Magnetic Resonance Elastography of the Human Brain

Magnetic resonance imaging (MRI) is one of the most essential imaging modalities in neuroscience. Routine MRI of the brain is often key supportive to successful diagnosis and monitoring of neurological diseases. Nevertheless, various neurological disorder characteristics may be undistinguishable by clinically established MRI contrasts, encouraging research and development of advanced MRI techniques. Many neurological diseases cause changes in the cerebral microstructure and concomitant alterations in the mechanical tissue properties. Motivated by manual palpation, where irregularities in the mechanical stiffness of tissue are assessed by hand, magnetic resonance elastography (MRE) enables visualization and quantification of mechanical tissue properties. Especially in neuroscience, the novel image contrast of MRE offers innovative possibilities for characterizing the brain tissue and for detecting tissue alterations. Even in encapsulated and deeper lying tissues such as the brain, which is usually not accessible to manual palpation in vivo. Technically, MRE probes the propagation of induced mechanical wave motions within the tissue at several points in time. The gathered image series is used to estimate the underlying mechanical properties such as the tissue shear modulus or stiffness. In this thesis, a novel MRE acquisition sequence scheme was developed for the investigation of the human brain. This imaging scheme allows sampling the propagation of the wave at several discrete time points in a single run. The benefits of the presented approach are short acquisition times coupled with reduced imaging artifacts and the use of low frequencies for the mechanical wave motion excitation. This is especially desirable in brain MRE to investigate deeper lying cerebral areas, as low frequency waves propagate deeper into the tissue. In a second approach, the scheme was enhanced to additionally acquire the three-dimensional motion components at once. Phantom experiments were performed to validate the proposed methods. The method was also validated in the brain of healthy subjects and regional shear moduli were assessed in different brain areas at an excitation frequency of 20 Hz. Due to the reduced acquisition time, the proposed MRE sequence can be integrated into a clinical MRI protocol and is currently part of an ongoing brain MRE study in patients with multiple sclerosis.