Correction of gradient pulse shape distortions in radial MRI

This paper focuses on the optimization of gradient-pulse shapes in MRI measurement. The main topic investigated in this paper is optimization of slice-selective gradient, where imperfections may produce phase distortion in the resulting image and also signal loss in the acquired signal. A method for correction is proposed based on the Nelder-Mead algorithm followed by greedy search in the neighborhood of the found solution. The adjustment is evaluated using a simple Fast low angle shot (FLASH) sequence with radial readout. The results show a significant improvement in the Free induction decay (FID) signal magnitude, echo stability, and an improvement in the homogeneity of image phase

Quantification of Relaxation Time T1 in DCE-MRI

Tato bakalářská práce se zabývá metodami kvantifikace relaxačního času T1 a korekce nehomogenity magnetického pole B1. Pro účely měření je vytvořen fantom pomocí 3D tisku. Nejprve je provedena rešerše na dané téma, následně je provedena analýza dat z měření standardními metodami kvantifikace relaxačního času T1. Tyto metody jsou následně mezi sebou porovnány. Další část bakalářské práce se věnuje korekci nehomogenity magnetického pole B1 a využití této korekce pro kvantifikaci T1 pomocí rychlých metod založených na proměnném sklápěcím úhlu. Pro korekci nehomogenity jsou zpracovány dva postupy a to korekce přímým mapováním relativní mapy B1 metodou double angle a mapování B1 na základě referenční metody. Pro účely výběru referenční metody je vytvořena jednoduchá simulace, která ukazuje náchylnosti jednotlivých referenčních metod na nepříznivé vlivy při akvizici. Na závěr jsou stanoveny výhody a nevýhody jednotlivých metod korekce nehomogenity B1 a srovnání přesnosti metod založených na proměnném sklápěcím úhlu s referenčními metodami.This bachelor thesis deals with methods of T1 relaxation time quantification and B1 magnetic field inhomogeneities corrections. For purposes of the required measurements, the phantom was created using a 3D printer. Firstly the thesis presents information about a given theme, then it hands out the results of measurements taken using standard quantification methods. These methods are consequently analyzed and compared. Next part of the bachelor thesis deals with the fast method based on a variable flip angle (VFA), which needs a magnetic field B1 correction maps to give reasonable T1 estimates. For this purpose, two methods of B1 correction are presented. The first method is a double angle method and second is based on mapping magnetic field B1 using a reference T1 map. To decide which T1 map of which method should be used as a reference, a simple simulation is proposed, which describes the impact of unfavorable effects, such as B1 itself, on the T1 estimate of standard methods. In the end, the conclusions are made and the results of VFA methods are compared with standard methods of quantification.

MRI of Rats - Quantification of T1 in Myocardium

Tato práce se zabývá tématikou snímání a kvantifikace relaxačního času T1 u srdce potkanů. Zaměřuje se na prostudování metod této kvantifikace a následnou aplikaci těchto metod při vývoji kvantifikačního nástroje. Se zvláštním zřetelem se věnuje metodám synchronizace akvizice vzhledem k dechu a srdeční aktivitě. Hlavním cílem této práce je vyvinout nástroj pro synchronizaci akvizice retrospektivním způsobem s využitím pouze nasnímaných dat, tedy bez vnějšího měření navigačního signálu získaného například z dechového polštářku či elektrokardiografu. Práce se zabývá jak synchronizací v případě tzv. steady-state akvizice, tak i v případě situací, kdy je rovnovážný stav vyrušen pomocí inverzních pulzů či podáním kontrastní látky.This thesis focuses on cardiac imaging and quantification of T1 relaxation time in rat hearts. Its main focus is to investigate available methods for such quantification and their application in the development of quantification tools. The large impact is given to methods of acquisition synchronization, mainly with respect to cardiac motion and breathing using retrospective gating, where the navigator signal is obtained solely from the acquired data, so without any external equipment such as the ECG or respiratory sensors. This paper takes into account situations where steady-state has been reached and also those where it has not, by means of contrast agent injection or by inversion pulses.

Improving DCE-MRI through unfolded low-rank + sparse optimisation

A method for perfusion imaging with DCE-MRI is developed based on two popular paradigms: the low-rank + sparse model for optimisation-based reconstruction, and the deep unfolding. A learnable algorithm derived from a proximal algorithm is designed with emphasis on simplicity and interpretability. The resulting deep network is trained and evaluated using a simulated measurement of a rat with a brain tumor, showing large performance gain over the classical low-rank + sparse baseline. Moreover, quantitative perfusion analysis is performed based on the reconstructed sequence, proving that even training based on a simple pixel-wise error can lead to significant improvement of the quality of the perfusion maps.Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

MRI of Rats - Quantification of T1 in Myocardium

This thesis focuses on cardiac imaging and quantification of T1 relaxation time in rat hearts. Its main focus is to investigate available methods for such quantification and their application in the development of quantification tools. The large impact is given to methods of acquisition synchronization, mainly with respect to cardiac motion and breathing using retrospective gating, where the navigator signal is obtained solely from the acquired data, so without any external equipment such as the ECG or respiratory sensors. This paper takes into account situations where steady-state has been reached and also those where it has not, by means of contrast agent injection or by inversion pulses

Analysis of time-frequency characteristics of signals

This thesis focuses on time-frequency analysis of discrete signals. The aim of this work is to compare the most well known methods for spectro/scalegram estimation. The two main topics discussed are: The compromise between time and frequency resolution and the effect of noise in input data on the quality of estimated spectrograms. To achieve this a database has been created. This database consists of real and artificial signals on which the analysis can be performed and evaluated. This database is used in created demonstration application. This application is also used in a created laboratory task

Correction of gradient pulse shape distortions in radial MRI

This paper focuses on the optimization of gradient-pulse shapes in MRI measurement. The main topic investigated in this paper is optimization of slice-selective gradient, where imperfections may produce phase distortion in the resulting image and also signal loss in the acquired signal. A method for correction is proposed based on the Nelder-Mead algorithm followed by greedy search in the neighborhood of the found solution. The adjustment is evaluated using a simple Fast low angle shot (FLASH) sequence with radial readout. The results show a significant improvement in the Free induction decay (FID) signal magnitude, echo stability, and an improvement in the homogeneity of image phase