Development of spoiled gradient echo sequences for MRI relaxometry

Magnetic Resonance Imaging (MRI) is a noninvasive, non-ionizing technique that allows both anatomical and functional imaging with tunable contrast among soft tissues. Although MRI is a significant source of images with differences in signal intensity defined by differences in specific physical parameters; for the vast majority of applications, images are being interpreted qualitatively in the clinical practice. Quantitative MRI (qMRI) is an umbrella term that encompasses the quantification of the parameters specifying signal intensity. Among others, the relaxation parameters T1 and T2 remain a main research topic in qMRI as the relaxation times have been used as biomarkers for tissue characterization and differentiation, and can be of great radiological support. Several methods for fast relaxometry have been proposed, however there is yet no general consensus on fast relaxometry mapping techniques that allow good image quality within clinically acceptable time and with good reproducibility and repeatability, also due to the dependency of these parameters on confounding factors, including the RF excitation field B1. This PhD thesis starts with a short history of the concept of Nuclear Magnetic Resonance and the development of the Magnetic Resonance Imaging technique, after which the main qMRI techniques dedicated to relaxometry mapping are reported. The contributions of this thesis focus on improvements for T1 mapping. As for most T1 mapping methodologies, an accurate estimation of the flip angle is necessary, we start with the development of a preparation module for a faster steady state approach and sampling of Actual Flip angle Imaging (AFI), a B1 mapping technique. Next, we extended B1-mapping with T1-mapping using Variable flip angle with AFI (VAFI) approach in which we introduce a slice profile correction as well as a 2D multislice stacking approach for AFI, which respectively provide a more accurate and faster way to acquire parametric maps. Then, a newly developed sequence, Relaxation Alternate Mapping of Spoiled Echo Signals (RAMSES) for multiparametric mapping including T1, B1, and T2* is introduced. This was achieved by adding a bipolar multi-gradient-echo readout to AFI, with no additional scanning time required. Lastly, another newly developed sequence, Echo Planar Imaging Fast Actual Nutation Imaging (EPIFANI) for ultrafast B1-corrected T1 mapping is introduced. This sequence is an echo planar imaging (EPI) version of AFI, which can be used to provide high time efficiency to a multiparametric acquisition, and represents one of the very few attempts to acquire multiparametric maps with EPI.