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

Influence of Magnetic Field Strength on Intravoxel Incoherent Motion Parameters in Diffusion MRI of the Calf

Background: The purpose of this study was to investigate the dependence of Intravoxel Incoherent Motion (IVIM) parameters measured in the human calf on B0. Methods: Diffusion-weighted image data of eight healthy volunteers were acquired using five b-values (0-600 s/mm2) at rest and after muscle activation at 0.55 and 7 T. The musculus gastrocnemius mediale (GM, activated) was assessed. The perfusion fraction f and diffusion coefficient D were determined using segmented fits. The dependence on field strength was assessed using Student's t-test for paired samples and the Wilcoxon signed-rank test. A biophysical model built on the three non-exchanging compartments of muscle, venous blood, and arterial blood was used to interpret the data using literature relaxation times. Results: The measured perfusion fraction of the GM was significantly lower at 7 T, both for the baseline measurement and after muscle activation. For 0.55 and 7 T, the mean f values were 7.59% and 3.63% at rest, and 14.03% and 6.92% after activation, respectively. The biophysical model estimations for the mean proton-density-weighted perfusion fraction were 3.37% and 6.50% for the non-activated and activated states, respectively. Conclusions: B0 may have a significant effect on the measured IVIM parameters. The blood relaxation times suggest that 7 T IVIM may be arterial-weighted whereas 0.55 T IVIM may exhibit an approximately equal weighting of arterial and venous blood.

Intravoxel incoherent motion imaging to assess the acute effects of moderate-intensity continuous training and high-intensity interval training on thigh muscles

This study investigated the use of intravoxel incoherent motion imaging (IVIM) to compare skeletal muscle perfusion during and after high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) to determine the impact on fat oxidation outcomes. Twenty overweight volunteers were recruited for the study. Each participant received one HIIT intervention and one MICT intervention using a cycling ergometer. Participants underwent a magnetic resonance imaging scan before, immediately after, and 1 and 2 h after each intervention. The IVIM parameters (D, fD*) of the rectus femoris, vastus lateralis, and biceps femoris long head were obtained. Changes in IVIM parameters of these muscles after both exercise interventions were compared using a two-factor repeated measures analysis of variance. In the rectus femoris, the fD* increased immediately after exercise intervention (d = 0.69 × 10-3  mm2 /s, p < 0.0083) and 2 h after exercise intervention (d = 0.64 × 10-3  mm2 /s, p < 0.0083) compared with before exercise. The increase in the fD* in the HIIT group was greater than that in the MICT group (d = 0.32, p = 0.023). In the vastus lateralis, the fD* increased immediately after the exercise intervention (d = 0.53 × 10-3  mm2 /s, p < 0.001) and returned to the pre-exercise level 1 h after exercising. The increase in the fD* in the HIIT group was lower than that in the MICT group (d = -0.21, p = 0.015). For the biceps femoris long head, the fD* was not significantly different between the two exercise interventions before and after exercise. Furthermore, the fD* 60 min after the HIIT intervention correlated with maximal oxygen consumption (VO2max), whereas fD* immediately after the MICT intervention correlated with VO2max. In summary, IVIM parameters can be used to evaluate differences in muscle perfusion between HIIT and MICT, and show a correlation with VO2max.

Emerging Quantitative Imaging Techniques in Sports Medicine

This article describes recent advances in quantitative imaging of musculoskeletal extremity sports injuries, citing the existing literature evidence and what additional evidence is needed to make such techniques applicable to clinical practice. Compositional and functional MRI techniques including T2 mapping, diffusion tensor imaging, and sodium imaging as well as contrast-enhanced US have been applied to quantify pathophysiologic processes and biochemical compositions of muscles, tendons, ligaments, and cartilage. Dual-energy and/or spectral CT has shown potential, particularly for the evaluation of osseous and ligamentous injury (eg, creation of quantitative bone marrow edema maps), which is not possible with standard single-energy CT. Recent advances in US technology such as shear-wave elastography or US tissue characterization as well as MR elastography enable the quantification of mechanical, elastic, and physical properties of tissues in muscle and tendon injuries. The future role of novel imaging techniques such as photon-counting CT remains to be established. Eventual prediction of return to play (ie, the time needed for the injury to heal sufficiently so that the athlete can get back to playing their sport) and estimation of risk of repeat injury is desirable to help guide sports physicians in the treatment of their patients. Additional values of quantitative analyses, as opposed to routine qualitative analyses, still must be established using prospective longitudinal studies with larger sample sizes.

