Tecniche avanzate di neuroimaging in risonanza magnetica nello studio delle alterazioni cerebrali in processi patologici e fisiologici

This thesis is focused on the development and application of advanced neuroimaging techniques in the study of experimental models of brain pathologies. Specifically, experimental models of brain aging and multiple sclerosis have been investigated. During the last ten years, many neuroimaging clinical investigations of brain aging based on advanced magnetic resonance techniques have been reported. Clinical studies are potentially affected by confounding factors such as lifestyle (e.g. physical exercise, caloric intake, stress) and genetic variability. Experimental models of aging may therefore be useful for the separation of physiological events from pathological ones, as well as for evaluation of the impact of environmental and genetic factors. Such models may also represent a valuable platform to study the efficacy of pharmacological or physical therapies for the delay of brain aging. In the first part of the work, I focused on the development and optimization of protocol for functional imaging (fMRI) applied in an experimental model of EAE (Experimental Autoimmune Encephalomyelitis) for chronicprogressive form of multiple sclerosis. By using fMRI techniques, it was possible to study brain plasticity, i.e. the ability of the brain to reorganize itself after damage caused by neurodegenerative disease. Brain plasticity enables the performance of a specific task even if the area, or areas, normally delegated to control this function are damaged. Functional imaging, that allows to monitor non-invasively the brain areas activated by a specific stimulus, has not yet used in EAE although several clinical studies have shown that fMRI can provide valuable information on the extent of spontaneous or therapy-induced cortical cerebral reorganization. Data obtained from this study have been published in The Journal of Neuroscience, July 8,2015 - 35(27):10088–10100. Subsequently, during the second and third year of my PhD, I have focused on the study of physiological brain aging. Similarly, to pathological conditions, physiological aging also causes alterations which can affect the architecture and the microstructural composition of the brain. The use of MRI innovative techniques could be important for characterize these alterations and their effects 7 on brain reorganization. The second year of my PhD was dedicated to the use of DTI (diffusion tensor imaging) and volumetric techniques. Due to its high sensitivity DTI allows to evaluate the microstructural organization and orientation of the white matter tracts, providing detailed information on the integrity of biological tissues. In this work, we carried out DTI acquisitions of the whole brain and evaluated total as well as white and gray matter volumes. Specifically, we evaluated FA (Fractional Anisotropy) and ADC (apparent diffusion coefficient) from DTI acquisitions. FA and ADC are considered reliable marker of brain tissue integrity. We found increased values of ADC and decreased values of FA with increasing age, in agreement with that observed in humans: this result confirms previous evidence of neural connections deterioration with aging. Moreover, our results reflected generalized alteration in the microstructure of the brain, rather than localized alterations. Considering the morphological study, we observed increased volume of CSF (CerebroSpinal Fluid) (young vs adult, young vs old) that reflects a well-documented atrophy of the brain observed in humans. Results obtained by DTI and volumetry in animal models are in agreement with literature data reported in humans, thus confirming the potential role of experimental models for the study of neurodegenerative processes. Finally, the aim of the last year was to confirm the results obtained in vivo by using histological techniques and to complete the study through the acquisition of localized proton spectroscopy (MRS) data. In the clinical literature, several studies investigated the alterations of cerebral metabolites in aging and generally reported a decreasing trend of the NAA metabolite (N-acetyl aspartate, a well-known neuronal marker) content with age. However, there is a certain degree of controversy in the literature about the changes in the cerebral concentration of this metabolite, potentially due to the difficulty in comparing studies between different laboratories also caused by different positioning of pixels. In order to have a comparison with clinical data, we acquired localized proton spectra in the experimental model of aging. Proton spectra were acquired in the hippocampus and in the parietal and frontal cortex and analyzed with the LCModel software to quantify the absolute concentration of 8 metabolites, taking as reference the water signal. Data from different age groups (young vs. older) were compared. A statistically significant increase of the absolute concentration (i.e. relatively water) of NAA with increasing age in the prefrontal cortex was detected. This result is surprising considering the opposite trend observed in humans. Localized spectroscopy data, then, being in disagreement with the clinical results, deserve further investigations.