Magnetization Preparations in MR Imaging

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Helsinki University of Technology | Diplomityö
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Lääketieteellinen tekniikka
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The purpose of this work was to implement a double inversion black blood preparation, a T2-preparation and segmented preparations on the Philips Panorama open low-field magnetic resonance (MR) scanners. The double inversion black blood preparation is well-known magnetization preparations used for making the blood appear black in cardiac MR imaging (MRI). This preparation has historically been time-inefficient, but the recently developed multi slice method removes this inefficiency. The T2-preparation is also a magnetization preparation, but it has the effect of suppressing liquids and tissues with short T2-constants and is therefore mainly used for suppressing muscle and bringing forth blood. The T2-preparation is used primarily in cardiac MRI. The segmented use of preparations allows any preparation to also be performed differently than prior to every imaging cycle. Only some of the many possible segmented preparation applications are covered in this work. The implementation of both the double inversion black blood and T2-preparation magnetization preparations were tested in order to confirm the correct general effect of these preparations on the acquired images. The double inversion black blood preparation was tested by imaging a flow phantom and some stationary phantoms. These images confirmed the correct suppression of the flowing liquid, while rendering the stationary tissues unaffected. Both the single and multi-slice preparations produced a correct contrast in all images, although the multi slice preparation was more time-efficient as it produced several images of perfect black blood contrast in the same time as the single slice preparation merely produced one. The testing of the T2-preparation was necessary in order to find the optimal combination of the T2-preparation features for reducing the sensitivity of the preparation to magnetic field in homogeneities. The most robust T2-preparation was a T2-preparation with an MLEV weighted composite refocusing pulse train and a composite "tip-up" pulse, where a spoiler gradient was also used and the initial excitation and "tip-up" pulse were phase-spoiled. SNR measurements confirmed that the T2-weighting caused by this robust T2-preparation was theoretically correct. The testing of the segmented preparations confirmed that these preparations were run correctly according to the user-defined parameters. Partially reconstructed images of the k-space were used to verify the relaxation behaviour of the net magnetization and the contrast improvement offered by the modified phase-encoding orders, which were developed in this work for these segmented preparations. Some clinical applications were also demonstrated. The results of this work suggest that both the single and multi-slice double inversion black blood preparations produce an equally good black blood contrast, whereas the multi slice preparation permits a greater amount of slices to be acquired in the same scan time. The results obtained with the robust T2-preparation would, in turn, suggest that this preparation is very useful, especially as a segmented preparation, for improving the blood-myocardium contrast in cardiac MRI. As the results obtained with the segmented preparations are concurrent with theory, the almost complete lack of restrictions for these segmented preparations will therefore most certainly bring about many new preparation applications as well as clinical applications.
Katila, Toivo
Thesis advisor
Pesola, Marko
magnetic resonance imaging, magnetresonans avbildning, magnetization preparations, magnetiseringspreparationer, segmented preparations, segmenterade preparationer
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