title: Lowering the B1 threshold for improved BEAR B1 mapping.
abstract: OBJECTIVE
  Accurate measurement of the nonuniform transmit radiofrequency field is necessary for magnetic resonance imaging applications. The radiofrequency field excitation amplitude (B1) is often obtained by acquiring a B1 map. We modify the B1 estimation using adiabatic refocusing (BEAR) method to extend its range to lower B1 magnitudes.
  
  
  METHODS
  The BEAR method is a phase-based B1 mapping method, wherein hyperbolic secant pulses induce a phase sensitivity to B1. The measurable B1 range is limited due to the adiabatic threshold of the pulses. We redesign the method to use flattened hyperbolic secant pulses, which have lower adiabatic thresholds. We optimize the flattened hyperbolic secant parameters to minimize phase sensitivity to frequency variations.
  
  
  RESULTS
  We validate the performance of the new method via simulation and in vivo at 3T, and show that for n ≤ 8, accurate B1 maps can be acquired using reduced nominal peak B1 values.
  
  
  CONCLUSIONS
  The adiabatic threshold for the BEAR method is reduced with flattened hyperbolic secant pulses, which are optimized for accurate phase-to-B1 mapping over a frequency range, and allow for lower nominal B1 values. At 3T, the nominal B1 is decreased by 52% and the sensitivity to B1 is increased by a factor of 3.8. This can improve the method's applicability for measurement of low B1.
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Kalina V
authors.initials: KV
authors.lastname: Jordanova
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Dwight G
authors.initials: DG
authors.lastname: Nishimura
authors.affiliation: Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.
authors.forename: Adam B
authors.initials: AB
authors.lastname: Kerr
mesh_terms: D001921:Brain / Q000000981:diagnostic imaging; D001931:Brain Mapping / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D012815:Signal Processing, Computer-Assisted
keywords: B1 mapping; adiabatic pulses; flattened hyperbolic secant pulses
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