Repeatability of multinuclear interleaved acquisitions with nuclear Overhauser enhancement effect in dynamic experiments in the calf muscle at 3T

Mitochondrial efficiency in resting skeletal muscle in vivo: a novel non-invasive approach using multinuclear magnetic resonance spectroscopy in humans

Mitochondrial efficiency is a critical metabolic parameter with far-reaching implications for tissue homeostasis. However, the direct measurement of oxygen consumption (VO2) and ATP production from a large tissue sample in vivo remains challenging. Using phosphorus (31P) and proton (1H) magnetic resonance spectroscopy (MRS), this study aimed to non-invasively quantify the skeletal muscle ATP synthesis rate and VO2 to determine mitochondrial efficiency at rest and during muscle contraction in humans. We assessed mitochondrial efficiency in the plantar flexor muscles of 12 healthy adults (21 ± 1 years) using 31P and 1H MRS within a 3T MR system. MRS data were acquired at rest and during constant workloads to quantify oxidative ATP synthesis (ATPox) rate and myoglobin-derived oxygen consumption (Mb-derived VO2). At rest, ATPox was 0.85 ± 0.24 mm min-1, and Mb-derived VO2 was 0.46 ± 0.11 mm min-1, resulting in a P/O ratio of 1.95 ± 0.68. During graded exercise, end-exercise PCr concentration decreased from 29 ± 5.7 mm to 18 ± 4.8 mm, and end-exercise Mb oxygenation declined linearly to 47 ± 11%. ATPox synthesis rate increased linearly with exercise workload (r = 0.65 ± 0.31), whereas there was no significant change in Mb-derived VO2 (r = -0.19 ± 0.60), leading to non-physiological P/O values during exercise (>3). The results indicate that combined 31P/1H-MRS at rest offers a promising approach for non-invasively quantifying mitochondrial efficiency in large muscle samples, suggesting its potential as a clinical endpoint of mitochondrial function. However, further refinement is needed for use during exercise. KEY POINTS: Mitochondrial efficiency, converting chemical energy from carbon fuels into ATP, is a vital metabolic parameter for tissue homeostasis, but measuring oxygen consumption (VO2) and ATP production in vivo has been challenging. This study used phosphorus (31P) and proton (1H) magnetic resonance spectroscopy (MRS) to non-invasively quantify the skeletal muscle ATP synthesis rate and VO2 at rest and during muscle contraction in humans. At rest, the oxidative ATP synthesis (ATPox) and myoglobin-derived VO2 (Mb-derived VO2) were measured, resulting in a P/O ratio of 1.95 in the plantar flexor muscles. During exercise, the ATPox rate increased with workload, but Mb-derived VO2 did not change significantly, leading to non-physiological P/O ratios. The findings suggest that 31P/1H-MRS at rest is a promising method for assessing mitochondrial efficiency and could be used as a clinical endpoint for mitochondrial function in vivo, although further refinement is needed for exercise conditions.

Multinuclear Interleaving of 1H CEST, Water T2*, and 23Na MRI at 3 T

MRI in vivo is a powerful clinical diagnosis tool as it allows acquiring noninvasively images with an ample range of contrasts. Advanced imaging techniques such as chemical exchange saturation transfer (CEST) allow measuring metabolic information including pH. Sodium tissue concentration, which can be measured by 23Na MRI, is sensitive to changes in different pathological conditions. The routine clinical application of these techniques is limited by the required additional scan time. Multinuclear interleaved techniques allow reducing the total acquisition scan time by performing the pulse sequence elements of a 1H imaging sequence during the idle times typically used in 23Na MRI to allow magnetization recovery and reduce T1 weighting. An interleaved radial amine CEST and sodium (INTERLACED) pulse sequence was developed on a clinical scanner to simultaneously map acidity or T2* decay with 23Na signal, reducing the total scan time by 46% relative to sequential mononuclear acquisitions and without introducing any significant bias, as demonstrated in vitro. Dynamic INTERLACED measures were performed in the leg during a 5-min plantar flexion exercise and during a second plantar flexion exercise immediately followed by a 5-min voluntary isometric contraction. The results showed increased T2* and 23Na signal during recovery in the gastrocnemius (GAS) while only an increase in 23Na signal was observed in the soleus (SOL). During the isometric contraction, T2* decreased in GAS, SOL, and the tibialis anterior; the 23Na signal increased in GAS and SOL; and the magnetization transfer asymmetry increased in GAS, in agreement with an increase of intracellular sodium and acidification of the extracellular space. Our approach requires no hardware modifications, facilitating its inclusion in clinical routine at 3 T. Furthermore, it could benefit functional studies by enabling the acquisition of dynamic multinuclear information simultaneously from the same transient state.

