Search

Skeletal muscle phosphorus‐31 31P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31P MRS methods can probe it. Here we consider basic signal‐acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near‐infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1H MRS measurements) can help in the physiological/metabolic interpretation of 31P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology. [ABSTRACT FROM AUTHOR]

The goal of this study was to evaluate the reproducibility and repeatability of tissue sodium concentration (TSC) measurements using 23Na MRI in skeletal muscle tissue. 23Na MRI was performed at 3 T on the right lower leg of eight healthy volunteers (aged 28 ± 4 years). The examinations were repeated at the same site after ~ 22 weeks to assess the variability over a medium‐term period. Additionally, they were scanned at a second site shortly before or shortly after the first visit (within 3 weeks) to evaluate the inter‐site reproducibility. Moreover, we analysed the effect of B0 correction on the variability. Coefficients of variations (CVs) from mean TSC values as well as Bland–Altman plots were used to assess intra‐site repeatability and inter‐site reproducibility. In phantom measurements, the B0 correction improved the quantitative accuracy. We observed differences of up to 4.9 mmol/L between the first and second visit and a difference of up to 3.7 mmol/L between the two different sites. The CV for the medium‐term repeatability was 15% and the reproducibility CV was 9%. The Bland–Altman plots indicated high agreement between the visits in all muscle regions. The systematic bias of −0.68 mmol/L between site X and Y (P = 0.03) was slightly reduced to −0.64 mmol/L after B0 correction (P = 0.04). This work shows that TSC measurements in healthy skeletal muscle tissue can be performed with good repeatability and reproducibility, which is of importance for future longitudinal or multicentre studies. [ABSTRACT FROM AUTHOR]

In recent years, quantitative nuclear magnetic resonance imaging and spectroscopy (NMRI and NMRS) have been used more systematically as outcome measures in natural history and clinical trial studies for Duchenne muscular dystrophy (DMD). Whereas most of these studies have emphasized the evaluation of the fat fraction as an assessment for disease severity, less focus has been placed on metabolic indices measured by NMRS. 31P NMRS in DMD reveals an alkaline inorganic phosphate (Pi) pool, originating from either leaky dystrophic myocytes or an increased interstitial space. ¹H NMRS, exploiting the pH-sensitive proton resonances of carnosine, an intracellular dipeptide, was used to distinguish between these two hypotheses. NMR data were obtained in 23 patients with DMD and 14 healthy subjects on a 3-T clinical NMR system. Both 31P and ¹H NMRS data were acquired at the level of the gastrocnemius medialis muscle. A multi-slice multi-echo imaging acquisition was performed for the determination of water T2 and fat fraction in the same region of interest. Whereas nearly all patients with DMD showed an elevated pH compared with healthy controls when using 31P NMRS, ¹H NMRS-determined pH was not systematically increased. As expected, the carnosine-based intracellular pH was never found to be alkaline in the absence of a concurrent Pi-based pH elevation. In addition, abnormal intracellular pH, based on carnosine, was never associated with normal water T2 values. We conclude that, in one group of patients, both ¹H and 31P NMRS showed an alkaline pH, originating from the intracellular compartment and reflecting ionic dysregulation in dystrophic myocytes. In the other patients with DMD, intracellular pH was normal, but an alkaline Pi pool was still present, suggesting an extracellular origin, probably revealing an expanded interstitial volume fraction, often associated with fibrotic changes. The data demonstrate that ¹H NMRS could serve as a biomarker to assess the normalization of intramyocytic pH and sarcolemmal permeability following therapy inducing dystrophin expression in patients with DMD. [ABSTRACT FROM AUTHOR]

