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Measurement of gradient magnetic field decay in MR tomograph
Kubásek, Ondřej ; Kubásek, Radek (referee) ; Bartušek, Karel (advisor)
The objective of my thesis is to measure magnetic field gradient decay in an MR tomograph, process and evaluate the data measured and analyse the issue. The theoretical section focuses on the reasons for the gradient distortion which has a negative impact on NMR measuring techniques and it also describes options to eliminate the negative effects, focusing mainly on pre-emphasis filtration. It also lists methods of gradient magnetic field measuring. The practical section contains results of magnetic field decay measurements taken with the tomograph in the Institute of scientific instruments of the Academy of sciences in Brno using the method of instantaneous frequency with and without pre-emphasis compensation. The data are processed with MAREVISI and MATLAB programmes. MATLAB is used to develop a modern method of filtration to eliminate noise in useful signal leading to more accurate measurements of MR magnetic fields. This filtration uses a method of time-dependent variable thresholds and two-channel filter banks. MATLAB is also used to design a programme for approximation of the measured gradient magnetic field decay. The decay must be approximated in order to obtain time constants and the degree of exponential decay, which serve to set up the pre-emphasis compensation.
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Measurement of gradient magnetic field decay in MR tomograph
Kubásek, Ondřej ; Kubásek, Radek (referee) ; Bartušek, Karel (advisor)
The objective of my thesis is to measure magnetic field gradient decay in an MR tomograph, process and evaluate the data measured and analyse the issue. The theoretical section focuses on the reasons for the gradient distortion which has a negative impact on NMR measuring techniques and it also describes options to eliminate the negative effects, focusing mainly on pre-emphasis filtration. It also lists methods of gradient magnetic field measuring. The practical section contains results of magnetic field decay measurements taken with the tomograph in the Institute of scientific instruments of the Academy of sciences in Brno using the method of instantaneous frequency with and without pre-emphasis compensation. The data are processed with MAREVISI and MATLAB programmes. MATLAB is used to develop a modern method of filtration to eliminate noise in useful signal leading to more accurate measurements of MR magnetic fields. This filtration uses a method of time-dependent variable thresholds and two-channel filter banks. MATLAB is also used to design a programme for approximation of the measured gradient magnetic field decay. The decay must be approximated in order to obtain time constants and the degree of exponential decay, which serve to set up the pre-emphasis compensation.
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High resolution multivoxel spectroscopy of human brain at 3 Tesla
Mlynárik, V. ; Gruber, S. ; Starčuk, Zenon ; Starčuk jr., Zenon ; Horký, Jaroslav ; Moser, E.
Localisation in in vivo NMR spectroscopy can be achieved using different concepts. Single voxel localized spectroscopy exploits three slice selective pulses defining a cube (or a parallelepiped) in their intersection. By proper combination of spoiling gradients the excited magnetization outside the cube is dephased and does not contribute to the NMR signal. Another way of localisation is combination of slice selection with phase encoding of the NMR signal in the other two dimensions by means of gradient magnetic fields. The latter method is referred to as spectroscopic imaging and provides an array of spectra corresponding to individual voxels defined by the phase encoding. Further subdivision of the voxels is possible by Hadamard encoding of the excitation pulses.
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