Exercise MR of Skeletal Muscles, the Heart, and the Brain

Magnetic resonance (MR) imaging (MRI) is routinely used to evaluate organ morphology and pathology in the human body at rest or in combination with pharmacological stress as an exercise surrogate. With MR during actual physical exercise, we can assess functional characteristics of tissues and organs under real-life stress conditions. This is particularly relevant in patients with limited exercise capacity or exercise intolerance, and where complaints typically present only during physical activity, such as in neuromuscular disorders, inherited metabolic diseases, and heart failure. This review describes practical and physiological aspects of exercise MR of skeletal muscles, the heart, and the brain. The acute effects of physical exercise on these organs are addressed in the light of various dynamic quantitative MR readouts, including phosphorus-31 MR spectroscopy (31P-MRS) of tissue energy metabolism, phase-contrast MRI of blood flow and muscle contraction, real-time cine MRI of cardiac performance, and arterial spin labeling MRI of muscle and brain perfusion. Exercise MR will help advancing our understanding of underlying mechanisms that contribute to exercise intolerance, which often proceed structural and anatomical changes in disease. Its potential to detect disease-driven alterations in organ function, perfusion, and metabolism under physiological stress renders exercise MR stress testing a powerful noninvasive imaging modality to aid in disease diagnosis and risk stratification. Although not yet integrated in most clinical workflows, and while some applications still require thorough validation, exercise MR has established itself as a comprehensive and versatile modality for characterizing physiology in health and disease in a noninvasive and quantitative way. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

Repeatability of alkaline inorganic phosphate quantification in the skeletal muscle using 31P-magnetic resonance spectroscopy at 3 T

The detection of a secondary inorganic phosphate (Pi) resonance, a possible marker of mitochondrial content in vivo, using phosphorus magnetic resonance spectroscopy (31P-MRS), poses technical challenges at 3 Tesla (T). Overcoming these challenges is imperative for the integration of this biomarker into clinical research. To evaluate the repeatability and reliability of measuring resting skeletal muscle alkaline Pi (Pialk) using with 31P-MRS at 3 T. After an initial set of experiments on five subjects to optimize the sequence, resting 31P-MRS of the quadriceps muscles were acquired on two visits (~4 days apart) using an intra-subjects design, from 13 sedentary to moderately active young male and female adults (22 ± 3 years old) within a whole-body 3 T MR system. Measurement variability attributed to changes in coil position, shimming procedure, and spectral analysis were quantified. 31P-MRS data were acquired with a 31P/-proton (1H) dual-tuned surface coil positioned on the quadriceps using a pulse-acquire sequence. Test-retest absolute and relative repeatability was analyzed using the coefficient of variation (CV) and intra-class correlation coefficients (ICC), respectively. After sequence parameter optimization, Pialk demonstrated high intra-subject repeatability (CV: 10.6 ± 5.4%, ICC: 0.80). Proximo-distal change in coil position along the length of the quadriceps introduced Pialk quantitation variability (CV: 28 ± 5%), due to magnetic field inhomogeneity with more distal coil locations. In contrast, Pialk measurement variability due to repeated shims from the same muscle volume (0.40 ± 0.09mM; CV: 6.6%), and automated spectral processing (0.37 ± 0.01mM; CV: 2.3%), was minor. The quantification of Pialk in skeletal muscle via surface coil 31P-MRS at 3 T demonstrated excellent reproducibility. However, caution is advised against placing the coil at the distal part of the quadriceps to mitigate shimming inhomogeneity.