Skeletal muscle inflammation/necrosis and fat infiltration are strong indicators of disease activity and progression in many neuromuscular disorders. They can be assessed by muscle T2 relaxometry and water-fat separation techniques, respectively. In the present work, we exploited differences between water and fat T1 and T2 relaxivities by applying a bi-component extended phase graph (EPG) fitting approach to simultaneously quantify the muscle water T2 and fat fraction from standard multi-slice multi-echo (MSME) acquisitions in the presence of stimulated echoes. Experimental decay curves were adjusted to the theoretical model using either an iterative non-negative least-squares (NNLS) procedure or a pattern recognition approach. Twenty-two patients (age, 49 ± 18 years) were selected to cover a large range of muscle fat infiltration. Four cases of chronic or subchronic juvenile dermatomyositis (age, 8 ± 3 years) were investigated before and 3 months following steroid treatment. For control, five healthy volunteers (age, 25 ± 2 years) were recruited. All subjects underwent the MSME sequence and EPG fitting procedure. The EPG fitting algorithm allowed a precise estimation of water T2 and fat fraction in diseased muscle, even in the presence of large B1+ inhomogeneities. In the whole cohort of patients, there was no overall correlation between water T2 values obtained with the proposed method and the fat fraction estimated inside muscle tissues (R2 = 0.02). In the patients with dermatomyositis, there was a significant decrease in water T2 (-4.09 ± 3.7 ms) consequent to steroid treatment. The pattern recognition approach resulted in a 20-fold decrease in processing time relative to the iterative NNLS procedure. The fat fraction derived from the EPG fitting approach correlated well with the fat fraction derived from a standard three-point Dixon method (≈1.5% bias). The bi-component EPG fitting analysis is a precise tool to monitor muscle tissue disease activity and is able to handle bias introduced by fat infiltration and B1+ inhomogeneities. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Many anomalies exist in the resting 31P muscle spectra of boys with Duchenne muscular dystrophy (DMD) but few have been reported in Golden Retriever muscular dystrophy (GRMD), the closest existing animal model for DMD. Because GRMD is recommended for preclinical evaluation of therapies and quantitative outcome measures are needed, we investigated anomalies of 31P NMRS in tibial cranial and biceps femoris muscles from 14 GRMD compared to 9 control (CONT) dogs. Alterations observed in DMD children - low phosphocreatine and high phospho-monoesters and -diesters - were all found in GRMD but increased pH was not. More surprisingly, inorganic phosphate (Pi) appeared to present a prominent splitting with an enhanced Pib resonance at 0.3 ppm downfield of Pia. Assuming that both resonances are Pi, the pH for Pia in GRMD corresponded to a physiological intracellular pHa (6.97 ± 0.05), while pHb approached the extracellular range (7.27 ± 0.10) and correlated with pHa in GRMD (R2 = 0.65). Both Pia and Pib were elevated compared to CONT and Pia increased with age for GRMD (R2 = 0.48, p < 0.001). Magnetisation transfer experiments between γATP and Pi were conducted to better characterise Pi pools. Equal T1 relaxation times for Pib and Pia did not support a mitochondrial origin of Pib. We suggest that Pib could originate from degenerating hypercontracted cells that have a leaky membrane and inadequate cell homeostasis and pH regulation. Pib showed minimal chemical exchange in all dogs, while the exchange rate of Pia was reduced in GRMD and might extraneously reflect low glycolytic activity in DMD. Taken together, the ensemble of 31P NMRS alterations identifies muscle dysfunction and could provide useful biomarkers of therapeutic efficacy. Furthermore, among these, two might relate more specifically to dystrophic processes and merit further investigation: one is the existence of the enhanced alkaline Pib pool; the other, mechanisms by which membrane disruption might increase phosphodiesters in dystrophy. