Synopsis

Keywords: Data Acquisition, Spectroscopy, Diffusion

³¹P diffusion magnetic resonance spectroscopy could assess the diffusion properties of high energy metabolites. In this study, a diffusion weighted (DW) STEAM sequence was implemented, and spectra were acquired in the human calf muscle of six healthy volunteers. Frequency and phase alignments were applied prior to spectral averaging. The ADC of phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi) and glycerol phosphorylcholine (GPC) were (0.24±0.02, 0.15±0.04, 0.43±0.14, 0.40±0.09)×10^-3 mm²/s. To the best of our knowledge, this is the first study reporting the ADCs of ATP, Pi, and GPC, and the second study reporting the ADC of PCr in human.

Introduction

³¹P MRS is a non-invasive technique for studying energy metabolism in vivo to track high-energy metabolites. Phosphocreatine (PCr), adenosine triphosphate (ATP), and inorganic phosphate (Pi) are the three main ³¹P MRS detectable metabolites essential for energy production and transport¹. Combining ³¹P and diffusion MRS to assess their diffusion properties could provide information regarding the diffusion-related energy supply mechanism in vivo. However, due to the low concentration of phosphorous metabolites, low sensitivity of ³¹P nuclei, short T₂ relaxation time, and J coupling effects, the measurement is difficult, especially that for ATP. In this study, we aim to investigate the diffusion coefficients of major phosphorous metabolites in the human calf muscle in vivo at 7 tesla.

Methods

A diffusion weighted STEAM (DW-STEAM) pulse sequence was implemented (fig. 1a). A 1600 μs long asymmetric Shinnar-Le Roux RF pulse (SLR pulse) and an inverse SLR pulse were implemented^2,3. Diffusion gradients were applied in three orthogonal directions [-0.5, 1, 1], [1, -0.5, 1], and [1, 1, -0.5]. To attenuate eddy currents induced by the diffusion gradients, bipolar gradients were implemented. Diffusion gradients of opposite polarities were applied in each direction to eliminate the effect of cross terms. VAPOR water suppression⁴ and outer volume suppression⁵ were applied prior to the STEAM localization. The ADC of water, Pi, PCr, and ATP were measured in vitro at room temperature (~18℃) to validate the sequence. ¹H experiments were carried out with a spherical phantom containing water (pH = 7.0) using a single-channel quadrature transmit and a 32-channel receive coil (Nova Medical Inc., MA, USA), with TE/TM/TR = 15ms / 750ms / 40s, number of averages (NA) = 4. ³¹P experiments were carried out with a phantom containing 31mM PCr, 31mM ATP, 8mM NaH₂PO₄ and 106mM NaCl (pH = 7.2) using a surface coil with a single ³¹P loop and a quadrature ¹H loops, with TE/TM/TR=15ms/750ms/60s, NA=4. Diffusion-weighted spectra at seven different b-values (0, 200, 400, 600, 800, 1000, 1200 s/mm²) were measured. Then, six healthy subjects (18-40 years old, 3 males and 3 females) were scanned for in vivo ADC measurements. Informed consent forms were obtained from all subjects before the scan. The MR spectra were acquired from a VOI of 60mm x 100mm x 120mm containing the calf muscle. Localized diffusion-weighted MR spectra at three b-values (0, 800, 1200 s/mm²) were acquired per diffusion direction (TE/TM/TR = 15ms / 750ms / 5s, NA = 32). The total in vivo data acquisition time was 40 min. All experiments were performed at a 7T/68cm MR scanner (Siemens Medical Solutions, Erlangen, Germany). The scanner is equipped with a gradient coil capable of reaching 80mT/m in total, and 50mT/m along each axis simultaneously. Phase and frequency drift correction, outlier removal were implemented prior to spectra averaging. The averaged spectra were quantified by LCModel.

Results

The measured ADC of water, PCr, Pi, γ-ATP and α-ATP were shown in table 1, coherent with the reported values⁶. The in vivo ADC values of Pi, PCr, ATP, and GPC were measured in the human calf muscle. High spectral SNR was achieved at all b-values with average linewidth being around 13.7 Hz. PCr was reliably quantified at all b-values with mean CRLB less than 1%, while γ-ATP, α-ATP, Pi and GPC less than 10%. The ADCs of PCr, Pi, γ-ATP and GPC were (0.24 ± 0.02, 0.43 ± 0.14, 0.15 ± 0.04, 0.40 ± 0.09) × 10^-3 mm²/s respectively with average R² being 0.97, 0.92, 0.81, 0.88 (table 2).

Discussion

The high sensitivity of 7T enabled the successful measurement of PCr, ATP, Pi, and GPC from the in vivo spectra at the high b-value, allowing for the first measurement of ADC of ATP, Pi, and GPC in human calf muscles. In this study, the ADCs of PCr and ATP in human calf muscle were in a good agreement with the values reported in rat skeletal muscles^7,8. However, the ADC of PCr was lower than that reported by Gabr, et al.⁹ in human calf muscles under similar diffusion time (~755 ms). The disagreement could lie in the differences during the post-processing process. Diffusion MRS is very sensitive to motions, which could lead to phase dispersion and thus the overestimation of ADC. Due to the low SNR of the individual scan in the study performed by Gabr, et al.10 at 3T, no phase correction was implemented and thus the PCr’s ADC was likely to be overestimated⁹. In the current study, scan-to-scan phase and frequency correction were implemented to mitigate the problem (fig. 2). The measured ADC of Pi was higher than that reported by K. Yoshizaki, et al. in bullfrog muscles ex vivo at room temperature (24℃)¹⁰. This discrepancy could be due to the different experimental conditions (human vs bullfrogs and in vivo vs ex vivo, 37℃ vs 24℃, respectively).

Conclusion

We conclude that it is feasible to measure the ADC of four major ³¹P metabolites (PCr, ATP, Pi and GPC) in vivo in the human calf muscle at 7T. This study paves the way to investigate ³¹P metabolite diffusion properties in health and disease on the clinical MR scanner.

Acknowledgements

This work was partially supported by the Swiss National Science Foundation (n° 320030_189064). We acknowledge accessto the facilities and expertise of the CIBM Center for Biomedical Imaging, a Swiss research center of excellence foundedand supported by Lausanne University Hospital (CHUV), University of Lausanne (UNIL), Ecole polytechnique fédérale deLausanne (EPFL), University of Geneva (UNIGE) and Geneva University Hospitals (HUG).