Sequence building block for magnetic resonance spectroscopy on Siemens VE-series scanners

We present a sequence building block (SBB) that embeds magnetic resonance spectroscopy (MRS) into another sequence on the Siemens VE platform without any custom hardware. This enables dynamic studies such as functional MRS (fMRS), dynamic shimming and frequency correction, and acquisition of navigator images for motion correction. The SBB supports nonlocalised spectroscopy (free induction decay), STimulated Echo Acquisition Mode single voxel spectroscopy, and 1D, 2D and 3D phase-encoded chemical shift imaging. It can embed 1H or X-nuclear MRS into a 1H sequence; and 1H-MRS into an X-nuclear sequence. We demonstrate integration into the vendor's gradient-recalled echo sequence. We acquire test data in phantoms with three coils (31P/1H, 13C/1H and 2H/1H) and in two volunteers on a 7-T Terra MRI scanner. Fifteen lines of code are required to insert the SBB into a sequence. Spectra and images are acquired successfully in all cases in phantoms, and in human abdomen and calf muscle. Phantom comparison of signal-to-noise ratio and linewidth showed that the SBB has negligible effects on image and spectral quality, except that it sometimes produces a nuclear Overhauser effect (NOE) signal enhancement for multinuclear applications in line with conventional 1H NOE pulses. Our new SBB embeds MRS into a host imaging or spectroscopy sequence in 15 lines of code. It allows homonuclear and heteronuclear interleaving. The package is available through the standard C2P procedure. We hope this will lower the barrier for entry to studies applying dynamic fMRS and for online motion correction and B0-shim updating.

Correlation between skeletal muscle acetylcarnitine and phosphocreatine metabolism during submaximal exercise and recovery: interleaved 1H/31P MRS 7 T study

Acetylcarnitine is an essential metabolite for maintaining metabolic flexibility and glucose homeostasis. The in vivo behavior of muscle acetylcarnitine content during exercise has not been shown with magnetic resonance spectroscopy. Therefore, this study aimed to explore the behavior of skeletal muscle acetylcarnitine during rest, plantar flexion exercise, and recovery in the human gastrocnemius muscle under aerobic conditions. Ten lean volunteers and nine overweight volunteers participated in the study. A 7 T whole-body MR system with a double-tuned surface coil was used to acquire spectra from the gastrocnemius medialis. An MR-compatible ergometer was used for the plantar flexion exercise. Semi-LASER-localized 1H MR spectra and slab-localized 31P MR spectra were acquired simultaneously in one interleaved exercise/recovery session. The time-resolved interleaved 1H/31P MRS acquisition yielded excellent data quality. A between-group difference in acetylcarnitine metabolism over time was detected. Significantly slower τPCr recovery, τPCr on-kinetics, and lower Qmax in the overweight group, compared to the lean group was found. Linear relations between τPCr on-kinetics, τPCr recovery, VO2max and acetylcarnitine content were identified. In conclusion, we are the first to show in vivo changes of skeletal muscle acetylcarnitine during acute exercise and immediate exercise recovery with a submaximal aerobic workload using interleaved 1H/31P MRS at 7 T.

Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (31P-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (kPCr) following contractions. In the quadricep muscles, several studies have quantified kPCr following 24-30 s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24 s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120°·s-1, 1 every 2 s) contractions. Muscle kPCr was ∼20% lower following the MVIC protocol compared with the MVDC protocol (p ≤ 0.001), though this was less evident in older adults (p = 0.073). Changes in skeletal muscle pH (p ≤ 0.001) and PME accumulation (p ≤ 0.001) were greater following the sustained MVIC protocol, and pH (p ≤ 0.001) and PME (p ≤ 0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.

