Constrained and Spectral-Spatial RF Pulse Design for Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a critical tool for modern medicine, providing a non-invasive glimpse inside the human body with excellent soft tissue contrast and no ionizing radiation. The radio frequency (RF) pulse in an MRI acquisition is integral to producing an image and can be tailored to particular applications. This thesis focuses on the design of RF pulses and explores the MRI physics, convex optimization problems, and experimental methodologies behind doing so.
First, we introduce constrained RF pulse design which enables efficient RF pulse design with meaningful, physical constraints such as peak RF amplitude and integrated RF power. We explore constrained RF pulse design for simultaneous multislice imaging, a powerful tool for accelerating MRI and combatting notoriously long acquisition times. Compared to a conventional simultaneous multislice pulse designed without constraints, our constrained pulses achieved lower magnitude normalized root mean squared error (NRMSE) for an equivalent RF pulse length, or alternatively, the same NRMSE for a shorter pulse length. Constrained RF pulse design forms a basis for the rest of the dissertation.
Secondly, we describe a special class of RF pulses, “prewinding pulses”, that help correct for intravoxel dephasing due to magnetic field inhomogeneity, that can lead to signal loss. We propose a spectral-spatial prewinding pulse that leverages a larger effective recovery bandwidth than equivalent, purely spectral pulses. In an in vivo experiment imaging the brain of a human volunteer, we designed spectral-spatial pulses with a complex NMRSE of 0.18, which is significantly improved from the complex NRMSE of 0.54 in the purely spectral pulse for the same experiment.
Finally, we consider a slab-selective prewinding pulse, that extends spectral and spectral-spatial prewinding pulses to a common 3D imaging method. Here we integrate optimal control optimization to further improve the slab-selective spectral pulse design and see an in vivo improvement of excitation NRMSE from 0.40 to 0.37 and a major reduction in mean residual magnetization magnitude after a tip-up pulse from 0.18 to 0.02 when adding optimal control. This method has the potential to connect prewinding pulse design from the MRI physicist engineering workspace to a clinical application.
In summary, we show that constrained RF pulse design provides an efficient way of improving MRI in terms of acquisition speed (via multislice imaging) and image quality (via signal recovery).
Magnetic resonance imaging (MRI) is a critical tool for modern medicine, providing a non-invasive glimpse inside the human body with excellent soft tissue contrast and no ionizing radiation. The radio frequency (RF) pulse in an MRI acquisition is integral to producing an image and can be tailored to particular applications. This thesis focuses on the design of RF pulses and explores the MRI physics, convex optimization problems, and experimental methodologies behind doing so.
First, we introduce constrained RF pulse design which enables efficient RF pulse design with meaningful, physical constraints such as peak RF amplitude and integrated RF power. We explore constrained RF pulse design for simultaneous multislice imaging, a powerful tool for accelerating MRI and combatting notoriously long acquisition times. Compared to a conventional simultaneous multislice pulse designed without constraints, our constrained pulses achieved lower magnitude normalized root mean squared error (NRMSE) for an equivalent RF pulse length, or alternatively, the same NRMSE for a shorter pulse length. Constrained RF pulse design forms a basis for the rest of the dissertation.
Secondly, we describe a special class of RF pulses, “prewinding pulses”, that help correct for intravoxel dephasing due to magnetic field inhomogeneity, that can lead to signal loss. We propose a spectral-spatial prewinding pulse that leverages a larger effective recovery bandwidth than equivalent, purely spectral pulses. In an in vivo experiment imaging the brain of a human volunteer, we designed spectral-spatial pulses with a complex NMRSE of 0.18, which is significantly improved from the complex NRMSE of 0.54 in the purely spectral pulse for the same experiment.
Finally, we consider a slab-selective prewinding pulse, that extends spectral and spectral-spatial prewinding pulses to a common 3D imaging method. Here we integrate optimal control optimization to further improve the slab-selective spectral pulse design and see an in vivo improvement of excitation NRMSE from 0.40 to 0.37 and a major reduction in mean residual magnetization magnitude after a tip-up pulse from 0.18 to 0.02 when adding optimal control. This method has the potential to connect prewinding pulse design from the MRI physicist engineering workspace to a clinical application.
In summary, we show that constrained RF pulse design provides an efficient way of improving MRI in terms of acquisition speed (via multislice imaging) and image quality (via signal recovery).
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