Frequency response analysis and reconstruction weighting schemes for MR elastography

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buir.advisorAtalar, Ergin
dc.contributor.authorArıyürek, Cemre
dc.date.accessioned2020-10-12T13:02:14Z
dc.date.available2020-10-12T13:02:14Z
dc.date.copyright2020-09
dc.date.issued2020-09
dc.date.submitted2020-09-30
dc.departmentDepartment of Electrical and Electronics Engineeringen_US
dc.descriptionCataloged from PDF version of article.en_US
dc.descriptionThesis (Ph.D.): Bilkent University, Department of Electrical and Electronics Engineering, İhsan Doğramacı Bilkent University, 2020.en_US
dc.descriptionIncludes bibliographical references (leaves 91-99).en_US
dc.description.abstractMagnetic resonance elastography (MRE) non-invasively and quantitatively assesses the elasticity of the in-vivo tissue. In MRE, shear waves are induced to the tissue by an actuator, while phase-contrast images are obtained by magnetic resonance imaging (MRI). Finally, elasticity maps are generated using displacement information carried by phase-contrast images. The direction and frequency of the induced shear waves could be crucial in MRE. Here, it is demonstrated by the frequency response MRE simulations that modes of the shear waves can be observed in the brain during MR elastography with high shear wave displacement values at the mode frequencies. High shear wave displacements, 10-20 times of the applied displacement, were observed at mode frequencies in phantom MRE experiments. The second part of the thesis focuses on weighting schemes to combine multiple elasticity maps reconstructed from data collected for different excitation frequencies and motion direction. A new weighting scheme, which maximizes the signal-to-noise ratio (SNR) of the final wave speed map, has been proposed for tomoelastography and Helmholtz inversions. For both inversion techniques, considering the noise on the complex MRI signal, the SNR of the reconstructed wave speed map was formulated by an analytical approach assuming a high SNR. Thus, with the proposed SNR weighting method, while not altering the accuracy or spatial resolution of the wave speed map, the SNR of the wave speed map has been improved by 2 and 1.6 times for tomoelastography and Helmholtz inversion, respectively. The bias occurring for low SNR data cases was eliminated in tomoelastography and reduced in Helmholtz inversion with the proposed SNR-weighted reconstructions. Similarly, a strain-based weighting for MRE reconstruction has been introduced. Experimental results demonstrated that strain weights could prevent artifacts at the boundaries of encapsulated tumors or tissues with membranes; however, further examination is required. In this thesis, two independent contributions have been made to the field of magnetic resonance elastography. By showing the existence of modes of the shear waves in the body, new fronts are opened in the MRE actuation methods and safety. The improvements in the elasticity map inversions could lead to the routine use of MRE in clinical practice.en_US
dc.description.degreePh.D.en_US
dc.description.statementofresponsibilityby Cemre Arıyüreken_US
dc.format.extentxviii, 99 leaves : illustrations, charts ; 30 cm.en_US
dc.identifier.itemidB160507
dc.identifier.urihttp://hdl.handle.net/11693/54204
dc.language.isoEnglishen_US
dc.publisherBilkent Universityen_US
dc.rightsinfo:eu-repo/semantics/openAccessen_US
dc.subjectMagnetic resonance elastographyen_US
dc.subjectModes of shear wavesen_US
dc.subjectSignal-tonoise ratioen_US
dc.subjectOctahedral shear strainen_US
dc.subjectShear wave speeden_US
dc.subjectElasticityen_US
dc.subjectFinite element method simulationsen_US
dc.subjectMonte Carlo simulationsen_US
dc.titleFrequency response analysis and reconstruction weighting schemes for MR elastographyen_US
dc.title.alternativeMR elastografi'de frekans yanıtı analizi ve geriçatım ağırlıklandırma yöntemlerien_US
dc.typeThesisen_US
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