Exploring T*2 decay : new methods for short echo time imaging and fat-water quantification

The major advantage of magnetic resonance imaging (MRI) over
other imaging modalities like computed tomography (CT) is that it
does not utilize ionizing radiation. A drawback of MRI in
comparison to CT is that in general it requires longer scan times
and for this reason fast scanning techniques have been proposed.
Fast MR imaging can refer to fast scan times or fast signal
acquisition. The first is important in various cases such as in
abdominal scans to decrease motion sensitivity, while short echo
times and short acquisition times allow visualization of tissues
with fast signal relaxation. One category of MR sequences that
allows fast scanning is gradient echo sequences. These sequences
do not use radiofrequency pulses to yield a signal echo and this
allows fast imaging, shorter echo times and scan times, while the
signal decays according to the apparent transverse relaxation T_{2}^{*}
. Gradient echo sequences can be used both for qualitative and
quantitative imaging and during this thesis an application in
each direction was explored.
The first part of this thesis is related to fast gradient echo
imaging for qualitative imaging of fast decaying signals. It is
focused on the development of a short echo time sequence that can
be easily translated to clinical settings. In the first chapter
of this part a novel short echo time sequence is being
introduced. Subsequently, two different applications are being
discussed. Firstly, the application of the sequence to
musculoskeletal imaging at high and ultra-high field is being
described. In the second chapter, the effect of fat suppression
on short T_{2}
tissues imaging is being considered. At the last
chapter of this part the sequence is adapted to be used for
molecular imaging of iron oxide labeled cells.
The second part of this thesis refers to quantitative gradient
echo imaging. The aim is tissue characterization based on the
analysis of the signal decay. A multi-echo sequence is adapted in
order to be used with a novel powerful fitting tool for
three-dimensional (3D) liver fat-water imaging. Preliminary
results are presented from a comparison with a standard two-point
Dixon technique.