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

3D ultra-short echo time 31P-MRSI with rosette k-space pattern: Feasibility and comparison with conventional weighted CSI

Phosphorus-31 magnetic resonance spectroscopic imaging (31P-MRSI) provides valuable non-invasive in vivo information on tissue metabolism but is burdened by poor sensitivity and prolonged scan duration. Ultra-short echo time (UTE) acquisitions minimize signal loss when probing signals with relatively short spin-spin relaxation time (T2), while also preventing first-order dephasing. Here, a three-dimensional (3D) UTE sequence with a rosette k-space trajectory is applied to 31P-MRSI at 3T. Conventional chemical shift imaging (CSI) employs highly regular Cartesian k-space sampling, susceptible to substantial artifacts when accelerated via undersampling. In contrast, this novel sequence's "petal-like" pattern offers incoherent sampling more suitable for compressed sensing (CS). These results showcase the competitive performance of UTE rosette 31P-MRSI against conventional weighted CSI with simulation, phantom, and in vivo leg muscle comparisons.

Assessing the Feasibility of Dynamic 31P Spectroscopy for Metabolic Studies with a 1.0T Extremity Scanner

Objective: The feasibility of conducting in vivo non-localized ³¹P Magnetic Resonance Spectroscopy (MRS) with a 1.0T extremity scanner and the potential to increase accessibility of this important diagnostic tool for low cost applications is revisited. Methods: This work presents a custom transmit-only quadrature birdcage, four-element receive coil array, and spectrometer interfaced to a commercial ONI 1.0T magnet for enabling multi-channel, non-1H frequency capabilities. A custom, magnetic resonance compatible plantar flexion-extension exercise device was also developed to enable exercise protocols. The coils were assessed with bench measurements and ³¹P phantom studies before an in vivo demonstration. Results: In pulse and acquire spectroscopy of a phantom, the array was found to improve the signal-to-noise ratio (SNR) by a factor of 1.31 and reduce the linewidth by 13.9% when compared to a large loop coil of the same overall size. In vivo testing results show that two averages and a four second repetition time for a temporal resolution of eight seconds was sufficient to obtain phosphocreatine recovery values and baseline pH levels aligned with expected literature values. Conclusion: Initial in vivo human skeletal muscle ³¹P MRS allowed successful monitoring of metabolic changes during an 18-minute exercise protocol. Significance: Adding an array coil and multinuclear capability to a commercial low-cost 1.0T extremity scanner enabled the observation of characteristic ³¹P metabolic information, such as the phosphocreatine recovery rate and underlying baseline pH.

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

Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. State-of-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques. These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only has been the enabling factor in high-resolution whole-brain 1 H-MRSI, but today is also common in non-proton MRSI (31 P, 2 H and 13 C) and applied in many different organs. In this process, MRSI techniques had to constantly adapt, but have also benefitted from the significant increase of magnetic field strength boosting the signal-to-noise ratio along with high gradient fidelity and high-density receive arrays. In combination with recent trends in image reconstruction and much improved computation power, these advances led to a number of novel developments with respect to MRSI acceleration. Today MRSI allows for non-invasive and non-ionizing mapping of the spatial distribution of various metabolites' tissue concentrations in animals or humans, is applied for clinical diagnostics and has been established as an important tool for neuro-scientific and metabolism research. This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques. In addition to 1 H-MRSI it also includes other relevant nuclei and is not limited to certain body regions or specific applications.

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

Background

Clinical assessments of peripheral artery disease (PAD) severity are insensitive to pathophysiological changes in muscle tissue oxygenation and energy metabolism distal to the affected artery.

Purpose

To quantify the blood oxygenation level‐dependent (BOLD) response and phosphocreatine (PCr) recovery kinetics on a clinical MR system during a single exercise‐recovery session in PAD patients.

Study Type

Case–control study.

Subjects

Fifteen Fontaine stage II patients, and 18 healthy control subjects

Field Strength/Sequence

Interleaved dynamic multiecho gradient‐echo ¹H T₂* mapping and adiabatic pulse‐acquire ³¹P‐MR spectroscopy at 3T.

Assessment

Blood pressure in the arms and ankles were measured to determine the ankle‐brachial index (ABI). Subjects performed a plantar flexion exercise‐recovery protocol. The gastrocnemius and soleus muscle BOLD responses were characterized using the T₂* maps. High‐energy phosphate metabolite concentrations were quantified by fitting the series of ³¹P‐MR spectra. The PCr recovery time constant (τ_PCr) was derived as a measure of in vivo mitochondrial oxidative capacity.

Statistical Tests

Comparisons between groups were performed using two‐sided Mann–Whitney U‐tests. Relations between variables were assessed by Pearson's r correlation coefficients.

Results

The amplitude of the functional hyperemic BOLD response in the gastrocnemius muscle was higher in PAD patients compared with healthy subjects (–3.8 ± 1.4% vs. –1.4 ± 0.3%; P < 0.001), and correlated with the ABI (r = 0.79; P < 0.001). PCr recovery was slower in PAD patients (τ_PCr = 52.0 ± 13.5 vs. 30.3 ± 9.7 sec; P < 0.0001), and correlated with the ABI (r = –0.64; P < 0.001). Moreover, τ_PCr correlated with the hyperemic BOLD response in the gastrocnemius muscle (r = –0.66; P < 0.01).

Data Conclusion

MR readouts of calf muscle tissue oxygenation and high‐energy phosphate metabolism were acquired essentially simultaneously during a single exercise‐recovery session. A pronounced hypoxia‐triggered vasodilation in PAD is associated with a reduced mitochondrial oxidative capacity.

Level of Evidence: 2

Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2020;51:98–107.