Effects of fitting methods, high b-values and image quality on diffusion and perfusion quantification and reproducibility in the calf

PURPOSES

The study aimed to optimize diffusion-weighted imaging (DWI) image acquisition and analysis protocols in calf muscles by investigating the effects of different model-fitting methods, image quality, and use of high b-value and constraints on parameters of interest (POIs). The optimized modeling methods were used to select the optimal combinations of b-values, which will allow shorter acquisition time while achieving the same reliability as that obtained using 16 b-values.

METHODS

Test-retest baseline and high-quality DWI images of ten healthy volunteers were acquired on a 3T MR scanner, using 16 b-values, including a high b-value of 1200 s/mm², and structural T1-weighted images for calf muscle delineation. Three and six different fitting methods were used to derive ADC from monoexponential (ME) model and D_d, f_p, and D_p from intravoxel incoherent motion (IVIM) model, with or without the high b-value. The optimized ME and IVIM models were then used to determine the optimal combinations of b-values, obtainable with the least number of b-values, using the selection criteria of coefficient of variance (CV) ≤10% for all POIs.

RESULTS

The find minimum multivariate algorithm was more flexible and yielded smaller fitting errors. The 2-steps fitting method, with fixed D_d, performed the best for IVIM model. The inclusion of high b-value reduced outliers, while constraints improved 2-steps fitting only.

CONCLUSIONS

The optimal numbers of b-values for ME and IVIM models were nine and six b-values respectively. Test-retest reliability analyses showed that only ADC and D_d were reliable for calf diffusion evaluation, with CVs of 7.22% and 4.09%.

The quantification of IntraVoxel incoherent motion - MRI maps cannot preserve texture information: An evaluation based on simulated and in-vivo images

BACKGROUND

Radiomics can be applied on parametric maps obtained from IntraVoxel Incoherent Motion (IVIM) MRI to characterize heterogeneity in diffusion and perfusion tissue properties. The purpose of this work is to assess the accuracy and reproducibility of radiomic features computed from IVIM maps using different fitting methods.

METHODS

200 digitally simulated IVIM-MRI images with various SNR containing different combinations of texture patterns were generated from ground truth maps of true diffusion D, pseudo-diffusion D* and perfusion fraction f. Four different methods (segmented least-square LSQ, Bayesian, supervised and unsupervised deep learning DL) were adopted to quantify IVIM maps from simulations and from two real images of liver tumor. Radiomic features were computed from ground truth and estimated maps. Accuracy and reproducibility among quantification methods were assessed.

RESULTS

Almost 50% of radiomic features computed from D maps using DL approaches, 36% using Bayes and 27% using LSQ presented errors lower than 50%. Radiomic features from f and D* maps were accurate only if computed using DL methods from histogram. High reproducibility (ICC>0.8) was found only for D maps among DL and Bayes methods, whereas features from f and D* maps were less reproducible, with LSQ approach in lower agreement with the others.

CONCLUSIONS

Texture patterns were preserved and correctly estimated only on D maps, except for LSQ approach. We suggest limiting radiomic analysis only to histogram and some texture features from D maps, to histogram features from f maps, and to avoid it on D* maps.

A supervised deep neural network approach with standardized targets for enhanced accuracy of IVIM parameter estimation from multi-SNR images

