Novel imaging using a MEG scanner, and MRI homogeneity improvement techniques

Vella, Ingrid (2017) Novel imaging using a MEG scanner, and MRI homogeneity improvement techniques. PhD thesis, University of Nottingham.

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The general aims of the work in this thesis are to locate and quantify magnetic dipoles using a Magnetoencephalography (MEG) system based on Superconducting Quantum Interference Device (SQUID) sensors, and to generate various target magnetic fields using magnetic dipoles.

MEG provides direct, real-time measurements of magnetic fields at sub-millisecond temporal resolution and femtoTesla sensitivity. It is typically used to describe sources in terms of current dipoles, but here we adapt a different approach and use it to characterise magnetic dipoles. In the first part of this thesis, we describe initial experiments which were carried out in order to demonstrate the feasibility of using the high sensitivity of MEG SQUID sensors to detect extremely small magnetic field shifts due to magnetised samples, and to then locate and quantify the magnetic dipoles. We show that a standard MEG system can be used to measure magnetic field shifts due to susceptibility effects from samples exposed to an Ultra Low Field (ULF), as well as to detect and image the distribution of decaying longitudinal nuclear magnetisation from pre-polarised samples. During our experiments, we also identified a long-lived magnetisation in biological samples, whose magnetisation orientation is fixed by the sample orientation. This finding led us to carry out experiments on samples including human tissue (the hand, wrist, and foot) using MEG, and to characterise the magnetisation behaviour.

Even though ULF Magnetic Resonance Imaging (MRI) has several benefits, it is difficult for it to compete with Ultra High Field (UHF) MRI since the higher the field is, the larger does the SNR tend to be. Yet, higher fields increase the effects of intrinsic magnetic susceptibility differences, which in turn leads to field inhomogeneities. Thus, in the second part of this thesis, we aim at improving the quality of high field MR images. We show how magnetic dipoles can be used to generate different target fields that can be used to shim different inhomogeneous magnetic fields at UHF. These magnetic dipoles can be realised using either an array of orthogonal coils or pieces of strongly diamagnetic material.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Bowtell, Richard
Glover, P.M.
Keywords: MRI, MEG, magnetic dipoles, imaging, nuclear magnetisation, isothermal remanent magnetisation, shimming
Subjects: Q Science > QC Physics > QC501 Electricity and magnetism
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 39553
Depositing User: Vella, Ingrid
Date Deposited: 12 Jul 2017 04:40
Last Modified: 12 Oct 2017 14:28

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