De Vita, Enrico; (2003) Development and optimisation of high-field MRI techniques. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

High field (>1.5 Tesla) magnetic resonance (MR) scanners are increasingly used due to their potential advantages in clinical and neuroscientific applications. However full exploitation of high field MR requires overcoming technical difficulties, optimising pulse sequences and developing new contrast mechanisms. This thesis presents four projects, carried out mainly on a recently installed 4.7T whole body MR scanner and on a small bore 2.35T scanner: B1 becomes increasingly inhomogeneous at high fields resulting in image contrast non-uniformity. Compensation techniques greatly benefit from accurate B1 maps. A novel EPI-based B1 mapping method was implemented that is faster than pre-existing methods. A quick, automated Fourier-based analysis was also introduced that is robust to experimental errors. T2-weighted Fast Spin Echo (FSE) imaging is very common in clinical use. However its high field application has been hampered by its large rf power deposition. A 2D-FSE sequence was modified for use at 4.7T within safety limits. It provides high-quality high-resolution images of the human brain within reasonable examination times. Susceptibility artefacts in echo planar imaging (EPI) are exacerbated at high fields. A novel technique (TWIST) was developed that reduces blurring, geometric distortion and drop-out in EPI using conventional hardware. Pairs of aliased images are acquired in fast succession, each with half the number of phase encoding steps and a halved field of view. Image reconstruction entails processing similar to that of 2-coil SENSE. TWIST was validated in phantoms and used in vivo (including an fMRI study). The possible integration of electrical impedance tomography and MRI was explored by measuring the effect of alternating current on the longitudinal relaxation time in the rotating frame (T1p). When the AC frequency matches the B1 Larmor frequency. T1p is reduced by the locally induced magnetic fields and may therefore provide information on local impedance.

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