Biomedical Engineering - Theses
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ItemOptimised techniques for ultra-high field functional magnetic resonance imaging( 2022)Functional Magnetic Resonance Imaging (fMRI) has become broadly used to study brain function since its invention in the late 20th century, applicable to both the study of healthy controls and patient groups. Different fMRI methodologies such as Blood Oxygenation Level Dependent (BOLD) fMRI and Arterial Spin Labelling (ASL) fMRI have been developed to non-invasively measure multiple physiological information related to neuronal activity in the brain. In the last two decades, fMRI at ultra-high field (UHF) (i.e. >= 7T) has gained an increasing amount of interest, driven by the increased signal-to-noise ratio brought by the increased field strength. However, ultra-high field fMRI suffers from challenges such as signal degradation caused by increased spatial inhomogeneity of the static magnetic field (i.e. B0) and the radio-frequency (RF) field (i.e. B1+), as well as limitations caused by increased power deposition. This thesis covers three optimised techniques for fMRI at ultra-high field by focusing on BOLD fMRI and ASL, aiming to overcome aforementioned challenges. Starting with signal formation, the Hybrid Adiabatic Pulse with asYmmetry (HAPY), a new class of optimised RF pulse for Pulsed ASL (PASL), is introduced to overcome B0 and B1+ inhomogeneity and increased power deposition. The presented technique offers robust cerebral blood flow measurement with reduced energy deposition under the effect of B0 and B1+ inhomogeneity. The second technique presented in this thesis is an optimised BOLD fMRI data acquisition protocol matched to analysis settings, termed Smoothing-Matched EPI. High spatial resolution fMRI at ultra-high field faces challenges caused by the long acquisition window with increased sensitivity to B0 inhomogeneity. This technique provides enhanced BOLD sensitivity in high spatial resolution BOLD fMRI by tailoring the k-space coverage to the spatial smoothing settings, permitting optimisation of the acquisition parameters. Lastly, an optimised pulse sequence design for fMRI data acquisition, namely Field Mapping Embedded EPI (FME-EPI), that improves B0 inhomogeneity-induced distortion is presented. This modified EPI pulse sequence is able to acquire both functional data and B0 inhomogeneity information concurrently, which can be used in EPI reconstruction to improve the spatial fidelity of the reconstructed images.