Extraction of intravoxel incoherent motion (IVIM) parameters from noisy diffusion-weighted (DW) images using a biexponential fitting model is computationally challenging, and the reliability of the estimated perfusion-related quantities represents a limitation of this technique. Artificial intelligence can overcome the current limitations and be a suitable solution to advance use of this technique in both preclinical and clinical settings. The purpose of this work was to develop a deep neural network (DNN) approach, trained on numerical simulated phantoms with different signal to noise ratios (SNRs), to improve IVIM parameter estimation. The proposed approach is based on a supervised fully connected DNN having 3 hidden layers, 18 inputs and 3 targets with standardized values. 14 × 10³ simulated DW images, based on a Shepp-Logan phantom, were randomly generated with varying SNRs (ranging from 10 to 100). 7 × 10³ images (1000 for each SNR) were used for training. Performance accuracy was assessed in simulated images and the proposed approach was compared with the state-of-the-art Bayesian approach and other DNN algorithms. The DNN approach was also evaluated in vivo on a high-field MRI preclinical scanner. Our DNN approach showed an overall improvement in accuracy when compared with the Bayesian approach and other DNN methods in most of the simulated conditions. The in vivo results demonstrated the feasibility of the proposed approach in real settings and generated quantitative results comparable to those obtained using the Bayesian and unsupervised approaches, especially for D and f, and with lower variability in homogeneous regions. The DNN architecture proposed in this work outlines two innovative features as compared with other studies: (1) the use of standardized targets to improve the estimation of parameters, and (2) the implementation of a single DNN to enhance the IVIM fitting at different SNRs, providing a valuable alternative tool to compute IVIM parameters in conditions of high background noise.

IVIM Imaging of Paraspinal Muscles Following Moderate and High-Intensity Exercise in Healthy Individuals

Background

Quantification of the magnitude and spatial distribution of muscle blood flow changes following exercise may improve our understanding of the effectiveness of various exercise prescriptions. Intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) is a technique that quantifies molecular diffusion and microvascular blood flow, and has recently gained momentum as a method to evaluate a muscle's response to exercise. It has also been shown to predict responses to exercise-based physical therapy in individuals with low back pain. However, no study has evaluated the sensitivity of IVIM-MRI to exercise of varying intensity in humans. Here, we aimed to evaluate IVIM signal changes of the paraspinal muscles in response to moderate and high intensity lumbar extension exercise in healthy individuals.

Methods

IVIM data were collected in 11 healthy volunteers before and immediately after a 3-min bout of moderate and high-intensity resisted lumbar extension. IVIM data were analyzed to determine the average perfusion fraction (f), pseudo-diffusion coefficient (D^*), and diffusion coefficient (D) in the bilateral paraspinal muscles. Changes in IVIM parameters were compared between the moderate and high intensity exercise bouts.

Results

Exercise increased all IVIM parameters, regardless of intensity (p < 0.003). Moderate intensity exercise resulted in a 11.2, 19.6, and 3.5% increase in f, D^* and D, respectively. High intensity exercise led to a similar increase in f (12.2%), but much greater changes in D^* (48.6%) and D (7.9%).

Conclusion

IVIM parameter increases suggest that both the moderate and high-intensity exercise conditions elicited measurable changes in blood flow (increased f and D^*) and extravascular molecular diffusion rates (increased D), and that there was a dose-dependence of exercise intensity on D^* and D.

Intravoxel Incoherent Motion Magnetic Resonance Imaging in Skeletal Muscle: Review and Future Directions

Throughout the body, muscle structure and function can be interrogated using a variety of noninvasive magnetic resonance imaging (MRI) methods. Recently, intravoxel incoherent motion (IVIM) MRI has gained momentum as a method to evaluate components of blood flow and tissue diffusion simultaneously. Much of the prior research has focused on highly vascularized organs, including the brain, kidney, and liver. Unique aspects of skeletal muscle, including the relatively low perfusion at rest and large dynamic range of perfusion between resting and maximal hyperemic states, may influence the acquisition, postprocessing, and interpretation of IVIM data. Here, we introduce several of those unique features of skeletal muscle; review existing studies of IVIM in skeletal muscle at rest, in response to exercise, and in disease states; and consider possible confounds that should be addressed for muscle-specific evaluations. Most studies used segmented nonlinear least squares fitting with a b-value threshold of 200 sec/mm2 to obtain IVIM parameters of perfusion fraction (f), pseudo-diffusion coefficient (D*), and diffusion coefficient (D). In healthy individuals, across all muscles, the average ± standard deviation of D was 1.46 ± 0.30 × 10-3  mm2 /sec, D* was 29.7 ± 38.1 × 10-3  mm2 /sec, and f was 11.1 ± 6.7%. Comparisons of reported IVIM parameters in muscles of the back, thigh, and leg of healthy individuals showed no significant difference between anatomic locations. Throughout the body, exercise elicited a positive change of all IVIM parameters. Future directions including advanced postprocessing models and potential sequence modifications are discussed. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

