Lisa Leroi^{1}, Jacques Stout^{1}, Arthur Coste^{1}, Ludovic de Rochefort^{2}, Mathieu D. Santin^{3}, Romain Valabrègue^{3}, Franck Mauconduit^{4}, Cécile Rabrait-Lerman^{1}, Fawzi Boumezbeur^{1}, and Alexandre Vignaud^{1}

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 (T_{1}
and T_{2}). An acquisition time of 20 minutes was sufficient for a 10 mm^{3}
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.

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 bound^{8} 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 ^{1}H/^{2}3Na
RF birdcage coil (Rapid Biomedical, Rimpar, Germany). After B_{0} shimming, images were
acquired using a Cartesian sampling scheme, in transverse orientation with FOV=320x320x240mm^{3},
voxel size 10x10x10mm^{3}, 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 T_{1}-weighted
anatomical reference was acquired using MP2RAGE at a 0.75 mm^{3}
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 M_{0}, T_{1}
and T_{2} using Matlab (The MathWorks, Natick, USA). The flip angle was
considered equal to the targeted FA throughout the brain, since the B_{1}^{+}
profile of our ^{23}Na quadrature birdcage can be considered
homogeneous. The TSC was estimated from M_{0}, using our external
references of concentration. For visualization and further analysis, our TSC, T_{1}
and T_{2} quantitative maps were interpolated and aligned with our T_{1}-weighted
anatomical reference using SPM^{14}.

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Figure 1 : 7T 3D QuiCS quantitative extraction (10x10x10mm^{3}) TSC, T_{1}
and T_{2} (from left to right) from a 20 minutes acquisition. The red
lines show the depicted slices in the three orientations. As expected, TSC and
relaxations times are higher in CSF than in tissues. Values are in the range of
what is usually reported in literature^{10}.

Table 1 : Acquisition setup resulting from the optimization process for an
acquisition of 10 contrasts for a 10mm^{3} isotropic resolution

Table 2 : Quantitative results obtained in the two Falcon
reference tubes containing 3 and 0% agar in 150mM NaCl.

Table
3 : Quantitative results obtained in vivo in
different tissues such as white matter, grey matter and Cerebral Spinal Fluid.