Dynamic ³¹P spectroscopic imaging of skeletal muscles combining flyback echo‐planar spectroscopic imaging and compressed sensing

Abstract

Purpose Dynamic phosphorus MR spectroscopic imaging (31P‐MRSI) experiments require temporal resolution on the order of seconds to concurrently assess different muscle groups. A highly accelerated pulse sequence combining flyback echo‐planar spectroscopic imaging (EPSI) and compressed sensing was developed and tested in a phantom and healthy humans during an exercise‐recovery challenge of the lower leg muscles, using a clinical 3T MRI. Methods A flyback EPSI readout designed to achieve 2.25 × 2.25 cm2 resolution over a 18 × 18 cm2 field of view (i.e., 8 × 8 matrix) was combined with compressed sensing through the inclusion of pseudorandom gradient blips to sub‐sample the ky‐kt dimensions by a factor of 2.7×, achieving a temporal resolution of 9 s. The sequence was first tested in a phantom to assess performance compared to fully sampled EPSI (fidEPSI) and phase encoded chemical shift imaging (fidCSI). Then, tests were performed in 11 healthy volunteers during an exercise‐recovery challenge of the lower leg muscles. Voxels containing tissue from different muscle groups were evaluated measuring percentage phosphocreatine (%PCr) depletion, time constant of PCr recovery (τPCr) and intracellular pH at rest and following exercise. Results The sequence was capable to track the dynamic PCr response of multiple muscles simultaneously. No statistical differences were found in the metabolite ratio, pH or linewidth when compared with fidEPSI and fidCSI in the phantom study. Dynamic experiments showed differences in PCr depletion when comparing soleus with gastrocnemius muscles. Intracellular pH, τPCr and %PCr decrease were consistent with reported values. Conclusion Highly accelerated 31P‐MRSI combining flyback EPSI and compressed sensing is capable of assessing concurrent energy metabolism in multiple muscle groups using a clinical 3T MR system.

Dynamic magnetic resonance measurements of calf muscle oxygenation and energy metabolism in peripheral artery disease

“…A quantitative comparison of patients before and after a therapeutic intervention, such as revascularization or vasodilatory medication with cilostazol, will be instrumental to demonstrate any added value of this approach for treatment monitoring. We anticipate that with ongoing developments in MR hardware such as dual‐frequency 1 H/ 31 P coil arrays, acquisition schemes and sequences, and the emerging use of 7T MR systems, the sensitivity of BOLD MRI and 31 P‐MRS to dynamic changes and pathophysiologic alterations as well as the spatial coverage of the calf musculature will increase. In addition, simultaneous readouts of tissue high‐energy phosphate metabolism with 31 P‐MRS and measures obtained with 1 H‐MRI will be of benefit for studies of other organs including the brain and the heart, such that any correlations between different physiological parameters can truly be obtained during the same challenge or stimulus as we demonstrated here.…”

Accelerated MR spectroscopic imaging—a review of current and emerging techniques

“…At 3 T fully scanner-integrated solutions for spiral MRSI 153,155 have recently facilitated a stronger clinical use for brain tumor, 149,151,152 neurodegenerative 150,160 and demyelinating disorders, 161 as well as psychiatric research. 148,162 Since the evolution of 1 H-MRSI towards whole-brain coverage 76,163 there is an increased need for mitigation of extra-cranial lipid bleeding artifacts. This has led to the development of variable-density spirals, which improve the SRF during the acquisition in an SNR-efficient manner without the need for inefficient post-processing k-space filters.…”

“…232 An actual 2D implementation for human brain 31 P-MRSI was shown in 2017. 148,233,234 CS has been recently combined with flyback-EPSI for highly accelerated 31 P-MRSI, 62 and two distinct reconstruction schemes-L1-norm minimization and low-rank Hankel matrix completion-have been compared. 138 The application of CS to 1 H-MRSI is complicated due to large water and lipid nuisance signals, which can mislead the reconstruction algorithm to exclude the lower-intensity metabolite peaks by misadjusted thresholding.…”

“…With these properties, spirals are ideal for application in hyperpolarized 13 C studies, where speed is critical to minimize T 1 relaxation‐related SNR losses, 37,142–146 but also more recently in dynamic 31 P‐MRSI in muscles, 30,147 where a high temporal resolution is essential. Spiral SSE is also playing an increasingly important role in 1 H‐MRSI of the brain 71,148–157 and prostate 141,158,159 to reach clinically attractive scan times.…”

“…The first CS 31 P‐MRSI implementation was shown in 2012 and was based on simulated 2D‐MRSI data 232 . An actual 2D implementation for human brain 31 P‐MRSI was shown in 2017 148,233,234 . CS has been recently combined with flyback‐EPSI for highly accelerated 31 P‐MRSI, 62 and two distinct reconstruction schemes—L1‐norm minimization and low‐rank Hankel matrix completion—have been compared 138 …”