31 P magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

Abstract

Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P 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 31 P 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 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P 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.

Keywords: 31P; MRI; exercise; metabolism; muscle; nuclear magnetic resonance spectroscopy; phosphorus MRS.

© 2020 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Figures

FIGURE 1

A typical 31P MR spectrum of the resting soleus muscle of a healthy volunteer acquired at 7 T, with the region between 2.5 and 6 ppm enlarged (right). Signals of an extra Pi pool and phosphodiesters (PDE) and phosphomonoesters (PME) are visible. Peak assignments: two signals for inorganic phosphate (Pi and Pi2), glycero‐3‐phosphocholine (GPC), glycero‐3‐phosphoethanolamine (GPE), phosphocreatine (PCr), three signals for ATP and pyridine nucleotides (NADPH/NADH). Data were acquired using a pulse‐acquire sequence with a block pulse of 200 μs with a 5‐cm surface‐coil (TR = 5 s, bandwidth = 5 kHz, 2048 data points; 128 averages). Figure adapted from 50

FIGURE 2

Spectra showing the principles of the saturation transfer experiment. In this example saturation of the γ‐ATP resonance (A, lower) yields a reduction in the signals of Pi (and PCr) due to chemical spin exchange during the indicated reaction, as shown in detail for Pi in the insert (B), when compared to control conditions (A, upper); the difference Δ is then used to quantify Pi → ATP flux (see text). Figure adapted from 54 which is licensed under CC‐BY 3.0

FIGURE 3

Time series of pulse‐acquire spectra (A) measured at 7 T during rest, plantar flexion exercise and post‐exercise recovery with a 10‐cm surface coil placed below the calf and using a pulse‐acquire scheme (250 μs block pulse) without further localisation (left) compared to semi‐LASER single voxel localised MRS (TE = 23 ms) from the gastrocnemius medialis muscle (right). Both series: TR = 6 s, bandwidth = 5 kHz, 2048 data points; no averaging, 30 Hz apodisation. Non‐localised spectra show higher SNR with broader linewidths but reflect less PCr depletion, as indicated by the arrows and visible in the time series of fitted PCr signal amplitudes (B). The inorganic phosphate peak is clearly detectable in all non‐localised spectra, even at rest and during recovery, but is contaminated by signals from inactive tissue with neutral pH or shows a split peak (A), leading to ambiguous pH quantification during exercise and recovery (C). Figure adapted from, 44 which is licensed under CC‐BY‐NC 2.5

FIGURE 4

The figure shows combinations of RF coil and pulse sequence which are likely to be useful at different scanner field strengths (indicated by colour: see key). Requirements, and therefore recommendations, are different for static (left) and dynamic acquisitions (right). ‘Surface coil’ designates loop coils and coil arrays that provide some degree of localisation via their sensitive volume, while ‘volume coil’ designates birdcage coils and similar designs that can encompass e.g. a limb comprising several muscles or muscle groups. Parentheses indicate possible, but less favourable, combinations. The diagram should be read as follows: Dynamic studies employing localisation schemes are possible with sufficient SNR at high and ultra‐high fields, preferably employing surface coils or arrays; at lower fields, employing a pulse‐acquire scheme providing high SNR is preferable, relying on a surface coil for localisation. For studies of resting muscle, differentiation of individual muscles may be less critical, allowing for large volumes to contribute to the signal with large surface or volume coils, for high SNR, even at low fields