title: Spin echoes in the regime of weak dephasing.
abstract: OBJECTIVE
  This article analyzes possibilities and limits of spin echoes beyond Hahn's theory. The regime of weak dephasing is explored with the purpose of combining the enhanced signal and reduced artifacts of spin echoes with the speed and flexibility of the fast low angle shot sequence.
  
  
  METHODS
  In the regime of weak dephasing, an upper boundary of the echo time is derived analytically. This limit is verified with optimal control pulses, which are also used to heuristically examine the transition to Hahn's regime of complete dephasing. The potential of the proposed pulses is demonstrated in proof-of-concept spin echo fast low angle shot images of a volunteer's lung and head.
  
  
  RESULTS
  It was found that the time between the end of the pulse and the spin echo can exceed the duration of the composite radio frequency-pulse, which stands in contrast to Hahn echoes where those measures are at most equal. The maximum echo time was found to mainly depend on the total amount of dephasing. In vivo spin echo fast low angle shot images show an increased pulmonary signal as well as reduced artifacts in areas affected by susceptibility differences.
  
  
  CONCLUSIONS
  The spin dynamics in the regime of weak dephasing was investigated and the feasibility of spin echo fast low angle shot imaging was demonstrated in vivo.
authors: Department of Radiology-Medical Physics, University Medical Center Freiburg, Freiburg, Germany.
authors: Jakob
authors: J
authors: Assländer
authors: Department of Chemistry, Technische Universität München, Munich, Germany.
authors: Steffen J
authors: SJ
authors: Glaser
authors: Department of Radiology-Medical Physics, University Medical Center Freiburg, Freiburg, Germany.
authors: Jürgen
authors: J
authors: Hennig
mesh_terms: D000818:Animals; D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D011846:Radio Waves; D011874:Radiometry / Q000379:methods*; D012542:Scattering, Radiation; D013113:Spin Labels
chemical_list: D013113:Spin Labels
keywords: delayed-focus pulse; fast low angle shot; lung; optimal control; prefocused pulse; spin echo; susceptibility