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978-3-8439-1806-0, Reihe Physik
Static Field Inhomogeneities in Magnetic Resonance Encephalography: Effects and Mitigation
133 Seiten, Dissertation Albert-Ludwigs-Universität Freiburg im Breisgau (2014), Hardcover, B5
Magnetic resonance encephalography (MREG) allows the observation of functional signal with a repetition time at the order of 100 ms for whole brain coverage. This 3D method necessitates a long readout time, which entails off-resonance artifacts. This dissertation addresses the analysis and mitigation of those off-resonance artifacts. By employing the concept of local k-space it is shown that the linear components of spatial variations of the Larmor frequency are the primary cause of signal attenuation. To meet this issue, a single shot spherical stack of spirals trajectory is proposed, which mitigates signal attenuation above the sinuses, where the off-resonance artifacts are most severe.
The second part of this thesis explores the possibilities of spin echo MREG. A theoretical framework is derived for appraising the maximum echo time. For the regime of weak dephasing, it is shown that the echo time can exceed the values predicted by Hahn’s established theory of spin echo formation. The presented theory can be considered as the counterpart to Hahn’s theory, which assumes strong dephasing. It is demonstrated that the experimental conditions of MREG result in the regime of strong dephasing, such that the proposed pulses are not applicable to MREG. However, the transition between those regimes is analyzed with optimal control pulses and the feasibility of the proposed pulses is shown in spin echo FLASH imaging in the lung and the head.