Parallel detection of multi-contrast MRI and Deuterium Metabolic Imaging (DMI) for time-efficient characterization of neurological diseases

Deuterium Metabolic Imaging (DMI) is a novel method that can complement traditional anatomical magnetic resonance imaging (MRI) of the brain. DMI relies on the MR detection of metabolites that become labeled with deuterium (2H) after administration of a deuterated substrate and can provide images with highly specific metabolic information. However, clinical adoption of DMI is complicated by its relatively long scan time. Here, we demonstrate a strategy to interleave DMI data acquisition with MRI that results in a comprehensive neuro-imaging protocol without adding scan time. The interleaved MRI-DMI routine includes four essential clinical MRI scan types, namely T1-weighted MP-RAGE, FLAIR, T2-weighted Imaging (T2W) and susceptibility weighted imaging (SWI), interwoven with DMI data acquisition. Phantom and in vivo human brain data show that MR image quality, DMI sensitivity, as well as information content are preserved in the MRI-DMI acquisition method. The interleaved MRI-DMI technology provides full flexibility to upgrade traditional MRI protocols with DMI, adding unique metabolic information to existing types of anatomical image contrast, without extra scan time.

Interleaved and simultaneous multi-nuclear magnetic resonance in vivo. Review of principles, applications and potential

Magnetic resonance signals from different nuclei can be excited or received at the same time,rendering simultaneous or rapidly interleaved multi-nuclear acquisitions feasible. The advan-tages are a reduction of total scan time compared to sequential multi-nuclear acquisitions or that additional information from heteronuclear data is obtained at thesame time and anatomical position. Information content can be qualitatively increased by delivering a more comprehensive MR-based picture of a transient state (such as an exercise bout). Also, combiningnon-proton MR acquisitions with 1 Hinformation (e.g., dynamic shim updates and motion correction) can be used to improve data quality during long scans and benefits image coregistration. This work reviews the literature on interleaved and simultaneous multi-nuclear MRI and MRS in vivo. Prominent use cases for this methodology in clinical and research applications are brain and muscle, but studies have also been carried out in other targets, including the lung, knee, breast and heart. Simultaneous multi-nuclear measurements in the liver and kidney have also been performed, but exclusively in rodents. In this review, a consistent nomenclature is proposed, to help clarify the terminology used for this principle throughout the literature on in-vivo MR. An overview covers the basic principles, the technical requirements on the MR scanner and the implementations realised either by MR system vendors or research groups, from the early days until today. Considerations regarding the multi-tuned RF coils required and heteronuclear polarisation interactions are briefly discussed, and fields for future in-vivo applications for interleaved multi-nuclear MR pulse sequences are identified.

31 P MRSI at 7 T enables high-resolution volumetric mapping of the intracellular magnesium ion content in human lower leg muscles

PURPOSE

The non-invasive determination of the free magnesium ion concentration ([Mg²⁺ _free ]) using ³¹ P MRSI in vivo is of interest in research on various pathologies, e.g. diabetes. The purpose of this study was to demonstrate the potential of ³¹ P MRSI at 7 T to enable volumetric, high-resolution mapping of [Mg²⁺ _free ].

METHODS

3D ³¹ P MRSI datasets from the lower leg of three healthy volunteers were acquired at B₀  = 7 T with a nominal spatial resolution of (8 × 8 × 16) mm³ in 56 min. Volumetric [Mg²⁺ _free ] maps were calculated based on the quantified local chemical shift difference between the α- and β-resonance of adenosine triphosphate (ATP) considering also local pH values. Mean [Mg²⁺ _free ] values from three different muscle groups were compared. To demonstrate the potential of reducing the measurement time, the analysis was repeated on the acquired MRSI data retrospectively reconstructed with fewer averages.

RESULTS

The generated [Mg²⁺ _free ] maps revealed local differences, and mean [Mg²⁺ _free ] values of (1.08 ± 0.03) mM were found in the tibialis anterior, (0.91 ± 0.04) mM in the soleus and (0.98 ± 0.03) mM in the gastrocnemius medialis. The time-reduced 28-min scan resulted in comparable [Mg²⁺ _free ] maps, and mean values being in agreement with the values from the 56-min scan.

CONCLUSION

³¹ P MRSI at 7 T enables volumetric, high-resolution mapping of free magnesium ion content in human lower leg muscles. The measurement time of the ³¹ P MRSI acquisition can be reduced to 28 min, opening the potential to apply volumetric [Mg²⁺ _free ] mapping for the investigation of pathologies with altered magnesium homeostasis.