Biomedical Engineering - Theses
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ItemB1 insensitive techniques for ultra-high field magnetic resonance imaging( 2021)Magnetic resonance imaging (MRI) is a powerful technology that is widely used in medical imaging and science. MRI is a flexible imaging modality that can be used to acquire a range of structural and functional information non invasively and without the use of ionising radiation. Ultra-high field (UHF) MRI allows for improved outputs from a range of imaging sequences due to increased signal-to-noise ratio and increases in contrast. However, due to wavelength effects, UHF imaging suffers from inhomogeneity of the RF transmit field, B1+. Nonuniformity of the B1+ field results in spatial variation in image intensity and contrast which compromises image utility. Existing approaches for overcoming the effects of B1+ inhomogeneity include specialised pulse design, use of multiple element transmit arrays and correction to images in post-processing. As each approach has limitations, none is used universally. This thesis seeks to improve the capabilities of B1+ insensitive imaging at UHF with three major contributions. In this thesis a new class of pulses is described, termed Spin Lock Adiabatic Correction pulses, aimed at decreasing the required energy deposition to achieve a given level of B1+ insensitivity in excitation or inversion. Additionally, theory is developed to describe an approximation for adiabatic pulse behaviour. A Fourier relationship is demonstrated for a range of well known adiabatic pulses and the strengths and limitations of the approximation are explored with simulation. Finally, this thesis presents a novel imaging pipeline for production of B1+ artifact-free images. The method combines super resolution imaging with measurement and correction of B1+ field inhomogeneity in order to achieve B1+ insensitive imaging for 2D slice stacks with no time penalty over uncorrected slice stacks.