Brain operates on a network level. Magnetic resonance imaging (MRI) provides structural and functional images noninvasively with large field of view and at high spatial resolution and thus assumes an extremely valuable role in studying brain networks. The objectives of this doctoral work were to develop and apply novel MRI methods on human and rodent brains, for in vivo and global assessments of functional brain networks at resting and task-evoked states. Firstly, the feasibility of passband balanced steady-state free precession (bSSFP) imaging for distortion-free and high-resolution resting-state fMRI (rsfMRI) was investigated. Resting-state networks (RSNs) derived from bSSFP images were shown spatially and spectrally comparable to those derived from conventional gradient-echo echo-planar imaging (GE-EPI) with considerable intra- and inter-subject reproducibility. High-resolution bSSFP corresponded well to the anatomical images, with RSNs exquisitely co-localized to gray matter. Furthermore, RSNs at areas of severe susceptibility were proved accessible including human anterior prefrontal cortex and rat piriform cortex. These findings demonstrated for the first time that passband bSSFP approach can be a promising alternative to GE-EPI for rsfMRI. It offers distortion-free and high-resolution RSNs and is potentially suited for high field studies. Secondly, to examine the macrovascular contributions to the spatial and spectral prosperities of resting-state networks, spin-echo echo-planar imaging (SE-EPI) with moderate diffusion weighting (DW) was proposed for rsfMRI. SE and DW suppressed the extravascular and intravascular contributions from macrovessels respectively. Significantly lower functional connectivity strength was observed in the posterior cingulate cortex of the default mode network derived from DW SE-EPI data comparing to that derived from SE-EPI, suggesting a confounding role played by the intravascular component from large veins, whereas no significant spectral difference was detected. Therefore, the DW SEEPI approach for rsfMRI may assist in better identifying and interpreting largescale brain networks with future improvement in temporal resolution by acceleration techniques and in sensitivity at higher field. Thirdly, rsfMRI was performed to evaluate the intrinsic functional networks in the corresponding anatomical visual brain connections traced by Mn-enhanced MRI (MEMRI). Strengths of resting-state functional connectivity appeared to couple with structural connectivity in MEMRI, demonstrating the sensitivity of these structural and functional connectivity MRI techniques for assessing the neuroarchitecture, neurophysiology and structural-functional relationships in the visual brain in vivo. Fourthly, the hypothesis that a regional activation identified via general linear model analysis of fMRI data reflects the summation of multiple distinct networks that carry different functional purposes was tested. Overlapping frontoparietal networks engaged in a simple single-digit multiplication task were found and their functional roles were evaluated through independent components analysis and contributive source analysis. Future studies incorporating different arithmetic tasks and resting state will shed more light upon how brain accomplishes arithmetic and more complex tasks in general. Lastly, benefiting from higher SNR, better spatial and temporal resolution at higher field, exploratory fMRI studies were conducted on rats at 7 T for in vivo assessments of 1) the effect of dark-rearing on postnatal visual development, 2) sound amplitude modulations and 3) sound frequency modulation sweep direction selectivity in auditory system. ( / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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Chemical remanent magnetisation (CRM) occurs as the result of the acquisition of remanence by a rock through chemical change or mineral growth. Despite the important role CRM plays in rock magnetic and palaeomagnetic processes (as a mechanism by which rocks acquire a primary remanence or as a feature of post- depositional alteration) there are few studies of this process to date. This dissertation investigates the effect of phase and chemical change on the magnetic and remanence carrying properties of some geologically important iron oxide minerals. The mineral systems studied are: the goethite-hematite, the magnetite-maghemile-hematite and the lepidocrocite-maghemite-hematite systems. The mineralogy of these systems is studied using transmision electron microscopy, thermogravimetry and x-ray diffraction methods. A transformation mechanism for the dehydration reaction of goethite is presented. The cation distribution of maghemite and its intrinsic magnetic properties are investigated. The process of CRM is simulated in the systems goethite-hematite and lepidocrocite-maghemite-hematite and the results correlated with theoretical predictions of CRM intensity, blocking volume and their dependence on mineral growth rate. A self-reversal in the maghemite-hematite system is reproduced. The low temperature oxidation of magnetite to maghemite and hematite (for example in basalts) and the low temperature dehydration reactions of goethite and lepidocrocite (as occur, for example, in sediments) are important mineral transformations. The results and conclusions of this study are extrapolated to geological environments and time scales, in particular with reference to red bed palaeomagnetism.

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