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Optimization of Basic Magnetic Field Homogeneity in MR Tomography
Hadinec, Michal ; Brančík, Lubomír (referee) ; Frollo, Ivan (referee) ; Bartušek, Karel (advisor)
This thesis is concerned with problems of measuring and mapping of magnetic field in MR tomograph, for purpose of magnetic flux density homogeneity optimization. Attention is paid to mapping techniques on rotary symmetric volume and to ways of magnetic fields optimization with utilization of passive and active correction systems. Theoretical analysis of magnetic field decomposition with utilization of spherical harmonics and numerical decomposition is made. Mapping and approximation techniques of basic magnetic field are verified by experiments in the laboratory at the Institute of Scientific Instruments AS CR in Brno.
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Advanced Methods of Perfusion Analysis in MRI
Macíček, Ondřej ; Frollo, Ivan (referee) ; Mikl, Michal (referee) ; Jiřík, Radovan (advisor)
This dissertation deals with quantitative perfusion analysis of MRI contrast-enhanced image time sequences. It focuses on two so far separately used methods -- Dynamic contrast-enhanced MRI (DCE-MRI) and Dynamic susceptibility contrast MRI (DSC-MRI). The common problem of such perfusion analyses is the unreliability of perfusion parameters estimation. This penalizes usage of these unique techniques on a regular basis. The presented methods are intended to improve these drawbacks, especially the problems with quantification in DSC in case of contrast agent extravasation and instability of the deconvolution process in DCE using advanced pharmacokinetic models. There are a few approaches in literature combining DCE and DSC to estimate new parameters of the examined tissue, namely the relaxivity of the vascular and of the interstitial space. Originally, in this scheme, the 2CXM DCE model was used. Here various models for DCE analysis are tested keeping in mind the DCE-DSC combination. The ATH model was found to perform better in this setting compared to 2CXM. Finally, the ATH model was used in alternating DCE-DSC optimization algorithm and then in a truly fully simultaneous DCE-DSC. The processing was tested using simulated and in-vivo data. According to the results, the proposed simultaneous algorithm performs better in comparison with sequential DCE-DSC, unleashing full potential of perfusion analysis using MRI.
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Analysis of Selected Artefacts in Diffusion-Based Magnetic Resonance Measurements
Marcoň, Petr ; Král, Bohumil (referee) ; Frollo, Ivan (referee) ; Bartušek, Karel (advisor)
The presented dissertation thesis analyses artefacts in diffusion-weighted images. In medical practice, the artefacts can impede the diagnostics of pathological tissues and, therefore, need to be eliminated. As the first step within the thesis, an analysis of the most frequent artefacts in diffusion-weighted images is performed, and the hitherto known approaches to artefact elimination are described. In order to facilitate the reduction of artefacts caused by the inhomogeneity of the static magnetic field and induced by eddy currents, a novel three-measurement method is shown. This technique will find application especially in measuring the diffusion coefficient of isotropic materials. At this point, it is important to note that a significant and commonly found problem is the magnetic susceptibility artefact; different magnetic susceptibility values at the boundary between two materials can cause magnetic field inhomogeneities and even complete loss of the signal. Therefore, we designed a novel method for the measurement of magnetic susceptibility in various samples of magnetically incompatible materials, which do not produce any MR signal. The technique was experimentally verified using a set of differently shaped diamagnetic and paramagnetic samples. In addition to the magnetic susceptibility problem, the thesis presents artefacts such as noise, motion-induced items, hardware limitations, chemical shift, and the dependence of the diffusion coefficient on the temperature. To enable precise measurement of the diffusion coefficient, we proposed a thermal system; in the experiment, it was determined that when the measurement error does not exceed 5%, the temperature change should not be higher than 0,1 °C. In the final sections of the thesis, practical application examples involving the designed methods are shown.