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The in vivo determination of peripheral vascular resistances (VR) is crucial for the assessment of arteriolar function. It requires simultaneous determination of organ perfusion (F) and arterial blood pressure (BP). A fully non-invasive method was developed to measure systolic and diastolic BP in the caudal artery of rats based on dynamic NMR angiography. A good agreement was found between the NMR approach and the gold standard techniques (linear regression slope = 0.98, R2 = 0.96). This method and the ASL-MRI measurement of skeletal muscle perfusion were combined into one single NMR experiment to quantitatively evaluate the local vascular resistances in the calf muscle of anaesthetized rats, in vivo and non-invasively 1) at rest: VR = 7.0 ± 1.0 mmHg·min 100 g·ml−1, F = 13 ± 3 ml min−1.100 g−1 and mean BP (MBP) = 88 ± 10 mmHg; 2) under vasodilator challenge (milrinone): VR = 3.7 ± 1.1 mmHg min.100 g ml−1, F = 21 ± 4 ml min−1.100 g−1 and MBP = 75 ± 14 mmHg; 3) under vasopressor challenge (norepinephrine): VR = 9.8 ± 1.2 mmHg min 100 g ml−1, F = 14 ± 3 ml min−1.100 g−1 and MBP = 137 ± 2 mmHg. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Early studies have demonstrated that (total) magnesium was decreased in skeletal muscle of Duchenne muscular dystrophy (DMD) patients. Free intramuscular Mg2+ can be derived from 31P NMRS measurements. The value of free intramuscular magnesium concentration ([Mg2+]) is highly dependent on precise knowledge of intracellular pH, which is abnormally alkaline in dystrophic muscle, possibly due to an expanded interstitial space, potentially causing an underestimation of [Mg2+]. We have recently shown that intracellular pH can be derived using 1H NMRS of carnosine. Our aim was to determine whether 31P NMRS‐based [Mg2+] is, in fact, abnormally low in DMD patients, taking advantage of the 1H NMRS‐based pH. A comparative analysis was, therefore, made between [Mg2+] values calculated with both 1H and 31P NMRS‐based approaches to determine pH in 25 DMD patients, on a 3‐T clinical NMR scanner. [Mg2+] was also assessed with 31P NMRS only in (forearm or leg) skeletal muscle of 60 DMD patients and 63 age‐matched controls. Additionally, phosphodiester levels as well as quantitative NMRI indices including water T2, fat fraction, contractile cross‐sectional area and one‐year changes were evaluated. The main finding was that the significant difference in [Mg2+] between DMD patients and controls was preserved even when the intracellular pH determined with 1H NMRS was similar in both groups. Consequently, we observed that [Mg2+] is significantly lower in DMD patients compared with controls in the larger database where only 31P NMRS data were obtained. Significant yet weak correlations existed between [Mg2+] and PDE, water T2 and fat fraction. We concluded that low [Mg2+] is an actual finding in DMD, whether intracellular pH is normal or alkaline, and that it is a likely consequence of membrane leakiness. The response of Mg2+ to therapeutic treatment remains to be investigated in neuromuscular disorders. Free [Mg2+] determination with 31P NMRS is highly dependent on a precise knowledge of intracellular pH. The pH of Duchenne muscular dystrophy (DMD) patients, as determined by 31P NMRS, is abnormally alkaline. We have recently shown that intracellular pH could be determined using 1H NMRS of carnosine, and that intracellular pH was alkaline in a proportion of, but not all, DMD patients with a 31P NMRS‐based alkaline pH. Taking advantage of this 1H NMRS‐based intracellular pH, we found that free intramuscular [Mg2+] is in fact abnormally low in DMD patients. [ABSTRACT FROM AUTHOR]

