Nafion has been the mostly used perfluorosulfonated proton exchange membrane (PEM) in fuel cell. Although a number of problems remain to be resolved on the application of Nafion as a PEM, a less expensive alternative PEM has not been found mainly because of its high proton conductivity. Therefore, much effort has been invested to modify it or find a better and inexpensive material. The exploration of the methods to counter degradation and aging of Nafion is also an important direction of research. In this work, the behavior of PEG in Nafion is investigated with solid state NMR spectroscopy. A series of samples with different PEG sizes and concentrations in Nafion was prepared and the variable temperature proton spectra and longitudinal relaxation times (T1) were measured on two spectrometers. Some interesting findings were made, e.g., the 1H chemical shift of water in concentrated PEG solution decrease while its T1 increase, the higher the concentration of PEG, the larger the increase (of water 1H chemical shift) or decrease (of water 1H T1). These findings provide valuable information on further improving the performance of Nafion in proton conductivity and durability.

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The area of ultrafast (sub-nanosecond) magnetization dynamics of ferromagnetic elements and thin films, usually driven by a strong femtosecond laser pulse, has experienced intense research interest. In this dissertation, laser-induced demagnetization is theoretically studied by taking into account interactions among electrons, spins, and lattice. We propose a microscopic approach under the three temperature framework and derive the equations that govern the demagnetization at arbitrary temperatures.To address the question of magnetization reversal at high temperatures, the conventional Landau-Lifshitz equation is obviously unsatisfactory, since it fails to describe the longitudinal relaxation. So by using the equation of motion for the quantum density matrix within the instantaneous local relaxation time approximation, we propose an effective equation that is capable of addressing magnetization dynamics for a wide range of temperatures. The longitudinal and transverse relaxations are analyzed, magnetization reversal processes near Curie temperatures is also studied. Furthermore, we compared our derived Self-consistent Bloch equation and Landau-Lifshitz-Bloch equation in detail. Finally, the demagnetzation dynamics for ferromagnetic and ferrimagnetic alloys is studied by solving the Self-consistent Bloch equation.

Fetal growth restriction (FGR) is a common pregnancy complication resulting in increased neonatal mortality and morbidity. The aetiology of fetal growth restriction is not fully understood, but abnormalities in placental development are, leading to abnormalities in placental structure which are thought to affect supply of oxygen to the fetus. The source of fetal hypoxia is unknown due to the difficulty in obtaining oxygenation data in the context of pregnancy using existing techniques. There is also an absence of data relating to oxygenation in FGR pregnancies. Oxygen-Enhanced MRI (OE-MRI) and Blood Oxygen-Level Dependent (BOLD) MRI permit noninvasive acquisition of data related to changes in the concentration of dissolved oxygen (pO2) and changes in hemoglobin saturation (sO2) under air- and oxygen- breathing (hyperoxic challenge).The aim of this project was to determine whether MRI methods can provide information relating to placental oxygenation in normal and FGR-compromised pregnancy, to investigate fetal brain oxygenation and to assess the potential confound of placental perfusion changes under hyperoxic challenge. After optimization of sequences in non-pregnant volunteers, similar pO2 and sO2 increases under hyperoxic challenge were seen in normal and FGR pregnancy. This suggested placental oxygenation was similar and that fetal extraction of oxygen may be a likelier cause of fetal hypoxia. Normal fetal brain oxygenation was found not to increase under hyperoxic challenge, which may be due to hemodynamic adaptation to limit cerebral hyperoxygenation. Finally, the robustness of these oxygenation results was supported by the lack of placental perfusion changes observed under hyperoxia using Arterial Spin Labeling (ASL).In conclusion, MRI methods successfully provided information on placental and fetal oxygenation in normal and abnormal pregnancy, obtaining novel data informing the aetiology of FGR and the physiology of the fetal brain.

