title: Fast three-dimensional inner volume excitations using parallel transmission and optimized k-space trajectories.
abstract: To design short parallel transmission (pTx) pulses for excitation of arbitrary three-dimensional (3D) magnetization patterns.
  
  
  
  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.
  
  
  
  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.
  
  
  
  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. © 2015 Wiley Periodicals, Inc.
authors.affiliation: Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany. mathias.davids@medma.uni-heidelberg.de.|A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States. mathias.davids@medma.uni-heidelberg.de.
authors.forename: Mathias
authors.initials: M
authors.lastname: Davids
authors.affiliation: Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany.
authors.forename: Lothar R
authors.initials: LR
authors.lastname: Schad
authors.affiliation: A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States.|Harvard-MIT, Division of Health Sciences and Technology, Cambridge, Massachusetts, United States.|Harvard Medical School, Boston, Massachusetts, United States.
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Wald
authors.affiliation: A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States.|Harvard Medical School, Boston, Massachusetts, United States.
authors.forename: Bastien
authors.initials: B
authors.lastname: Guérin
mesh_terms: D000465:Algorithms; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D010363:Pattern Recognition, Automated / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
keywords: B0 robustness; Parallel transmit; inner volume excitation; k-space trajectory optimization; spatially selective excitation