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Abstract

Magnetic resonance imaging (MRI) is moving towards higher and higher
eld strengths. After 1.5T MRI scanners b ecame commonplace, 3T scanners
were intro duced and once 3T scanners b ecame commonplace, ultra high eld
(UHF) scanners were intro duced. UHF scanners typically refer to scanners with
a eld strength of 7T or higher. The numb er of sites that utilise UHF scanners
is slowly growing and the rst 7T MRI scanners were recently CE certied for
clinical use.
Although UHF scanners have the b enet of higher signal-to-noise ratio (SNR),
they come with their own challenges. One of the many challenges is the prob-
lem of inhomogeneity of the main static magnetic eld (B0 eld). This thesis
addresses multiple asp ects asso ciated with the problem of B0 inhomogeneity.
The pro cess of homogenising the eld is called shimming. The fo cus of this
thesis is on active shimming where extra shim coils drive DC currents to gen-
erate extra magnetic elds sup erimp osed on the main magnetic eld to correct
for inhomogeneities. In particular, we lo oked at the following issues: algorithms
for calculating optimal shim currents; global static shimming using very high
order/degree spherical harmonic-based (VHOS) coils; dynamic slice-wise shim-
ming using VHOS coils compared to a lo calised multi-coil array shim system; B0
eld monitoring using an NMR eld camera; characterisation of the shim system
using a eld camera; and designing a controller based on the shim system mo del
for real-time feedback.
We hop e that, after reading this thesis, the reader will b ecome well-informed
in the practical implementation and limitations of B0 shimming at 9.4T in the
human brain.