Dynamic 31 P-MRSI using spiral spectroscopic imaging can map mitochondrial capacity in muscles of the human calf during plantar flexion exercise at 7T.

Ladislav Valkovič,Borjan Gagoski,Siegfried Trattnig,Martin Krššák,Marek Chmelík,Wolfgang Bogner,Christopher T Rodgers,Ovidiu C Andronesi,Martin Meyerspeer

doi:10.1002/nbm.3662

Abstract

Phosphorus MRSI (31P–MRSI) using a spiral‐trajectory readout at 7 T was developed for high temporal resolution mapping of the mitochondrial capacity of exercising human skeletal muscle.The sensitivity and localization accuracy of the method was investigated in phantoms. In vivo performance was assessed in 12 volunteers, who performed a plantar flexion exercise inside a whole‐body 7 T MR scanner using an MR‐compatible ergometer and a surface coil. In five volunteers the knee was flexed (~60°) to shift the major workload from the gastrocnemii to the soleus muscle.Spiral‐encoded MRSI provided 16–25 times faster mapping with a better point spread function than elliptical phase‐encoded MRSI with the same matrix size. The inevitable trade‐off for the increased temporal resolution was a reduced signal‐to‐noise ratio, but this was acceptable. The phosphocreatine (PCr) depletion caused by exercise at 0° knee angulation was significantly higher in both gastrocnemii than in the soleus (i.e. 64.8 ± 19.6% and 65.9 ± 23.6% in gastrocnemius lateralis and medialis versus 15.3 ± 8.4% in the soleus).Spiral‐encoded 31P–MRSI is a powerful tool for dynamic mapping of exercising muscle oxidative metabolism, including localized assessment of PCr concentrations, pH and maximal oxidative flux with high temporal and spatial resolution.

Highlights

Impaired energy metabolism in skeletal muscle mitochondria is indicative of muscular disorders (e.g. Duchenne muscular dystrophy[1] or mitochondrial myopathy2), systemic metabolic diseases and cardiovascular diseases (e.g. peripheral arterial disease (PAD)[5,6,7])
We propose dynamic spiral‐accelerated 31P–MRSI as an efficient method for simultaneous mapping of the temporal changes in PCr and intracellular pH during plantar flexion exercise of the calf muscle at 7 T
The performance of our rapid spiral‐encoded 31P–MRSI method and its feasibility for dynamic experiments was investigated in phantom experiments and in vivo in a group of 12 healthy volunteers

Summary

| INTRODUCTION

Impaired energy metabolism in skeletal muscle mitochondria is indicative of muscular disorders (e.g. Duchenne muscular dystrophy[1] or mitochondrial myopathy2), systemic metabolic diseases (e.g. diabetes mellitus3,4) and cardiovascular diseases (e.g. peripheral arterial disease (PAD)[5,6,7]). The temporal resolution of phosphorus MRSI (31P–MRSI) with Cartesian phase encoding is, insufficient for estimating mitochondrial capacity with high spatial resolution unless a dedicated exercise protocol (i.e. gated MRSI32,33) is applied, which considerably complicates and prolongs the examination and may impose limitations, such as the requirement of only mild pH changes. In this context, our aim was to develop and test a 31P–MRSI sequence using spiral readout trajectories with high temporal resolution for spatially resolved quantification of maximal oxidative ATP‐ synthase flux in the muscles of the human calf during plantar flexion exercise. The performance of the proposed sequence was tested in a localization phantom and in healthy volunteers

| METHODS

Findings

| DISCUSSION