title: B1 mapping of short T2 * spins using a 3D radial gradient echo sequence.
abstract: OBJECTIVE
  To develop a method to acquire a radiofrequency (B1 ) field map when the signal has a short T2 *.
  
  
  METHODS
  The method is based on the actual flip angle imaging (AFI) technique and a radial 3D gradient-echo sequence known as COncurrent Dephasing and Excitation (CODE), which preserves short T2 (*) signals. CODE was implemented with Gradient-modulated Offset-Independent Adiabaticity (GOIA) pulses to obtain high estimation sensitivity with AFI. The correlation method, which removes the quadratic phase from the frequency-modulated pulse excitation, was modified to handle gradient-modulated pulses. Validity of the modified correlation procedure was tested by Bloch simulations. CODE experiments with sinc, hyperbolic secant, and GOIA pulses were performed in order to see effects from the frequency and gradient modulation. Finally, GOIA-CODE AFI was conducted and compared with conventional AFI with 3D gradient echo (GRE).
  
  
  RESULTS
  The modified correlation method developed to accommodate frequency and gradient modulations of GOIA performed well as judged by the minimal impact on reconstructed image quality. GOIA-CODE AFI provided flip angle maps consistent with those measured by GRE AFI when the T2 * was long (>2 ms) and continued to perform well for short T2 * signals.
  
  
  CONCLUSIONS
  The proposed technique provides a means to obtain a 3D B1 field map when imaging spins with short T2 (*) .
authors.affiliation: Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota.
authors.forename: Naoharu
authors.initials: N
authors.lastname: Kobayashi
authors.forename: Michael
authors.initials: M
authors.lastname: Garwood
mesh_terms: D000465:Algorithms; 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; D013113:Spin Labels
chemical_list: D013113:Spin Labels
keywords: B1 mapping; actual flip angle imaging; concurrent dephasing and excitation; frequency-modulated pulse; gradient-modulated offset-independent adiabaticity; radial MRI; ultra-short TE