim: To examine the effects of low‐volume muscle endurance training on muscle oxidative capacity, endurance and strength of the forearm muscle during 21‐day forearm immobilization (IMM‐21d). Methods: The non‐dominant arm (n = 15) was immobilized for 21 days with a cast and assigned to an immobilization‐only group (Imm‐group; n = 7) or an immobilization with training group (Imm+Tr‐group; n = 8). Training comprised dynamic handgrip exercise at 30% of pre‐intervention maximal voluntary contraction (MVC) at 1 Hz until exhaustion, twice a week during the immobilization period. The duration of each exercise session was 51.7 ± 3.4 s (mean ± SE). Muscle oxidative capacity was evaluated by the time constant for phosphocreatine recovery (τ_offPCr) after a submaximal handgrip exercise using ³¹phosphorus‐magnetic resonance spectroscopy. An endurance test was performed at 30% of pre‐intervention MVC, at 1 Hz, until exhaustion. Results: τ _offPCr was significantly prolonged in the Imm‐group after 21 days (42.0 ± 2.8 and 64.2 ± 5.1 s, pre‐ and post‐intervention respectively; P < 0.01) but did not change for the Imm+Tr‐group (50.3 ± 3.0 and 48.8 ± 5.0 s, ns). Endurance decreased significantly for the Imm‐group (55.1 ± 5.1 and 44.7 ± 4.6 s, P < 0.05) but did not change for the Imm+Tr‐group (47.9 ± 3.0 and 51.7 ± 4.0 s, ns). MVC decreased similarly in both groups (P < 0.01). Conclusions: Twice‐weekly muscle endurance training sessions, each lasting approx. 50 s, effectively prevented a decrease in muscle oxidative capacity and endurance; however, there was no effect on MVC decline with IMM‐21d.