Structural Magnetic Resonance Imaging : Motion Correction and Improvement Strategies

Notwithstanding its great success in diagnostic routine and research, structural magnetic resonance imaging (MRI) still faces challenges. In this context, the present thesis studies new approaches to mitigate the motion susceptibility of MRI. Furthermore, it shows new possibilities to prevent inhomogeneities in T1-weighted imaging. Lastly, the performance of different MRI sequences with regard to lesion conspicuity is evaluated in a cohort of early-stage multiple sclerosis (MS) patients. Free-induction-decay (FID) navigators are first established as a novel approach to detect motion using multi-channel MR receive coils. It is demonstrated that a very short sample of the FID suffices to obtain information about the occurrence of head motion. A major advantage of FID navigators – arising from their short duration – is their negligible interference with the actual imaging procedures. This allows their incorporation in various acquisition schemes. In a first proof-of-concept demonstration, the FID navigators are employed in an anatomical imaging sequence. Subsequently, a comprehensive approach for retro- and prospective motion correction in MR diffusion imaging employing FID navigators is presented. Within the scope of T1-weighted imaging, the MP2RAGE sequence is introduced. This novel sequence enables the acquisition of homogeneous T1-weighted images at high and ultra-high fields by cancelling out the bias field which arises from inhomogeneities in the transmission and reception RF fields. In addition, it is shown that the very pure T1-weighted contrast obtained allows the derivation of quantitative T1-maps. Finally, the conspicuity of MS lesions in different routine and non-standard MR image contrasts is investigated in a cohort of ten early-stage MS patients. Thereby, the MP2RAGE sequence is directly applied in a clinical context. Together with a standard T2 measurement, its T1-mapping feature is used to provide a comprehensive picture of MR lesion characteristics in this patient group. An important finding of the study is that routine MS protocols may be considerably improved by employing new acquisition techniques. The insights gained in this study may hence contribute to optimise future clinical MS imaging protocols. To conclude, this thesis introduces different novel techniques to improve structural MRI. Besides the investigation of the theoretical aspects of the presented methods, their application is also evaluated in realistic settings.