title: Fast rotary nonlinear spatial acquisition (FRONSAC) imaging.
abstract: OBJECTIVE
  Nonlinear spatial encoding magnetic fields (SEMs) have been studied to reconstruct images from a minimum number of echoes. Previous work has also explored single shot trajectories in nonlinear SEMs. However, the search continues for optimal schemes that apply nonlinear SEMs to improve spatial encoding efficiency and image quality.
  
  
  METHODS
  We enhance the encoding efficiency of standard linear gradient trajectories by adding a rapidly rotating nonlinear SEM of moderate amplitude, the so called FRONSAC (Fast ROtary Nonlinear Spatial ACquisition) imaging. This additional gradient greatly improves the image quality of highly undersampled single-shot trajectories, including EPI, Spiral, and Rosette trajectories.
  
  
  RESULTS
  Our simulations, including noise and dephasing effects, test the effect of adding FRONSAC gradients, demonstrating the applicability of this approach. Performance is explained by demonstrating the additional k-space sampling the nonlinear gradient provides. Studies of the optimal amplitude and frequency of the additional FRONSAC field are presented, and the role of enhanced sampling during the readout demonstrated. Dynamic field mapping in a second-order gradient system shows the proposed gradient waveforms are feasible.
  
  
  CONCLUSIONS
  Images resulting from highly undersampled existing k-space trajectories, such as EPI, Spiral, and Rosette, are greatly enhanced simply by adding a rotating nonlinear SEM field.
authors.affiliation: Department of Diagnostic Radiology, Yale University, New Haven, Connecticut, USA.
authors.forename: Haifeng
authors.initials: H
authors.lastname: Wang
authors.affiliation: Department of Diagnostic Radiology, Yale University, New Haven, Connecticut, USA.
authors.forename: Leo K
authors.initials: LK
authors.lastname: Tam
authors.affiliation: Department of Diagnostic Radiology, Yale University, New Haven, Connecticut, USA.|Department of Biomedical Engineering, Yale University, New Haven, Connecticut, USA.|Department of Neurosurgery, Yale University, New Haven, Connecticut, USA.
authors.forename: R Todd
authors.initials: RT
authors.lastname: Constable
authors.affiliation: Department of Diagnostic Radiology, Yale University, New Haven, Connecticut, USA.
authors.forename: Gigi
authors.initials: G
authors.lastname: Galiana
mesh_terms: D001921:Brain / Q000000981:diagnostic imaging; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008954:Models, Biological; D017711:Nonlinear Dynamics; D019047:Phantoms, Imaging
keywords: EPI; Rosette; Spiral; accelerated imaging; nonlinear gradients; parallel imaging; single echo imaging