Myeloarchitecture is a prominent feature that can identify the primary motor and sensory areas in the cerebral cortex and is increasingly imaged in magnetic resonance imaging (MRI) studies of cortical parcellation in humans. However, MRI studies of cortical myeloarchitecture are technically difficult for two reasons: the cortex is only a few millimeters thick, and intracortical contrast due to myelin is much smaller than the overall anatomical contrast between cortical tissue and underlying white matter that is typically utilized in imaging. The research in this thesis thus presents specific MRI protocols to visualize intracortical myelin, image processing protocols to delineate the heavily myelinated cortex from the adjacent typical cortex and the application of these techniques in the precentral motor cortex to study morphology of the highly myelinated dorso-medial part, consisting of Brodmann area (BA) 4 and part of BA 6. Optimization of the MRI protocols involved determining the sequence parameters for a T1-weighted MRI sequence to obtain maximal intracortical contrast at 0.7 mm isotropic resolution in imaging time of 15 min, based on T1 differences between cortex that is myelinated (GMm) or unmyelinated (GM). As part of the optimization, T1 values were measured in the following brain tissues: GM, GMm and white matter (WM). The optimization was carried out by simulating the MRI signal for a 3D, magnetization prepared, gradient echo sequence, using the measured T1 values in the analytical signal equations. It was found that lengthening the time delay at the end of each inner phase encoding loop increased the intracortical contrast. The optimization of MRI protocols also included implementing techniques to reduce radio frequency field (B1) inhomogeneities. It was found that dividing the optimized, T1-weighted MRI with a predominantly proton density weighted image resulted in a ratio image with significantly reduced B1 inhomogeneities. The goal of the image processing protocols developed in this thesis was to visualize the variation of intracortical myelin across the precentral motor cortex and to delineate its well-myelinated dorso-medial part. The myeloarchitectonic feature that was selected to visualize the variation in intracortical myelination was the thickness of GMm in the deeper parts of the cortex relative to the cortical thickness, referred to as the proportional myelinated thickness (p). To measure p, the following processing steps were performed. The ratio image was segmented into four tissues: GM, GMm, WM and cerebrospinal fluid (CSF) using fuzzy C-means clustering technique. Using a level set approach, thickness of the cortex was determined as the distance between the outer boundaries of GM and WM and thickness of GMm or myelinated thickness (m) was determined as the distance between the outer boundaries of GMm and WM. The proportional myelinated thickness p, was calculated as follows: p= m/t. The well-myelinated dorso-medial part of the precentral cortex, referred to as Mm, was distinguishable from the adjacent cortex when the proportional myelinated thickness was projected on the outer cortical surface. The optimized MRI and image processing techniques developed in this thesis were used to investigate cortical plasticity in amputees. Two morphological features of the myeloarchitecture over Mm, the mean proportional myelinated thickness and area, were measured in four lower limb amputees and four matched controls. A comparison of these morphological features showed no statistically significant difference (p < 0.05) between the two groups. / Thesis / Doctor of Philosophy (PhD)

Myelin facilitates efficient neural signal transmission along axons. While predominantly present in white matter, myelin is also found in smaller amounts in the outermost layer of gray matter, known as the cortex, as well as deep subcortical structures. Quantitative magnetic resonance imaging (qMRI) metrics, including longitudinal relaxation rate (R1), can report myelin levels in vivo, although these measures are often estimated and can vary depending on pulse sequences and acquisition parameters used to collect the data. Multi-site approaches can increase the statistical power and ecological validity of studies by recruiting participants from larger and more diverse samples, but considerable variability has previously been reported in qMRI estimates of myelin computed using data from different MRI scanners. Here, we assessed intra- and inter-vendor variability in R1 maps acquired at three sites using scanners from two manufacturers, General Electric (GE) and Siemens. Two ‘traveling heads’ scans, where a participant visits and completes a scan at each site, were acquired to directly assess site effects independent of biological variability. Histogram-based site-specific scaling factors determined from the ‘traveling heads’ scans were then applied to harmonize the main dataset. Before site correction, comparable inter- and intra-vendor differences were observed in both the main dataset and the ‘traveling heads’ scans. Our method of harmonization reduced the inter-site variability considerably, while remaining sensitive to age effects in R1. We then characterized age trajectories of R1 across the whole brain using linear regressions. In the age range of 16-36, age was significantly associated with mean R1s of bilateral putamen, left pallidum, and left amygdala, in line with past research. Cortical analyses also replicated earlier findings, with higher age associations in regions of the motor and lateral frontal cortices. The findings contribute to the growing research in multi-site harmonization, while providing normative estimates and trajectories of whole-brain R1s that can be referred to in future clinical studies. / Thesis / Master of Science (MSc) / Myelin is crucial for efficient signal transmission in nerve cells, and advancements in magnetic resonance imaging (MRI) have allowed us to indirectly estimate myelin levels in the brain. Collaborative multi-site investigations have become increasingly popular due to their ability to recruit larger and more diverse samples, making their findings more generalizable to the public. However, differences in scanner hardware and configurations across sites can introduce site-dependent biases to MRI scans. In this thesis, we found considerable inter-site variability in our whole-brain MRI-based estimate of myelin based on longitudinal relaxation rate (R1). We thus developed a method that can remove unwanted site-dependent differences from images by comparing repeated scans across sites. Using the corrected data, we showed how myelin amounts in different brain structures changes with age during early adulthood. Overall, we highlight the importance of carefully considering site-specific differences in multi-site MRI data and characterized how myelin changes with age in young adults.