A Clustering Approach to Improve IntraVoxel Incoherent Motion Maps from DW-MRI Using Conditional Auto-Regressive Bayesian Model

The Intra-Voxel Incoherent Motion (IVIM) model allows to estimate water diffusion and perfusion-related coefficients in biological tissues using diffusion weighted MR images. Among the available approaches to fit the IVIM bi-exponential decay, a segmented Bayesian algorithm with a Conditional Auto-Regressive (CAR) prior spatial regularization has been recently proposed to produce more reliable coefficient estimation. However, the CAR spatial regularization can generate inaccurate coefficient estimation, especially at the interfaces between different tissues. To overcome this problem, the segmented CAR model was coupled in this work with a k-means clustering approach, to separate different tissues and exclude voxels from other regions in the CAR prior specification. The proposed approach was compared with the original Bayesian CAR method without clustering and with a state-of-the-art Bayesian approach without CAR. The approaches were tested and compared on simulated images by calculating the estimation error and the coefficient of variation (CV). Furthermore, the proposed method was applied to some illustrative real images of oncologic patients. On simulated images, the proposed innovation reduced the average error of 47%, 21% and 58% for D, f and D*, respectively, compared to the state-of-the-art Bayesian approach, and of 48% and 34% for D and f, respectively, compared to the original CAR, while it achieved the same error for D*. The clustering approach was also able to consistently reduce the CV for each coefficient. On real images, the novel approach did not alter the IVIM maps obtained by the original CAR method, with the advantage of reducing their typical blotchy appearance at the boundaries. The proposed approach represents a valuable improvement over the state-of-the-art Bayesian CAR method and provides more reliable IVIM coefficient estimation, and is less sensitive to bias and inconsistency at tissue/tissue and tissue/background interfaces.

Acute Effects of Foam Rolling on Hamstrings After Half-Marathon: A Muscle Functional Magnetic Resonance Imaging Study

Purpose: Foam rolling (FR) is widely used for post-exercise muscle recovery; yet, the effects of FR on skeletal muscle inflammation and microvascular perfusion following prolonged exercise are poorly understood. We aim to address the gap in knowledge by using magnetic resonance imaging (MRI) T2 mapping and intravoxel incoherent motion (IVIM) sequences to study the acute effects of FR on hamstrings following half-marathon running in recreational runners.

Methods: Sixteen healthy recreational marathon runners were recruited. After half-marathon running, FR was performed on the hamstrings on the dominant side, while the other limb served as a control. MRI T2 and IVIM scans were performed bilaterally at baseline (pre-run), 2–3 h after running (post-run), immediately after FR (post-FR0), 30 min after FR (post-FR30) and 60 min after FR (post-FR60). T2, a marker for inflammatory edema, as well as IVIM microvascular perfusion fraction index f for biceps femoris long head (BFL), semitendinosus (ST) and semimembranosus (SM) were determined. Total Quality Recovery (TQR) scale score was also collected.

Results: Both T2 and f were higher at post-run compared to pre-run in all hamstrings on both sides (all p < 0.05; all d > 1.0). For the FR side, T2 decreased, and f increased significantly at post-FR0 and post-FR30 compared to post-run in all muscles (p < 0.05; all d > 0.4) except for f at BFL and SM at post-FR30 (both p > 0.05), though f at BFL was still marginally elevated at post-FR30 (p = 0.074, d = 0.91). Both parameters for all muscles returned to post-run level at post-FR60 (all p > 0.05; all d < 0.4) except for T2 at SM (p = 0.037). In contrast, most MRI parameters were not changed at post-FR0, post-FR30 and post-FR60 compared to post-run for the control side (p < 0.05; d < 0.2). TQR scores were elevated at post-FR0 and post-FR30 compared to post-run (both p < 0.05; both d > 1.0), and returned to the post-run level at post-FR60 (p > 0.99; d = 0.09). Changes in TQR scores compared to post-run at any time points after FR were correlated to T2 for ST at post-FR30 (r = 0.50, p = 0.047) but not T2 for other muscles and any changes in f values.