Early studies have demonstrated that (total) magnesium was decreased in skeletal muscle of Duchenne muscular dystrophy (DMD) patients. Free intramuscular Mg 2+ can be derived from 31 P NMRS measurements. The value of free intramuscular magnesium concentration ([Mg 2+ ]) is highly dependent on precise knowledge of intracellular pH, which is abnormally alkaline in dystrophic muscle, possibly due to an expanded interstitial space, potentially causing an underestimation of [Mg 2+ ]. We have recently shown that intracellular pH can be derived using 1 H NMRS of carnosine. Our aim was to determine whether 31 P NMRS-based [Mg 2+ ] is, in fact, abnormally low in DMD patients, taking advantage of the 1 H NMRS-based pH. A comparative analysis was, therefore, made between [Mg 2+ ] values calculated with both 1 H and 31 P NMRS-based approaches to determine pH in 25 DMD patients, on a 3-T clinical NMR scanner. [Mg 2+ ] was also assessed with 31 P NMRS only in (forearm or leg) skeletal muscle of 60 DMD patients and 63 age-matched controls. Additionally, phosphodiester levels as well as quantitative NMRI indices including water T 2 , fat fraction, contractile cross-sectional area and one-year changes were evaluated. The main finding was that the significant difference in [Mg 2+ ] between DMD patients and controls was preserved even when the intracellular pH determined with 1 H NMRS was similar in both groups. Consequently, we observed that [Mg 2+ ] is significantly lower in DMD patients compared with controls in the larger database where only 31 P NMRS data were obtained. Significant yet weak correlations existed between [Mg 2+ ] and PDE, water T 2 and fat fraction. We concluded that low [Mg 2+ ] is an actual finding in DMD, whether intracellular pH is normal or alkaline, and that it is a likely consequence of membrane leakiness. The response of Mg 2+ to therapeutic treatment remains to be investigated in neuromuscular disorders. Free [Mg 2+ ] determination with 31 P NMRS is highly dependent on a precise knowledge of intracellular pH. The pH of Duchenne muscular dystrophy (DMD) patients, as determined by 31 P NMRS, is abnormally alkaline. We have recently shown that intracellular pH could be determined using 1 H NMRS of carnosine, and that intracellular pH was alkaline in a proportion of, but not all, DMD patients with a 31 P NMRS-based alkaline pH. Taking advantage of this 1 H NMRS-based intracellular pH, we found that free intramuscular [Mg 2+ ] is in fact abnormally low in DMD patients., (© 2019 John Wiley & Sons, Ltd.)

In recent years, quantitative nuclear magnetic resonance imaging and spectroscopy (NMRI and NMRS) have been used more systematically as outcome measures in natural history and clinical trial studies for Duchenne muscular dystrophy (DMD). Whereas most of these studies have emphasized the evaluation of the fat fraction as an assessment for disease severity, less focus has been placed on metabolic indices measured by NMRS. 31 P NMRS in DMD reveals an alkaline inorganic phosphate (P i ) pool, originating from either leaky dystrophic myocytes or an increased interstitial space. 1 H NMRS, exploiting the pH-sensitive proton resonances of carnosine, an intracellular dipeptide, was used to distinguish between these two hypotheses. NMR data were obtained in 23 patients with DMD and 14 healthy subjects on a 3-T clinical NMR system. Both 31 P and 1 H NMRS data were acquired at the level of the gastrocnemius medialis muscle. A multi-slice multi-echo imaging acquisition was performed for the determination of water T 2 and fat fraction in the same region of interest. Whereas nearly all patients with DMD showed an elevated pH compared with healthy controls when using 31 P NMRS, 1 H NMRS-determined pH was not systematically increased. As expected, the carnosine-based intracellular pH was never found to be alkaline in the absence of a concurrent P i -based pH elevation. In addition, abnormal intracellular pH, based on carnosine, was never associated with normal water T 2 values. We conclude that, in one group of patients, both 1 H and 31 P NMRS showed an alkaline pH, originating from the intracellular compartment and reflecting ionic dysregulation in dystrophic myocytes. In the other patients with DMD, intracellular pH was normal, but an alkaline P i pool was still present, suggesting an extracellular origin, probably revealing an expanded interstitial volume fraction, often associated with fibrotic changes. The data demonstrate that 1 H NMRS could serve as a biomarker to assess the normalization of intramyocytic pH and sarcolemmal permeability following therapy inducing dystrophin expression in patients with DMD., (Copyright © 2017 John Wiley & Sons, Ltd.)

