Preclinical MRS advancements

Magnetic Resonance Spectroscopy (MRS) is the main technique allowing to measure simultaneously a high number of metabolites in vivo non-invasively. In parallel, it allows to measure metabolite concentrations and different biochemical processes (e.g., metabolite fluxes) in different organs in normal or pathological conditions¹. These unique advantages enable the longitudinal monitoring of disease progression and/or effect of treatment thus making a bridge between basics research and clinical diagnosis.
In this context, the use of different rodent models mimicking multiple aspects of the human disease is extremely useful². While different cell cultures can be used for answering basic cellular/metabolic questions, in vivo rodent models are needed for more complex questions related to the living organism².

In vivo MRS studies can be performed using different nuclei (1H, 31P, 13C, 15N,17O) and can be more or less complex depending on the nuclei under investigation³. Often complementary information on the same metabolites can be measured when combining different nuclei: phosphocreatine in 1H and 31P spectra; glutamate and glutamine in 1H and 13C spectra or 15N spectra.
1H MRS allows the detection of 19 metabolites involved in: osmoregulation (taurine (Tau), inositol (Ins), creatine (Cr)), myelination/cell proliferation (phosphocholine (PCho), glycerophosphocholine (GPC), phosphoethanolamine (PE), N-acetylaspartate (NAA), N-acetylaspartylglutamate (NAAG)), energy metabolism (lactate (Lac), glucose (Glc), alanine (Ala), phosphocreatine (PCr), Cr), and neurotransmitters and metabolites involved in neurotransmission (glutamate (Glu), glutamine (Gln), aspartate (Asp), glycine (Gly), γ-aminobutyrate (GABA)), antioxidants (glutathione (GSH) and ascorbate (Asc)).
31P MRS provides complementary information on the three phosphates of adenosine triphosphate (ɣ,β,α-ATP), PCr and inorganic phosphate (Pi) and chemical reaction rates between them (i.e. creatine kinase and ATP synthase). Cell membrane precursors and degradation products (phosphomonoesters (PCho, PE); diesters (GPC, glycerol-phosphoethanolamine), the resonance of nicotinamide adenine dinucleotide (NAD+), nicotinamide adenine dinucleotide phosphate (NADP) and NAD+/NADH redox state can be measured.
13C or 15N MRS with infusions of isotopically labeled substrates are used to monitor the flow of the isotope of interest into different metabolic intermediates. 13C MRS enables the determination of biochemical fluxes like glycolytic rate, tricarboxylic cycle flux, exchange of Glu and Gln through neurotransmission processes. 15N MRS is an alternative approach to study glutamate-glutamine metabolism while 17O MRS is also used to measure energy metabolism.
Preclinical in vivo MRS has undergone an enormous evolution from the first unlocalized experiments to the robust technique which is today: advancements in localization, spectral resolution, water and outer volume suppression, minimization of the static B0 magnetic field variations, artifact suppression, spectral editing, number of detected metabolites, spectral fitting and quantification precision^2–11.
The tendency is to perform acquisitions at ultra-high magnetic field strengths (UHF, B0≥7T) leading to the following benefits:
· increased intrinsic signal-to-noise ratio (SNR) (i.e. sensitivity)
· increased chemical shift dispersion (i.e. spectral resolution)
· decreased strong J-coupling effects
These benefits combined with the usage of ultra-short echo times:
· are useful for low concentrated and/or strongly overlapped metabolites and for metabolites having complex J-coupled spectral patterns
· led to an enormous increase in the number of detectable metabolites increasing thus the amount of biochemical information
· led to improved quantification precision and accuracy

Preclinical MRS challenges and strategies

Single voxel MRS (SVS) is measuring metabolites in one single preselected volume (VOI of mm size). The most used SVS sequences are STEAM, PRESS, SPECIAL, LASER and they all lead to high spectral quality (e.g. good localization with low chemical shift displacement error, outer volume and water suppression)². However, only one single spectrum per VOI is usually acquired.
In contrast, Magnetic Resonance Spectroscopic Imaging (MRSI) is a powerful non-invasive imaging tool to map simultaneously the brain regional distribution of multiple metabolites offering a characterization of the regional differences in the studies organ. The current drawback of MRSI is the long measurement time, while fast MRSI in preclinical settings has not been used very often mainly due to the complexity of the technique. To the best of our knowledge, preclinical high resolution 1H-MRSI data are still acquired using traditional phase encoded MRSI^12–16. Fast MRSI has been implemented for dynamic nuclear polarization with lower spatial resolution, higher SNR, few metabolites investigated with almost no spectral overlap¹⁷.
SVS 1H MRS in nowadays widely used while MRSI, X-nuclei MRS^2,3 and diffusion weighted MRS¹⁸ are more complex MRS techniques with several challenges:
· long measurement times
· low concentrated metabolites and SNR
· hardware limitations (B0 and gradient strength, RF coils, B0 inhomogeneities)
· requirement for advanced in-house developed pulse sequences and post-processing methods
· quality assessment of a huge number of spectra
· estimation of the precision and reliability of derived metabolite maps or modelling approaches.
Several strategies are worth to be mentioned to overcome these challenges:
· Specific technical recommendations for preclinical MRS have been provided in the consensus manuscript on preclinical MRS²
· FID-MRSI is particularly suited for brain metabolites mapping due to negligible J-coupling and T2 related signal loss, resulting in better suitability for fast MRSI at UHF in preclinical settings^19–23
· Post-processing methods aiming at minimizing the variance of the MRS signals are needed^24–28. · The MRSHub (https://www.mrshub.org) should be used for harmonization of preclinical MRS acquisitions, processing and data sharing.

Acknowledgements

Supported by the SNSF projects no 310030_173222 and 310030_201218. We acknowledge access to the facilities and expertise of the CIBM Center for Biomedical Imaging founded and supported by Lausanne University Hospital (CHUV), University of Lausanne (UNIL), Écôle polytechnique fédérale de Lausanne (EPFL), University of Geneva (UNIGE) and Geneva University Hospitals (HUG).

References

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