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Abstract

Nuclear magnetic resonance (NMR) is a physical phenomenon that is widely used in the biomedical field due to its non-invasive and non-destructive properties, which make it an optimal tool for the in vivo investigation of living organs such as the brain. This thesis focused on the development of 1H magnetic resonance (MR) techniques at ultra-high magnetic field strength to improve the measurement and quantification of the 1H MR signal in the rodent brain, and to accurately assess the alterations of the metabolite and water concentrations in several cerebral metabolic disorders. In rodents, 1H MR spectroscopy (MRS) allows the assessment of the concentration of up to 20 metabolites that take part in many cerebral metabolic processes such as neurotransmission, cell energy metabolism, cell growth and osmosis. 1H MRS thus provides a unique tool to non-invasively investigate the cerebral metabolism in healthy and pathological conditions in vivo. However, it is essential for reliable quantification that systematic errors and overlap of the measured metabolite signals are minimized. To that aim, the impact of a potential additional short T2 relaxation time component, which might affect the glutamine quantification at long echo times, was assessed in this thesis. The J-difference editing technique MEGA-SPECIAL was optimized to obtain the unequivocal detection of the glutamine signal at moderate echo time. As a control, the glutamine concentration obtained with this method was then compared to short echo time 1H MR spectroscopy measurements. Since the two measurements of the glutamine concentration at short and moderate echo time did not result in significant differences, this study concluded that there is a low probability of an effect of a short T2 relaxation time component on the glutamine concentration measurement. Since a reliable quantification of 1H spectra partly relies on the accurate assessment of the overlapping macromolecule contribution to the metabolites, an optimized method for the post-processing of the measured macromolecule signal was developed to ensure an accurate assessment of its contribution. This method was applied to investigate potential regional differences in the mouse brain macromolecule signals that may affect metabolite quantification when not taken into account, as well as for the assessment of macromolecule alterations in a human-glioma mouse model. No regional macro- molecule variation was found to significantly affect the metabolite quantification in the healthy mouse brain, which supports the common use of a general macromolecule spec- trum for healthy rodent brain 1H spectra quantification. However, several alterations of the macromolecule spectrum, some of which were reported for the first time, were observed in glioma tissues, and their accurate assessment was shown to be necessary for reliable metabolite quantification. Localized 1H MR spectroscopy techniques that are designed to acquire the1H MR signal from a 3D volume can also be combined with MR imaging (MRI) to acquire information on the spatial distribution of the individual metabolites, which is often of interest when heterogeneous tissues are investigated (such as in pathological conditions). This MR spectroscopic imaging (MRSI) technique is however limited in its conventional implementation by the long measurement time that is necessary to acquire the spatial information. A fast MRSI technique, spectroscopic RARE (spRARE), was therefore implemented at high magnetic field strength in this thesis in order to benefit from the advantage of high magnetic fields in addition to the rapid spatial encoding scheme offered by spRARE. Several optimizations of the metabolite signal detection were implemented in spRARE, before the efficiency of the technique was validated at 9.4 T through a comparison of the acquired in vitro metabolite signals with numerical simulations using the density matrix formalism. The in vivo quantification of the water content is also of interest as it offers perspec- tives for investigating cerebral disease progression that is associated with brain water content alterations. In addition, when the water signal is used as an internal reference for metabolite concentration quantification in MRS, such knowledge can also contribute to a more reliable quantification. A method that is based on the multi-spin-echo MRI technique was therefore developed for the absolute quantification of the water content in the rodent brain, with the aim of assessing alterations of its content in pathological conditions. It was then applied on a bile-duct-ligation rat model of chronic hepatic encephalopathy (HE) to assess the brain water content changes associated with the disease as well as its metabolic alterations. The absence of significant brain water content changes detected in the investigated HE rat model, which was supported by post-mortem brain water content measurements using gravimetry and the dry/wet weight-ratio technique, potentially opens the way for investigating cerebral metabolic processes in chronic HE in absence of edema formation. Localized 1H MRS and conventional MRSI were also applied for the investigation of rodent models of brain disorders in order to assess the metabolic profile associated with those disorders. These studies for the first time allowed the assessment of GABA as a potential early Parkinson’s disease marker, while they also provided new insights into the role of the PPAR-β brain receptor in the early neuroprotective effects following ischemia.

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