Skeletal muscle inflammation/necrosis and fat infiltration are strong indicators of disease activity and progression in many neuromuscular disorders. They can be assessed by muscle T2 relaxometry and water-fat separation techniques, respectively. In the present work, we exploited differences between water and fat T1 and T2 relaxivities by applying a bi-component extended phase graph (EPG) fitting approach to simultaneously quantify the muscle water T2 and fat fraction from standard multi-slice multi-echo (MSME) acquisitions in the presence of stimulated echoes. Experimental decay curves were adjusted to the theoretical model using either an iterative non-negative least-squares (NNLS) procedure or a pattern recognition approach. Twenty-two patients (age, 49 ± 18 years) were selected to cover a large range of muscle fat infiltration. Four cases of chronic or subchronic juvenile dermatomyositis (age, 8 ± 3 years) were investigated before and 3 months following steroid treatment. For control, five healthy volunteers (age, 25 ± 2 years) were recruited. All subjects underwent the MSME sequence and EPG fitting procedure. The EPG fitting algorithm allowed a precise estimation of water T2 and fat fraction in diseased muscle, even in the presence of large B1(+) inhomogeneities. In the whole cohort of patients, there was no overall correlation between water T2 values obtained with the proposed method and the fat fraction estimated inside muscle tissues (R(2) = 0.02). In the patients with dermatomyositis, there was a significant decrease in water T2 (-4.09 ± 3.7 ms) consequent to steroid treatment. The pattern recognition approach resulted in a 20-fold decrease in processing time relative to the iterative NNLS procedure. The fat fraction derived from the EPG fitting approach correlated well with the fat fraction derived from a standard three-point Dixon method (≈1.5% bias). The bi-component EPG fitting analysis is a precise tool to monitor muscle tissue disease activity and is able to handle bias introduced by fat infiltration and B1(+) inhomogeneities., (Copyright © 2016 John Wiley & Sons, Ltd.)

Many anomalies exist in the resting (31) P muscle spectra of boys with Duchenne muscular dystrophy (DMD) but few have been reported in Golden Retriever muscular dystrophy (GRMD), the closest existing animal model for DMD. Because GRMD is recommended for preclinical evaluation of therapies and quantitative outcome measures are needed, we investigated anomalies of (31) P NMRS in tibial cranial and biceps femoris muscles from 14 GRMD compared to 9 control (CONT) dogs. Alterations observed in DMD children - low phosphocreatine and high phospho-monoesters and -diesters - were all found in GRMD but increased pH was not. More surprisingly, inorganic phosphate (Pi) appeared to present a prominent splitting with an enhanced Pi(b) resonance at 0.3 ppm downfield of Pi(a) . Assuming that both resonances are Pi, the pH for Pi(a) in GRMD corresponded to a physiological intracellular pH(a) (6.97 ± 0.05), while pH(b) approached the extracellular range (7.27 ± 0.10) and correlated with pH(a) in GRMD (R(2)  = 0.65). Both Pi(a) and Pi(b) were elevated compared to CONT and Pi(a) increased with age for GRMD (R(2)  = 0.48, p < 0.001). Magnetisation transfer experiments between γATP and Pi were conducted to better characterise Pi pools. Equal T1 relaxation times for Pi(b) and Pi(a) did not support a mitochondrial origin of Pi(b) . We suggest that Pi(b) could originate from degenerating hypercontracted cells that have a leaky membrane and inadequate cell homeostasis and pH regulation. Pi(b) showed minimal chemical exchange in all dogs, while the exchange rate of Pi(a) was reduced in GRMD and might extraneously reflect low glycolytic activity in DMD. Taken together, the ensemble of (31) P NMRS alterations identifies muscle dysfunction and could provide useful biomarkers of therapeutic efficacy. Furthermore, among these, two might relate more specifically to dystrophic processes and merit further investigation: one is the existence of the enhanced alkaline Pi(b) pool; the other, mechanisms by which membrane disruption might increase phosphodiesters in dystrophy., (Copyright © 2012 John Wiley & Sons, Ltd.)