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Advanced Methods of Perfusion Analysis in MRI
Macíček, Ondřej ; Frollo, Ivan (referee) ; Mikl, Michal (referee) ; Jiřík, Radovan (advisor)
This dissertation deals with quantitative perfusion analysis of MRI contrast-enhanced image time sequences. It focuses on two so far separately used methods -- Dynamic contrast-enhanced MRI (DCE-MRI) and Dynamic susceptibility contrast MRI (DSC-MRI). The common problem of such perfusion analyses is the unreliability of perfusion parameters estimation. This penalizes usage of these unique techniques on a regular basis. The presented methods are intended to improve these drawbacks, especially the problems with quantification in DSC in case of contrast agent extravasation and instability of the deconvolution process in DCE using advanced pharmacokinetic models. There are a few approaches in literature combining DCE and DSC to estimate new parameters of the examined tissue, namely the relaxivity of the vascular and of the interstitial space. Originally, in this scheme, the 2CXM DCE model was used. Here various models for DCE analysis are tested keeping in mind the DCE-DSC combination. The ATH model was found to perform better in this setting compared to 2CXM. Finally, the ATH model was used in alternating DCE-DSC optimization algorithm and then in a truly fully simultaneous DCE-DSC. The processing was tested using simulated and in-vivo data. According to the results, the proposed simultaneous algorithm performs better in comparison with sequential DCE-DSC, unleashing full potential of perfusion analysis using MRI.
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The Study of Artifacts in MR Tomographic Images for Medical and Technical Applications
Al Khaddour, Mouin ; Frollo, Ivan (referee) ; Jiřík, Radovan (referee) ; Fiala, Pavel (advisor)
Tato disertační práce analyzuje artefakty v NMR obrazech. V lékařské praxi mohou artefakty zabránit diagnostice patologických tkání, a proto je třeba tyto nežádoucí jevy odstranit. Vzhledem k nutnosti vyloučit artefakty způsobené nehomogenitou statického magnetického pole jsou v dané souvislosti diskutovány také nové možnosti měření deformací. Práce popisuje metodu pro měření základního pole i RF pole a jejich nehomogenit na základě měření MR obrazů T2 a T2*. Metoda kombinuje akviziční postupy pro spinové a gradientní echo za účelem rozlišení relaxačních časů T2 a T2*. V této souvislosti také vyvstává potřeba provést exponenciální aproximaci relaxačního procesu. Experimentální výsledky pro plastové i měděné vzorky jsou prezentovány v příslušné části práce. Pokud jde o vlastní členení popisovaných jevů, je třeba uvést, že významný problém představují artefakty způsobené magnetickou susceptibilitou. Různé hodnoty magnetické vodivosti na rozhraní mezi dvěma materiály mohou způsobit deformaci magnetického pole, přičemž může dojít i k úplné ztrátě signálu.
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Analysis of Selected Artefacts in Diffusion-Based Magnetic Resonance Measurements
Marcoň, Petr ; Král, Bohumil (referee) ; Frollo, Ivan (referee) ; Bartušek, Karel (advisor)
The presented dissertation thesis analyses artefacts in diffusion-weighted images. In medical practice, the artefacts can impede the diagnostics of pathological tissues and, therefore, need to be eliminated. As the first step within the thesis, an analysis of the most frequent artefacts in diffusion-weighted images is performed, and the hitherto known approaches to artefact elimination are described. In order to facilitate the reduction of artefacts caused by the inhomogeneity of the static magnetic field and induced by eddy currents, a novel three-measurement method is shown. This technique will find application especially in measuring the diffusion coefficient of isotropic materials. At this point, it is important to note that a significant and commonly found problem is the magnetic susceptibility artefact; different magnetic susceptibility values at the boundary between two materials can cause magnetic field inhomogeneities and even complete loss of the signal. Therefore, we designed a novel method for the measurement of magnetic susceptibility in various samples of magnetically incompatible materials, which do not produce any MR signal. The technique was experimentally verified using a set of differently shaped diamagnetic and paramagnetic samples. In addition to the magnetic susceptibility problem, the thesis presents artefacts such as noise, motion-induced items, hardware limitations, chemical shift, and the dependence of the diffusion coefficient on the temperature. To enable precise measurement of the diffusion coefficient, we proposed a thermal system; in the experiment, it was determined that when the measurement error does not exceed 5%, the temperature change should not be higher than 0,1 °C. In the final sections of the thesis, practical application examples involving the designed methods are shown.
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Optimization of Basic Magnetic Field Homogeneity in MR Tomography
Hadinec, Michal ; Brančík, Lubomír (referee) ; Frollo, Ivan (referee) ; Bartušek, Karel (advisor)
This thesis is concerned with problems of measuring and mapping of magnetic field in MR tomograph, for purpose of magnetic flux density homogeneity optimization. Attention is paid to mapping techniques on rotary symmetric volume and to ways of magnetic fields optimization with utilization of passive and active correction systems. Theoretical analysis of magnetic field decomposition with utilization of spherical harmonics and numerical decomposition is made. Mapping and approximation techniques of basic magnetic field are verified by experiments in the laboratory at the Institute of Scientific Instruments AS CR in Brno.
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