Conclusions: Hamstrings inflammatory edema and microvascular perfusion were elevated following half-marathon running, which were detectable with MRI T2 mapping and IVIM sequences. FR resulted in acute alleviation in inflammation and greater microvascular perfusion; however, the effects seemed to last only for a short period of time (30–60 min). FR can provide short-term benefits to skeletal muscle after prolonged running.

Four functional magnetic resonance imaging techniques for skeletal muscle exploration, a systematic review

BACKGROUND

The study of muscle health has become more relevant lately, due to global aging and a higher incidence of musculoskeletal pathologies. Current exploration techniques, such as electromyography, do not provide accurate spatial information.

OBJECTIVE

The objective of this work is to perform a systematic review of the literature to synthesize the contributions that can offer functional MRI techniques commonly used in neuroimaging, applied to skeletal muscle: Blood Oxygen Level Dependent (BOLD), IntraVoxel Incoherent Motion (IVIM), Arterial Spin Labeling (ASL) and Dynamic Contrast Enhanced (DCE).

EVIDENCE ACQUISITION

Web of Science and Medline databases were searched, over the last 10 years, focused on the use of BOLD, ASL, IVIM or DCE in skeletal muscle.

EVIDENCE SYNTHESIS

59 articles were included after applying the selection criteria. 37 studies were performed in healthy subjects, and 22 in patients with different pathologies: in peripheral arterial disease, systemic sclerosis, diabetes, osteoporosis, adolescent idiopathic scoliosis, and dermatomyositis. Reference values in healthy subjects still vary in some cases.

CONCLUSION

The studies show the feasibility of implementing functional MRI through BOLD, ASL, IVIM or DCE imaging in several muscles and their possible utility in different pathologies. A synthesis of how to implement such exploration is given here.

CLINICAL IMPACT

These four techniques are based on sequences already present in clinical MRI scanners, therefore, their use for functional muscle exploration does not require additional investment. These techniques allow visualization and quantification of parameters associated with the vascular health of the muscles and represent interesting support for musculoskeletal exploration.

A Multi-Variate framework to assess reliability and discrimination power of Bayesian estimation of Intravoxel Incoherent Motion parameters

PURPOSE

To propose a multivariate multi-step framework for a systematic assessment of the estimation reliability and discriminability of Intravoxel Incoherent Motion (IVIM) model parameters.

METHODS

Monte-Carlo simulations were generated on a range of SNRs and in different IVIM combinations considering: i) a dense discretization with 24 b-values; ii) a discretization with 9 b-values. A state-of-the-art Bayesian fitting method was adopted. The framework assessed: i) the best model between mono- and bi-exponential, through the BIC index; ii) the fitting accuracy; iii) the power in discriminating two different IVIM parameters distributions of estimated coefficients, using a multivariate test. Exemplificative oncologic cases were also presented.

RESULTS

The bi-exponential fitting was reliable for perfusion fraction higher than 5%, with high accuracy in D estimation, acceptable error for f, but high uncertainty in D*. The discrimination of two distributions is generally feasible if differences in D values (at least 0.3 x10^-3 mm²/s) are present; in the case of similar D values, a minimal difference of 5% in f can be discriminated just in case of balanced sample size and dense b-values discretization, whereas the impact of D* is quite negligible. These results were also supported by clinical examples.

CONCLUSIONS

IVIM model is generally accurate in estimating diffusion, but uncertainties related to perfusion estimation are not negligible and compromise the discrimination power when different populations should be differentiated. The proposed framework should be adopted as interpretative guidelines to better understand when IVIM model applied on real data can provide reliable findings.

