The work in this thesis utilizes the method of finite element modelling,
to examine and reduce the effects of a small radio frequency coil on an
external magnetic field. When a small radio frequency coil is place d in
a constant magnetic field the susceptibility differences in the coil cause
the field to be perturbed. These perturbations have an implication on thelineshpae
and resolution that is achievable. thsu the necessity for optimization
of the coil to reduce these effects. The optimization technique that had
been performed is based on the idea o ftrying to compensate for the magnetic
suscebility differences by plating the coil with another material to try
and cancel the magnetic susceptibility to zero.
The theis introduction covers some of the theory of magnetic resonance
imaging, including topics such as net magnetization, the principles of
magnetic resonance, relaxation and the basic principles of imaging. The
effects of inhomogeneity are then discussed and the concept of shimming
is then intriduced. Several possible different techniques for th coil optimization
are outlined. An introduction to the use of the finite element method is
also given before details of the work that has been performed are explained.
Initila investigations were carried out to try and discover the exact nature
of the effects that the coil geometry had on the perturbations. The coil
was based on the biplanar geometry, with the dimensions being varied to
determine the whether there was a dependence on the coil size. The coil
material was modelled as copper (diamagnetic). They revealed that the least
perturbations were experienced for a coil with the dimensions 15 x 1 x
25 mm, corresponding ti the x, y and z axes respectively, with the external
B0 field applied in the z direction.
These dimensions were then used for the optimization modelling, which
modelled the effects of layers of various thickness of both rhodium and
aluminum (paramagnetic materials) plating. The results that were obtained
revealed a dramatic decrease in the B0 field perturbations. The implications
of the optimization, which has been carreid out, is now to be viewed on
the lineshape. It is then to be combined with the study of Eng Whee Yeo,
on the radio frequency characteristics of such coils with intent of producing
one at the centre for magnetic resonance sometime next year.
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