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@PHDTHESIS{Schwerter:781281,
author = {Schwerter, Michael},
othercontributors = {Shah, Nadim Joni and Stahl, Achim},
title = {{A}dvanced software and hardware control methods for
improved static and dynamic ${B}_{0}$ shimming in magnetic
resonance imaging},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2020-01201},
pages = {1 Online-Ressource (xii, 135 Seiten) : Illustrationen,
Diagramme},
year = {2019},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2020; Dissertation, RWTH Aachen University, 2019},
abstract = {Magnetic resonance imaging (MRI) is a non-invasive
tomographic imaging technique and a powerful tool applied in
many fields of medicine and research. Methodological
developments have vastly broadened the spectrum of possible
applications and turned the classical MR scanner into an
inherently multi-modal imaging device. Simultaneous hardware
advancements have been striving for obtaining better data
quality at reduced acquisition times. Here, a central role
is taken by the improvement of the MRI magnets, which are
required to generate a strong, homogeneous and temporally
stable static magnetic field. Any perfectly homogeneous
magnetic field, however, is inevitably distorted by the
unique magnetic susceptibility distribution of an
examination subject. Thus, subject-specific magnetic field
shimming technology, which homogenizes the magnetic field,
is indispensable and still forms an integral part of current
MR research. However, this is non-trivial, because human MR
acquisitions are faced with complex distortion field
patterns and, thus, many of today’s applications still
suffer from strong uncompensated inhomogeneities.
Consequently, it was the purpose of this work to identify
and overcome existing challenges in subject-specific static
magnetic field shimming with a focus on human brain
acquisitions. Conventional shim approaches are typically
based on 2nd order spherical harmonic shim coils, whose
driving currents are adjusted so as to correct for
previously measured field inhomogeneities. To gain full
control over this process, a comprehensive field mapping and
shim current optimization framework was implemented.
Accurate shim field characterizations were performed and
included into custom-written software. Conducted simulations
as well as phantom and in vivo measurements showed, that the
shimming framework reaches optimal field homogeneity over
user-defined volumes within one minute and without the need
for iterations. This way, an average whole-brain field
homogeneity of 18.4 ± 2.5 Hz was achieved at a main field
strength of 3 T. The shim quality was further improved via
the application of a shim coil insert, which can generate
very high-order spherical harmonic shim fields. Means of
communication with its shim controller were implemented and
integrated into the shimming software. Simulations,
indicating an improve in whole-brain field homogeneity to
15.1 ± 3.8 Hz, were confirmed in experiments. This residual
inhomogeneity was identified as being beyond the correction
capabilities even of the high-order shim set and indicates
the feasible limit of static spherical harmonic shimming. In
contrast, dynamic shim updates during data acquisition can
further improve the achievable B0 homogeneity. However,
rapid shim changes evoke eddy current-induced distortion
fields and, thus, require pre-emphasis corrections. For
this, an image-based measurement scheme was developed, which
is applicable to capture full 4D eddy current field
evolutions. Requiring 10 min of acquisition time, a six-fold
acceleration compared to an existing alternative was
achieved. Combined with a novel model-based pre-emphasis
reconstruction, the shim-induced eddy currents were
effectively suppressed and enabled a dynamic operation of
the shim hardware. Based on this, a key finding of this work
is that the dynamic shim currents and their temporal
variation can be strongly constrained while negligibly
compromising achievable field homogeneity. Incorporation of
these constraints into the shim optimization led to a
23-fold reduction of average maximum and an 18-fold
reduction of the average mean inter-slice current changes.
Nonetheless, a whole-brain field homogeneity of 8.76 ± 0.32
Hz was achieved, as compared to 8.10 ± 0.31 Hz for the
unconstrained case. The associated benefits are manifold,
including an intrinsic eddy current reduction, decreased
power supply demands and more accurate shim simulations. In
conclusion, novel and efficient means to increase the
magnetic field homogeneity in MRI measurements were
developed within the scope of this work. Especially the
methods to measure and reduce shim-induced eddy currents
improve dynamic shimming implementations significantly.},
cin = {535000-5 / 133510 / 130000},
ddc = {530},
cid = {$I:(DE-82)535000-5_20140620$ / $I:(DE-82)133510_20140620$ /
$I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2020-01201},
url = {https://publications.rwth-aachen.de/record/781281},
}
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