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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 dynamic quality of gradient magnetic fields
Kabrda, Miroslav ; Kubásek, Radek (referee) ; Bartušek, Karel (advisor)
In the first chapter of bachelor thesis I focused on description the tomograph construction. The next chapter deals with the measurement of gradient magnetic fields, using methods based on instantaneous frequency. The thesis describes: the IF method, the IFSE method ( the method of measuring the instantaneous frequency of the spin echo), the IFSES method (the instantaneous frequency measurement method for a series of spins) and the method of Gradient decay measurement in NMR tomography. Each method describes the basic principles of the measurement, the excitation pulse sequence, obtainment of instantaneous frequency and the method of evaluation of the obtained signal. This bachelor thesis includes experimental measurements using the method of Gradient decay measurement in NMR tomography. The experimental data obtained are given and evaluated in the last chapter at the end of the thesis.
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Measurement of dynamic quality of gradient magnetic fields
Kabrda, Miroslav ; Kubásek, Radek (referee) ; Bartušek, Karel (advisor)
In the first chapter of bachelor thesis I focused on description the tomograph construction. The next chapter deals with the measurement of gradient magnetic fields, using methods based on instantaneous frequency. The thesis describes: the IF method, the IFSE method ( the method of measuring the instantaneous frequency of the spin echo), the IFSES method (the instantaneous frequency measurement method for a series of spins) and the method of Gradient decay measurement in NMR tomography. Each method describes the basic principles of the measurement, the excitation pulse sequence, obtainment of instantaneous frequency and the method of evaluation of the obtained signal. This bachelor thesis includes experimental measurements using the method of Gradient decay measurement in NMR tomography. The experimental data obtained are given and evaluated in the last chapter at the end of the thesis.
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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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