published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Magnetic resonance (MR) is a well-known non-invasive technique that provides spectra (by MR spectroscopy, MRS) and images (by magnetic resonance imaging, MRI) of the examined tissue with detailed metabolic, structural, and functional information. This doctoral work is focused on advanced methodologies and applications of MRS for probing cellular and molecular changes in vivo. A single-voxel diffusion-weighted (DW) MRS method was first developed for monitoring the size changes of intramyocellular lipid droplets in vivo. This DWMRS technique was then utilized for exploring the vascular origins of the functional blood-oxygen-level-dependent (BOLD) signal. Magnetic resonance spectroscopic imaging (MRSI) enables simultaneous MRS acquisition in multiple voxels. However, MRSI is conventionally time-consuming. Therefore, a compressed sensing (CS) method was proposed in this thesis to accelerate the acquisition speed of the in vivo MRSI. It holds the potential for promoting the realization of multiple-voxel DW-MRS experiments, though the latter is still constrained by hardware in the present. The single-voxel DW-MRS method for probing lipid diffusion was first developed and evaluated in oil and muscle phantoms. The experimental sequence was demonstrated to be sensitive to diffusion restriction and free of significant artifacts. Experiments were then performed in rat hindlimb muscles in vivo. The restricted lipid diffusion behavior was characterized by apparent diffusion coefficient (ADC) changes and utilized for quantifying the sizes of intramyocellular lipid (IMCL) droplets in normal, fasting, diabetic and obese rats. The sizes of IMCL droplets reflect their vital roles in muscle energy metabolism. The IMCL droplet size estimated by ADC here was closely correlated with that measured by transmission electron microscopy. IMCL ADC was sensitive to metabolic alterations, decreasing in the fasting and diabetic groups while increasing in the obese group. These results clearly demonstrate DW MRS as a new means to examine the dynamics of IMCL metabolism in vivo. The DW-MRS technique was then utilized to characterize water ADC during neuronal activation to explore the vascular origins of the BOLD signal in rat brains. MRS experiments with acoustic stimulation were performed with a dynamic point-resolved spectroscopy (PRESS) acquisition on conditions with or without the diffusion gradient for blood suppression in the same voxel and same experimental session, which enabled the simultaneous T2/T2*/diffusion measurements. The T2*% changes with and without diffusion gradient showed no significant difference, while the spin echo (SE)-BOLD% (T2%) change significantly decreased after applying the diffusion gradient, suggesting an intravascular component in the SE-BOLD signal. This intravascular component was not venous blood, as the T2* of this component was comparable with the T2* of the brain tissue. These results provide new insights into the vascular origins of BOLD signals. A CS approach was developed to accelerate in vivo magnetic resonance spectroscopic imaging (MRSI) which enables multi-voxel MRS measurements. The CS undersampling was performed by acquiring a pseudo-random and density-varying subset of phase encodings. The proposed CS approach preserved the spectral and spatial resolution, while substantially reduced the number of phase encodings with accelerations up to seven fold for phantom and up to six fold for in vivo rat brains. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Diffusion weighted magnetic resonance images offer unique insights into the neural networks of in vivo human brain. In this study, we investigate estimation and statistical analysis of multi-compartment models in high angular resolution diffusion imaging (HARDI) involving the Rician noise model. In particular, we address four important issues in multi-compartment diffusion model estimation, namely, the modelling of Rician noise in diffusion weighted (DW) images, the automatic determination of the number of compartments in the diffusion signal, the application of spatial prior on multi-compartment models, and the evaluation of parameter indeterminacy in diffusion models. We propose an expectation maximization (EM) algorithm to estimate the parameters of a multi-compartment model by maximizing the Rician likelihood of the diffusion signal. We introduce a novel scheme for automatically selecting the number of compartments, via a sparsity-inducing prior on the compartment weights. A non-local weighted maximum likelihood estimator is proposed to improve estimation accuracy utilizing repetitive patterns in the image. Experimental results show that the proposed algorithm improves estimation accuracy in low signal-to-noise-ratio scenarios, and it provides better model selection than several alternative strategies. In addition, we derive the Cram´er-Rao Lower Bound (CRLB) of the maximum Rician likelihood estimator for the balland-stick model and general differentiable diffusion models. The CRLB provides a general theoretical tool for comparing diffusion models and examining parameter indeterminacy in the maximum likelihood estimation problem. / published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

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Conventional MRI relies on a strong fixed magnetic field B0 which is stable during the imaging process. By contrast, field-cycling MRI switches the strength of B0 up or down during an experiment. In this way, field-cycling provides access to endogenous information not accessible to standard MRI, such as enhanced T1 relaxation at certain NMR frequencies due to interactions between hydrogen and nitrogen nuclei in proteins. However, biomedical research of T1 dispersion is limited by the unavailability of equipment and rapid software methods. Strategies are presented to address these deficiencies. A removable electromagnet was designed and implemented for use with a 59-mT vertical-field, permanent-magnet based imager. The resistive magnet locally offsets the primary field over a small projected region to enable field-cycling relaxometry on an otherwise-conventional imager. Radiofrequency coils were constructed to suit the electromagnet’s configuration. T1 dispersion measurements were demonstrated for, separately, the finger joints and forearm of a human volunteer. Prior to this work, producing graphs of T1 dispersion from a volume of interest required lengthy T1 mapping at each field strength step. A new pulse sequence combining SR/IR T1 determination with field-cycling and point-resolved spectroscopy localization enables the measurement of dispersion curves of a volume selected from a pilot image. Its advantages include less partial voluming than whole-sample relaxometry, as well as better SNR and faster acquisition times than image-based techniques. The sequence’s sensitivity is sufficient to reveal distinctive ‘quadrupole dips’ in dispersion curves. To the author’s knowledge, it is the first pulse sequence to enable the relationship between T1 and field strength to be examined in times which are feasible for clinical investigations. Used together as presented in this thesis, the hardware and software developed represent a step towards field-cycling being used to reveal useful diagnostic information inaccessible to conventional MRI.

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published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

A number of effects of low to medium strength (< 2000 gauss photospheric) magnetic fields on otherwise normal stars are proposed and examined. We consider magnetic perturbations to the standard stellar structure and evolutionary calculations in the core, the deep envelope, and the extreme outer envelope in intermediate to high mass stars. In the stellar core the gas pressure probably far exceeds the (B²/8π) magnetic field pressure term so that the only effect of such a field may come from its inhibiting convection in the core. We present isochrones of both convective and radiative core models of 2-5 M(⊙). In the deep envelope, we may expect to see mixing of partially nuclear processed material driven by rising and falling magnetic flux tubes. The effects of this mixing will be brought to the surface during the deep convection phase of the star's tenure as a red giant. We use this model to predict a signature for magnetic mixing based on the CNO isotope and abundance ratios. In the outer envelope the gas pressure is low enough that we might expect to see a perturbation of the stellar structure due to the magnetic field pressure itself. We calculate this perturbation under several physical models for intermediate and high mass stars and determine that sufficient magnetic field energy may be available in the outer envelope to expand a star by about 20% over its unperturbed radius. Finally we consider the evidence for the existence of non-magnetic neutron stars, concluding that while no non-magnetic neutron stars have ever been positively identified, we have no evidence that prevents the existence of at least as many non-magnetic as magnetic neutron stars.