Date

2021-6-22

Author

Şişman, Mert

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The magnetohydrodynamic (MHD) effect is a result of the interaction between orthogonal electric and magnetic fields in low viscosity media. Fluids inside such media start to form flow patterns in accordance with the Lorentz force density distributions caused by these orthogonal fields. In Magnetic Resonance Current Density Imaging (MRCDI) and Electrocardiogram (ECG) triggered Magnetic Resonance Imaging (MRI) applications, the MHD effect is observed and classified as artifacts that distort the measured signals. However, recently, the MHD flow velocity imaging is proposed as a novel imaging modality that may become an alternative to the blood oxygenation level-dependent functional Magnetic Resonance Imaging (fMRI). The aim of this thesis study is to realize MHD flow velocity imaging using a Spin Echo-based pulse sequence, to analyze the sensitivity of the obtained MHD flow velocity images on the current injection and MR image acquisition parameters, and to propose image acquisition and reconstruction techniques for multi-contrast imaging that contains MHD flow velocity imaging. As the result of the MR experiments conducted with a cylindrical phantom, it is determined that the magnitude of the MHD flow velocity distribution inside a homogeneous medium is increasing with the increasing area of the injected current pulse with an approximate power of 2.3. Moreover, it is observed that the MHD flow can reach higher velocity values in the horizontal flow direction than the vertical flow direction under the effect of the same current pulse because of the gravitational force. Lastly, with semi-analytical analyses of the relations between the SNR levels of the MHD flow velocity images and the parameters of the flow encoding gradients, it is derived that a b-value of 224 s/mm2 is optimal for the water at room temperature. Finally, a flow encoding gradient set is proposed for the simultaneous image acquisition of the imaging modalities MRCDI, Diffusion Tensor Imaging (DTI), and MHD flow velocity imaging. With the utilization of the proposed flow encoding gradient set, the data required for the reconstruction of all three contrast distributions can be collected with a total of 14 acquisitions. This provides a 50% decrease in the total image acquisition time in comparison with the separate acquisitions of all three imaging modalities. Furthermore, the proposed image acquisition technique is 100% efficient since all the acquired MR magnitude and phase images are utilized for the image reconstruction. The image reconstruction methods to obtain all three contrast distributions from the data acquired with the proposed multi-contrast imaging technique are also provided in this thesis. Finally, a numerical analysis is conducted in order to estimate the minimum required injected current amplitudes to obtain detectable MHD flow velocity signals. For the homogeneous cylindrical phantom and with the MR image acquisition parameters utilized during the sensitivity analysis, the minimum required current amplitude values are estimated as 1.43 mA and 1.94 mA for two different flow encoding gradient sets.

Subject Keywords

Magnetohydrodynamic Flow Velocity Imaging, Current Density Imaging, Diffusion Tensor Imaging, Multi-Contrast Imaging, Sensitivity Analysis

URI

https://hdl.handle.net/11511/91238

Collections

Graduate School of Natural and Applied Sciences, Thesis