Effect of compression garments on muscle perfusion in delayed-onset muscle soreness: A quantitative analysis using intravoxel incoherent motion MR perfusion imaging

The aim of this prospective cohort study was to evaluate the effect of compression garments under resting conditions and after the induction of delayed-onset muscle soreness (DOMS) by MR perfusion imaging using intravoxel incoherent motion (IVIM). Magnetic resonance imaging of both lower legs of 16 volunteers was performed before and after standardized eccentric exercises that induced DOMS. A compression garment (21-22 mmHg) was worn during and for 6 h after exercise on one randomly selected leg. IVIM MR imaging, represented as total muscle perfusion D*f, perfusion fraction f and tissue diffusivity D, were compared between baseline and directly, 30 min, 6 h and 48 h after exhausting exercise with and without compression. Creatine kinase levels and T2-weighted images were acquired at baseline and after 48 h. DOMS was induced in the medial head of the gastrocnemius muscle (MGM) in all volunteers. Compression garments did not show any significant effect on IVIM perfusion parameters at any time point in the MGM or the tibialis anterior muscle (p > 0.05). Microvascular perfusion in the MGM increased significantly in both the compressed and noncompressed leg between baseline measurements and those taken directly after and 30 min after the exercise: the relative median f increased by 31.5% and 24.7% in the compressed and noncompressed leg, respectively, directly after the exercise compared with the baseline value. No significant change in tissue perfusion occurred 48 h after the induction of DOMS compared with baseline. It was concluded that compression garments (21-22 mmHg) do not alter microvascular muscle perfusion at rest, nor do they have any significant effect during the regeneration phase of DOMS. In DOMS, only a short-term effect of increased muscle perfusion (30 min after exercise) was observed, with normalization occurring during regeneration after 6-48 h. The normalization of perfusion independently of compression after 6 h may have implications for diagnostic and therapeutic strategies and for the better understanding of pathophysiological pathways in DOMS.

Parsimonious modeling of skeletal muscle perfusion: Connecting the stretched exponential and fractional Fickian diffusion

PURPOSE

To develop an anomalous (non-Gaussian) diffusion model for characterizing skeletal muscle perfusion using multi-b-value DWI.

THEORY AND METHODS

Fick's first law was extended for describing tissue perfusion as anomalous superdiffusion, which is non-Gaussian diffusion exhibiting greater particle spread than that of the Gaussian case. This was accomplished using a space-fractional derivative that gives rise to a power-law relationship between mean squared displacement and time, and produces a stretched exponential signal decay as a function of b-value. Numerical simulations were used to estimate parameter errors under in vivo conditions, and examine the effect of limited SNR and residual fat signal. Stretched exponential DWI parameters, α and D , were measured in thigh muscles of 4 healthy volunteers at rest and following in-magnet exercise. These parameters were related to a stable distribution of jump-length probabilities and used to estimate microvascular volume fractions.

RESULTS

Numerical simulations showed low dispersion in parameter estimates within 1.5% and 1%, and bias errors within 3% and 10%, for α and D , respectively. Superdiffusion was observed in resting muscle, and to a greater degree following exercise. Resting microvascular volume fraction was between 0.0067 and 0.0139 and increased between 2.2-fold and 4.7-fold following exercise.

CONCLUSIONS

This model captures superdiffusive molecular motions consistent with perfusion, using a parsimonious representation of the DWI signal, providing approximations of microvascular volume fraction comparable with histological estimates. This signal model demonstrates low parameter-estimation errors, and therefore holds potential for a wide range of applications in skeletal muscle and elsewhere in the body.

Post-exercise intramuscular O2 supply is tightly coupled with a higher proximal-to-distal ATP synthesis rate in human tibialis anterior

Key points

The post‐exercise recovery of phosphocreatine, a measure of the oxidative capacity of muscles, as assessed by ³¹P MR spectroscopy, shows a striking increase from distal to proximal along the human tibialis anterior muscle.

To investigate why this muscle exhibits a greater oxidative capacity proximally, we tested whether the spatial variation in phosphocreatine recovery rate is related to oxygen supply, muscle fibre type or type of exercise.

We revealed that oxygen supply also increases from distal to proximal along the tibialis anterior, and that it strongly correlated with phosphocreatine recovery. Carnosine level, a surrogate measure for muscle fibre type was not different between proximal and distal, and type of exercise did not affect the gradient in phosphocreatine recovery rate.

Taken together, the findings of this study suggest that the post‐exercise spatial gradients in oxygen supply and phosphocreatine recovery are driven by a higher intrinsic mitochondrial oxidative capacity proximally.

