Mathias Davids1,2, Lothar R Schad3, Lawrence L Wald4,5,6, Bastien Guérin4,6. 1. Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany. mathias.davids@medma.uni-heidelberg.de. 2. A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States. mathias.davids@medma.uni-heidelberg.de. 3. Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany. 4. A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States. 5. Harvard-MIT, Division of Health Sciences and Technology, Cambridge, Massachusetts, United States. 6. Harvard Medical School, Boston, Massachusetts, United States.
Abstract
PURPOSE: To design short parallel transmission (pTx) pulses for excitation of arbitrary three-dimensional (3D) magnetization patterns. METHODS: We propose a joint optimization of the pTx radiofrequency (RF) and gradient waveforms for excitation of arbitrary 3D magnetization patterns. Our optimization of the gradient waveforms is based on the parameterization of k-space trajectories (3D shells, stack-of-spirals, and cross) using a small number of shape parameters that are well-suited for optimization. The resulting trajectories are smooth and sample k-space efficiently with few turns while using the gradient system at maximum performance. Within each iteration of the k-space trajectory optimization, we solve a small tip angle least-squares RF pulse design problem. Our RF pulse optimization framework was evaluated both in Bloch simulations and experiments on a 7T scanner with eight transmit channels. RESULTS: Using an optimized 3D cross (shells) trajectory, we were able to excite a cube shape (brain shape) with 3.4% (6.2%) normalized root-mean-square error in less than 5 ms using eight pTx channels and a clinical gradient system (Gmax = 40 mT/m, Smax = 150 T/m/s). This compared with 4.7% (41.2%) error for the unoptimized 3D cross (shells) trajectory. Incorporation of B0 robustness in the pulse design significantly altered the k-space trajectory solutions. CONCLUSION: Our joint gradient and RF optimization approach yields excellent excitation of 3D cube and brain shapes in less than 5 ms, which can be used for reduced field of view imaging and fat suppression in spectroscopy by excitation of the brain only. Magn Reson Med 76:1170-1182, 2016.
PURPOSE: To design short parallel transmission (pTx) pulses for excitation of arbitrary three-dimensional (3D) magnetization patterns. METHODS: We propose a joint optimization of the pTx radiofrequency (RF) and gradient waveforms for excitation of arbitrary 3D magnetization patterns. Our optimization of the gradient waveforms is based on the parameterization of k-space trajectories (3D shells, stack-of-spirals, and cross) using a small number of shape parameters that are well-suited for optimization. The resulting trajectories are smooth and sample k-space efficiently with few turns while using the gradient system at maximum performance. Within each iteration of the k-space trajectory optimization, we solve a small tip angle least-squares RF pulse design problem. Our RF pulse optimization framework was evaluated both in Bloch simulations and experiments on a 7T scanner with eight transmit channels. RESULTS: Using an optimized 3D cross (shells) trajectory, we were able to excite a cube shape (brain shape) with 3.4% (6.2%) normalized root-mean-square error in less than 5 ms using eight pTx channels and a clinical gradient system (Gmax = 40 mT/m, Smax = 150 T/m/s). This compared with 4.7% (41.2%) error for the unoptimized 3D cross (shells) trajectory. Incorporation of B0 robustness in the pulse design significantly altered the k-space trajectory solutions. CONCLUSION: Our joint gradient and RF optimization approach yields excellent excitation of 3D cube and brain shapes in less than 5 ms, which can be used for reduced field of view imaging and fat suppression in spectroscopy by excitation of the brain only. Magn Reson Med 76:1170-1182, 2016.
Authors: William Grissom; Chun-yu Yip; Zhenghui Zhang; V Andrew Stenger; Jeffrey A Fessler; Douglas C Noll Journal: Magn Reson Med Date: 2006-09 Impact factor: 4.668
Authors: Kawin Setsompop; Vijayanand Alagappan; Borjan A Gagoski; Andreas Potthast; Franz Hebrank; Ulrich Fontius; Franz Schmitt; L L Wald; E Adalsteinsson Journal: Magn Reson Med Date: 2009-02 Impact factor: 4.668