title: Composite pulses for RF phase encoded MRI: A simulation study.
abstract: In B1encoded MRI, a realistic non-linear phase RF encoding coil will generate an inhomogeneous B1field that leads to spatially dependent flip angles. The non-linearity of the B1phase gradient can be compensated for in the reconstruction, but B1inhomogeneity remains a problem. The effect of B1inhomogeneity on tip angles for conventional, B0encoded MRI, may be minimized using composite pulses. The objective of this study was to explore the feasibility of using composite pulses with non-linear RF phase encoding coils and to identify the most appropriate composite pulse scheme.
  
  
  
  RF encoded signals were simulated via the Bloch equation for various symmetric, asymmetric and antisymmetric composite pulses. The simulated signals were reconstructed using a constrained least squares method.
  
  
  
  Root mean square reconstruction errors varied from 6% (for an asymmetric composite pulse) to 9.7% (for an antisymmetric composite pulse).
  
  
  
  An asymmetric composite pulse scheme created images with fewer artifacts than other composite pulse schemes in inhomogeneous B0and B1fields making it the best choice for decreasing the effects of spatially varying flip angles. This is contrary to the conclusion that antisymmetric composite pulses are the best ones to use for spin echo sequences in conventional, B0encoded, MRI.
authors.affiliation: Division of Biomedical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada. Electronic address: s.salajeghe@usask.ca.
authors.forename: Somaie
authors.initials: S
authors.lastname: Salajeghe
authors.affiliation: Division of Biomedical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; Department of Medical Imaging, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: Paul
authors.initials: P
authors.lastname: Babyn
authors.affiliation: Division of Biomedical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; Department of Psychology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Sarty
mesh_terms: D016477:Artifacts; D003198:Computer Simulation; D005240:Feasibility Studies; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011674:Pulse; D011846:Radio Waves
keywords: Composite pulses; Low-field MRI; Portable MRI; RF encoding