Abstract

Phosphorus magnetic resonance spectroscopy (³¹P MRS) of human tibialis anterior (TA) revealed a strong proximo‐distal gradient in the post‐exercise phosphocreatine (PCr) recovery rate constant (k_PCr), a measure of muscle oxidative capacity. The aim of this study was to investigate whether this k_PCr gradient is related to O₂ supply, resting phosphorylation potential, muscle fibre type, or type of exercise. Fifteen male volunteers performed continuous isometric ankle dorsiflexion at 30% maximum force until exhaustion. At multiple locations along the TA, we measured the oxidative PCr resynthesis rate (V_PCr = k_PCr × PCr depletion) by ³¹P MRS, the oxyhaemoglobin recovery rate constant (k_O2Hb) by near infrared spectroscopy, and muscle perfusion with MR intravoxel incoherent motion imaging. The k_O2Hb, k_PCr, V_PCr and muscle perfusion depended on measurement location (P < 0.001, P < 0.001, P = 0.032 and P = 0.003, respectively), all being greater proximally. The k_O2Hb and muscle perfusion correlated with k_PCr (r = 0.956 and r = 0.852, respectively) and V_PCr (r = 0.932 and r = 0.985, respectively), the latter reflecting metabolic O₂ consumption. Resting phosphorylation potential (PCr/inorganic phosphate) was also higher proximally (P < 0.001). The surrogate for fibre type, carnosine content measured by ¹H MRS, did not differ between distal and proximal TA (P = 0.884). Performing intermittent exercise to avoid exercise ischaemia, still led to larger k_PCr proximally than distally (P = 0.013). In conclusion, the spatial k_PCr gradient is strongly associated with the spatial variation in O₂ supply. It cannot be explained by exercise‐induced ischaemia nor by fibre type. Our findings suggest it is driven by a higher proximal intrinsic mitochondrial oxidative capacity, apparently to support contractile performance of the TA.

The post‐exercise recovery of phosphocreatine, a measure of the oxidative capacity of muscles, as assessed by ³¹P MR spectroscopy, shows a striking increase from distal to proximal along the human tibialis anterior muscle.

To investigate why this muscle exhibits a greater oxidative capacity proximally, we tested whether the spatial variation in phosphocreatine recovery rate is related to oxygen supply, muscle fibre type or type of exercise.

We revealed that oxygen supply also increases from distal to proximal along the tibialis anterior, and that it strongly correlated with phosphocreatine recovery. Carnosine level, a surrogate measure for muscle fibre type was not different between proximal and distal, and type of exercise did not affect the gradient in phosphocreatine recovery rate.

Taken together, the findings of this study suggest that the post‐exercise spatial gradients in oxygen supply and phosphocreatine recovery are driven by a higher intrinsic mitochondrial oxidative capacity proximally.

A novel bayesian approach with conditional autoregressive specification for intravoxel incoherent motion diffusion-weighted MRI

The Intra-Voxel Incoherent Motion (IVIM) model is largely adopted to estimate slow and fast diffusion coefficients of water molecules in biological tissues, which are used in cancer applications. The most reported fitting approach is a voxel-wise segmented non-linear least square, whereas Bayesian approaches with a direct fit, also considering spatial regularization, were proposed too. In this work a novel segmented Bayesian method was proposed, also in combination with a spatial regularization through a Conditional Autoregressive (CAR) prior specification. The two segmented Bayesian approaches, with and without CAR specification, were compared with two standard least-square and a direct Bayesian fitting methods. All approaches were tested on simulated images and real data of patients with head-and-neck and rectal cancer. Estimation accuracy and maps noisiness were quantified on simulated images, whereas the coefficient of variation and the goodness of fit were evaluated for real data. Both versions of the segmented Bayesian approach outperformed the standard methods on simulated images for pseudo-diffusion (D^∗ ) and perfusion fraction (f), whilst the segmented least-square fitting remained the less biased for the diffusion coefficient (D). On real data, Bayesian approaches provided the less noisy maps, and the two Bayesian methods without CAR generally estimated lower values for f and D^∗ coefficients with respect to the other approaches. The proposed segmented Bayesian approaches were superior, in terms of estimation accuracy and maps quality, to the direct Bayesian model and the least-square fittings. The CAR method improved the estimation accuracy, especially for D^∗ .

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.

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.