Deux méthodes d'imagerie par résonance magnétique sont proposées pour analyser in vivo le tissu cérébral de la matière blanche. La première méthode permet l'acquisition ultra-rapide de cartes des métabolites cérébraux par une lecture de l'espace réciproque répétée à des intervalles de quelques millisecondes à l'aide d'une nouvelle trajectoire excentrée, combinée à un gradient de retour. Une procédure de correction de phase, pour prévenir la formation d'artéfacts de repliement dans l'image et le spectre, est introduite sur la base de paramètres déterminés à partir du signal des protons de l'eau. Une acquisition des cartes métaboliques tridimensionnelles de la créatine, de la choline, du N-acétylaspartate, du glutamate et du myo-inositol ont été déterminées de manière fiable dans la substance blanche humaine à 3 Tesla avec une matrice de taille 32 × 32 × 16 et une résolution isotropique de 7 mm. La deuxième méthode permet l'acquisition d'un train de 32 images échantillonnées géométriquement le long d'une courbe de recroissance, en employant une série d'échos de gradient excités par un angle de bascule de 5° pour éviter des effets de saturation. Après transformée inverse de Laplace utilisant une régularisation spatiale, on obtient une distribution continue des temps de relaxation spin-réseau, T1. Dans la région de T1 entre 100 ms et 230 ms, on distingue un pic attribué à l'eau hydratant les membranes de la myéline. La fraction apparente de cette composante de l'eau de myéline augmente en fonction de l'intensité du champ magnétique, de 8,3 % à 3 Tesla, à 11,3 % à 4 Tesla, pour atteindre 15,0 % à 7 Tesla / Two magnetic resonance imaging methods are proposed for the in vivo investigation of human brain white matter tissue. The first method allows the ultra-fast acquisition of maps of brain metabolites by repeating the sampling of k-space at intervals of a few milliseconds, with a center-out trajectory combined with flyback gradients. A phase-correction procedure is introduced to prevent the formation of aliasing artifacts in the image and in the spectrum, on the basis of parameters determined from the signal of the ubiquitous water protons. An acquisition of threedimensional metabolite maps of creatine, choline, N-acetylaspartate, glutamate, and myo-inositol were determined reliably in human brain white matter at 3 Tesla with a 32 × 32 × 16 matrix and a 7-mm isotropic resolution. The second method enables the acquisition of a train of 32 images geometrically sampled along an inversion-recovery curve, using a series of gradient echoes excited by a low 5° flip angle to avoid saturation effects. After inverse Laplace transform, using a spatial regularization, a continuous distribution of the spin-lattice relaxation times, T1, is obtained. In the region of T1 between 100 ms and 230 ms, a small component is attributed to water hydrating myelin membranes. The apparent fraction of this myelin water component increases with the strength of the magnetic field, from 8.3% at 3 Tesla, to 11.3% at 4 Tesla, and 15.0% at 7 Tesla