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Whole-brain 23Na multi-parametric mapping at 7 Tesla1CEA - Neurospin, Paris, France, 2CRMBM, UMR 7339, Aix-Marseille University, Paris, France, 3CENIR, ICM, Hôpital Pitié-Salpêtrière, Paris, France, Paris, France, 4Siemens Healthineers, Saint-Denis, France
Synopsis
Quantifying physical properties of sodium could be of benefit to assess more specifically changes in cellular homeostasis accompanying neuroinflammatory or neurodegenerative processes. Here, we performed whole-brain Quantitative Imaging using Configuration States (QuICS) in vivo at 7 Tesla to assess simultaneously Total Sodium Concentration (TSC) and relaxation times (T1 and T2). An acquisition time of 20 minutes was sufficient for a 10 mm3 isotropic resolution. In the future, the use of non-Cartesian trajectories could further reduce the overall acquisition time, opening the way to the additional estimation of the trace apparent diffusion coefficient.
Introduction
Quantifying physical properties of sodium could be of benefit to assess more specifically changes in cellular homeostasis accompanying neuroinflammatory or neurodegenerative processes. Due to its lower in vivo concentration and NMR sensitivity than 1H, 23Na MRI remains challenging, resulting in images with relatively low SNR and resolution. Thus, 23Na MRI benefits from the advent of ultra-high field scanners and novel pulse sequences. Lately, simultaneous multi-parametric approaches have become of high interest1–5, primarily for quantitative 1H MRI. A preliminary study demonstrated the possibility to perform 23Na Quantitative Imaging using Configuration States (QuICS) in vitro at 7 Tesla6. This technique allows to assess simultaneously NMR and physical properties such as Total Sodium Concentration (TSC) and relaxation times (T1 and T2)7. Here, we translate this in vitro feasibility study into a clinically viable in vivo sodium imaging protocol.Materials & Methods
This quantitative imaging technique is based on a spoiled Steady-State Free Precession (SSFP) sequence that is able to generate multiple contrasts using different flip angle and RF spoiling increments. To restrict the total acquisition time, an optimization algorithm based on the Cramér-Rao lower bound8 was used to select the 10 most informative contrasts. Resulting acquisition parameters are shown in Table 1.
MRI acquisitions were performed on a 7T Magnetom scanner (Siemens Healthineers, Erlangen, Germany) using a dual-resonance 1H/23Na RF birdcage coil (Rapid Biomedical, Rimpar, Germany). After B0 shimming, images were acquired using a Cartesian sampling scheme, in transverse orientation with FOV=320x320x240mm3, voxel size 10x10x10mm3, TR/TE 20ms/3.4ms, BW 220Hz/px, 8 TR of dummy scan time and 8 averages, leading to a total acquisition time of 20min31s. Two 50 mL Falcon tubes were used as external references of concentrations (150 mmol/L NaCl, with 0 and 3% agar gel respectively) and were positioned at the rim of our volume coil.
A T1-weighted anatomical reference was acquired using MP2RAGE at a 0.75 mm3 isotropic resolution in a 240x225x170mm3 FOV, with TR/TE=6000/3.0ms and a bandwidth of 240Hz/px.
The acquired data from different contrasts presented in Table 1 were fitted voxel-wised to Bloch-Torrey equations to estimate 3D maps of M0, T1 and T2 using Matlab (The MathWorks, Natick, USA). The flip angle was considered equal to the targeted FA throughout the brain, since the B1+ profile of our 23Na quadrature birdcage can be considered homogeneous. The TSC was estimated from M0, using our external references of concentration. For visualization and further analysis, our TSC, T1 and T2 quantitative maps were interpolated and aligned with our T1-weighted anatomical reference using SPM14.
Results
Mean T1/T2 relaxation times in reference tube with water and 3% agar are reported in Table 2, and show a very good agreement with values reported in the literature9,10. The whole-brain parametric maps are displayed in Figure 1 overlaid with their anatomical reference. Associated relaxation times and TSC in the brain are reported in Table 3. While all parametric maps suffered from significant partial volume effects due to our moderate 10 mm isotropic resolution, relaxation times in brain parenchyma are estimated around 40 ms for T1 and 25ms for T2, which is consistent with previously reported values in the literature10. Likewise, TSC values are consistent with the expected concentrations of about 40 mM in brain tissues. However, the CSF concentration, expected around 140 mM, is underestimated here, due to partial volume with tissues with significantly lower concentrations.Discussion/Conclusion
These results demonstrate that the QuICS approach can be used in vivo at 7 Tesla to estimate 23Na total sodium concentration and T1/T2 relaxation times. Considering the modest amount of signal from 23Na, this technique exhibits a satisfactory time-efficiency as only the needed information is acquired by optimizing the different contrasts. By lengthening the acquisition time, we could either acquire maps at a higher resolution, or explore more contrasts, leading to the assessment of a trace apparent diffusion coefficient (ADC) map. Indeed, QuICS has already proven to be able to retrieve a one-dimensional ADC map in vitro6, but it was discarded in this study as human brain is an anisotropic media. A proper trace ADC estimation would require at least 3 spoiling directions. To improve upon this proof-of-concept acquisition, non-Cartesian k-space trajectories are being implemented to reduce the acquisition time12. Finally, we believe that the combination of these complementary NMR parameters could help in the interpretation of early physio-pathological events using 23Na MRI, in echo to comparable strategies successfully applied with multi-parametric 1H MRI3,13.Acknowledgements
This work received financial support from the French program ‘Investissement d’Avenir’ run by the ‘Agence Nationale pour la Recherche’, grant ‘Infrastructure d’avenir en Biologie Santé – ANR-11-INBS-0006’, from the ERPT equipment program of the Leducq Foundation, and from the European Union Horizon 2020 Research and Innovation program under Grant Agreement No. 736937.References
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