  Magnetic resonance spectroscopic imaging (MRSI) enables the simultaneous
non-invasive acquisition of MR spectra from multiple spatial locations inside
the brain. While 1H-MRSI is increasingly used in the human brain, it is not yet
widely applied in the preclinical settings, mostly because of difficulties
specifically related to very small nominal voxel size in the rodent brain and
low concentration of brain metabolites, resulting in low signal-to-noise ratio
SNR.
  In this context, we implemented a free induction decay 1H-MRSI sequence
(1H-FID-MRSI) in the rat brain at 14.1T. We combined the advantages of
1H-FID-MRSI with the ultra-high magnetic field to achieve higher SNR, coverage
and spatial resolution in the rodent brain, and developed a custom dedicated
processing pipeline with a graphical user interface: MRS4Brain toolbox.
  LCModel fit, using the simulated metabolite basis-set and in-vivo measured
MM, provided reliable fits for the data at acquisition delays of 1.3 and 0.94
ms. The resulting Cram\'er-Rao lower bounds were sufficiently low (<30%) for
eight metabolites of interest, leading to highly reproducible metabolic maps.
Similar spectral quality and metabolic maps were obtained between 1 and 2
averages, with slightly better contrast and brain coverage due to increased SNR
in the latter case. Furthermore, the obtained metabolic maps were accurate
enough to confirm the previously known brain regional distribution of some
metabolites. The acquisitions proved high reproducibility over time.
  We demonstrated that the increased SNR and spectral resolution at 14.1T can
be translated into high spatial resolution in 1H-FID-MRSI of the rat brain in
13 minutes, using the sequence and processing pipeline described herein.
High-resolution 1H-FID-MRSI at 14.1T provided reproducible and high-quality
metabolic mapping of brain metabolites with significantly reduced technical
limitations.
