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

Abstract

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.