title: B1+-mapping with the transient phase of unbalanced steady-state free precession.
abstract: OBJECTIVE
  A novel B1+-mapping technique (B1-TRAP) is presented, which derives the actual flip angle from the frequency of signal oscillations, observed in the transient phase of unbalanced steady-state free precession sequences.
  
  
  BACKGROUND
  For short repetition times (TR), the angular frequency of distinct oscillations in the transient phase of steady-state free precession sequences is proven to be approximately proportional to the actual flip angle: ω⋅TR≈α. The result is not influenced by off-resonance and it can be shown that deviations are only of second order in the small parameter TR/T2.
  
  
  METHODS
  B1-TRAP makes use of this effect through a frequency analysis of the transient phase of a train of steady-state free precession signals.
  
  
  RESULTS
  In terms of reliability and time efficiency, a two-dimensional multislice implementation was found to be optimal. Unlike many steady-state B1+-mapping methods, the accuracy of B1-TRAP was not impaired by imperfect slice profiles.
  
  
  CONCLUSIONS
  Simulations, phantom, and in vivo measurements showed that B1-TRAP offers a good compromise with respect to speed, robustness, and accuracy.
authors: Department of Radiology, Klinikum rechts der Isar, Technische Universität München, Munich, Germany.
authors: Carl
authors: C
authors: Ganter
authors: Marcus
authors: M
authors: Settles
authors: Isabel
authors: I
authors: Dregely
authors: Francesco
authors: F
authors: Santini
authors: Klaus
authors: K
authors: Scheffler
authors: Oliver
authors: O
authors: Bieri
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D001931:Brain Mapping / Q000379:methods*; D006801:Humans; 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*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
keywords: B1-mapping; SSFP; actual flip angle; off-resonance; transient phase