0000000001032038
AUTHOR
Hélène Ratiney
Impulsions d'excitation basse énergie robustes aux inhomogénéités B1
International audience
MRI phase control with Optimal Control Theory
International audience
Contrôle Optimal appliqué au contrôle de la phase en IRM : simulations et expériences sur fantômes
National audience; IntroductionLes techniques IRM utilisant la phase du signal IRM, à la place ou en complément de l’amplitude, sont de plus en plus nombreuses. Dans ces techniques, la phase est gérée par l’application de gradients. Nous proposons ici de contrôler la phase du signal directement avec des impulsions RF. Pour cela, nous avons utilisé la théorie du contrôle optimal1 et calculé des impulsions RF optimisées pour atteindre des états cibles (dans notre cas, des motifs de phase) préalablement définis. Dans cette étude, une preuve de faisabilité de contrôle de la phase IRM par impulsions RF est présentée au travers de simulations (avec le logiciel ODIN2) ainsi que d’expériences IRM s…
Constant gradient magnetic resonance elastography experiments on phantom and bovine liver
International audience; SynopsisMagnetic Resonance Elastography (MRE) is performed by the application of motion-sensitive gradients. In this study, RF pulses are designed with an optimal control algorithm to obtain a desired magnetization phase distribution. Such pulse, in presence of a constant gradient, allows tosimultaneously perform spatially selective excitation and motion encoding. This offers some advantages when compared to standard MRE encoding strategy. Simulations, phantom and ex vivo experiments show that phase-to-noise ratios are improved. These results demonstrate that optimal control-based pulses can be used to encode motion in the MRE excitation phase with relevant advantage…
Magnetic resonance elastography without oscillating gradients
International audience
BEEEP: B1-robust Energy Efficient Excitation Pulses
International audience; SynopsisThis study introduces a new family of broadband B1-robust excitation (90°) pulses for MRI with large enough bandwidth (+/- 1 kHz) to account forstatic field inhomogeneities, and minimal energy deposition. RF pulses are designed with a regularized optimal control algorithm, which is able toadapt the pulse B1-robustness range to fit the coil limits in terms of peak amplitude and energy. In vitro acquisitions using an endoluminal-shapedRF transmit coil show comparable excitation profiles than BIR4 pulses, although BEEEP pulses deposit 5.2 times less energy.https://index.mirasmart.com/ISMRM2019/PDFfiles/4623.html
A Simplified Framework for Contrast Optimization in MRI
International audience
Contrôle optimal appliqué à l’Elastographie par Résonance Magnétique avec un gradient constant: expériences ex vivo et in vitro
International audience
Optimal Control Pulse Design for Contrast in MRI: in vivo applications
International audience; Optimal control RF pulse design has recently been proposed to address the optimization of image contrast in MRI - in order to explore the theoretical contrast bound of a given imaged system. Their use has recently been validated on a real MRI scanner to contrast various in vitro samples. This abstract extends these results to in vivo applications, and shows that contrasts obtained with standard weighting strategies on rat and mouse brains can be improved or inverted. This demonstrates both the interest and flexibility that one can get when using optimal contrast pulses for in vitro and in vivo applications.
Optimal control design of preparation pulses for contrast optimization in MRI
Abstract This work investigates the use of MRI radio-frequency (RF) pulses designed within the framework of optimal control theory for image contrast optimization. The magnetization evolution is modeled with Bloch equations, which defines a dynamic system that can be controlled via the application of the Pontryagin Maximum Principle (PMP). This framework allows the computation of optimal RF pulses that bring the magnetization to a given state to obtain the desired contrast after acquisition. Creating contrast through the optimal manipulation of Bloch equations is a new way of handling contrast in MRI, which can explore the theoretical limits of the system. Simulation experiments carried out…
Optimizing MRI contrast with optimal control theory
International audience; Magnetic Resonance Imaging (MRI) uses the difference in tissue relaxation times to create contrast. Various image weightings can be obtained by tuning acquisition parameters which are usually empirically defined. In this article, optimal control theory is used to design excitation pulses that produce the optimal contrast between given tissues. The designed pulses are tested on numerical phantoms with and without magnetic field inhomogeneities and for the first time in vitro on a small-animal MRI. The reasonable match between simulation and real experiments is promising for the development of such pulses in further in vivo applications.
Contrast Preparation Pulses Robust to B1 and B0 inhomogeneities: an Optimal Control Approach
International audience
A simplified framework to optimize MRI contrast preparation
PURPOSE This article proposes a rigorous optimal control framework for the design of preparation schemes that optimize MRI contrast based on relaxation time differences. METHODS Compared to previous optimal contrast preparation schemes, a drastic reduction of the optimization parameter number is performed. The preparation scheme is defined as a combination of several block pulses whose flip angles, phase terms and inter-pulse delays are optimized to control the magnetization evolution. RESULTS The proposed approach reduces the computation time of B 0 -robust preparation schemes to around a minute (whereas several hours were required with previous schemes), with negligible performance loss. …