Abstract
Field imperfections are normally undesirable in magnetic resonance imaging. They degrade the quality of the images by wrong depiction of the anatomy and decrease of the signal-to-noise ratio. Furthermore, for velocity, flow and diffusion quantification, measurement errors related to these imperfections are induced. Analysis of the imperfections can be used
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to correct for the associated artifacts and errors. Next to that, it can create new contrast mechanisms, speed up the acquisition procedure or provide functional and structural information. In this thesis, field imperfections were analyzed for three purposes, viz. to quantify and correct for biased flow values at off-center locations, to provide structural information of foreign materials by susceptibility analysis and to visualize interventional devices. With regard to the first purpose, it was demonstrated that for increasing off-center positions, flow values get more biased because of increasing inhomogeneity of the main field and nonlinearity of the gradient fields for spatial and velocity encoding. By field mapping with local scaling factors, the spatial dependency of the flow biases was analyzed. The same scaling factors were used for correction. The second purpose of susceptibility analysis was twofold and included the detection and quantification of crystal structure transformations in implant alloys and the characterization of foreign objects. The magnetic susceptibility of implant alloys and the accompanying susceptibility artifacts in MR images depend on the microcrystalline structure of the alloy. It was shown that microcrystalline transformation could be detected and quantified by analysis of the susceptibility artifacts, which facilitates to reconstruct the thermo-mechanical state of the material in order to detect early failure or to measure interaction between tissue and implant. In this thesis, the alloys stainless steel AISI 304L and Nitinol were used. The characterization of foreign objects was done with susceptibility analysis of metallic foreign particles. By use of simulation, it was shown that, on basis of clinically used spin echo and gradient echo MR images, no distinction could be made between the volume and the susceptibility of the particles. This gap could be filled up by X-ray, because with this modality the size of a particle could be measured. Computed tomography failed in this respect as the size of the particle was overestimated. With regard to the third purpose, the visualization of interventional devices by exploitation of locally induced artifacts (markers), simulations were performed in order to quantify the field strength dependency of the markers on a device. The material to create a marker has to be easily magnetizable in order to invoke a satisfactory artifact and, preferably, the magnetization has to be field strength independent. Among others, the ferromagnetic stainless steel AISI 410 was chosen as marker material as it was the most effective for marker construction. In vivo
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