title: Ultrafast volumetric B1 (+) mapping for improved radiofrequency shimming in 3 tesla body MRI.
abstract: OBJECTIVE
  To evaluate the use of the recently proposed ultrafast B1 (+) mapping approach DREAM (Dual Refocusing Echo Acquisition Mode) for a refinement of patient adaptive radiofrequency (RF) shimming.
  
  
  METHODS
  Volumetric DREAM B1 (+) calibration scans centered in the upper abdomen were acquired in 20 patients and three volunteers with written informed consent at a clinical dual source 3 Tesla (T) MR system. Based on these data, RF transmit settings were optimized by central-slice based RF-shimming (CS-RF shim) and by a refined, multi-slice adaptive approach (MS-RF shim). Simulations were performed to compare flip angle accuracy and B1 (+) homogeneity (cv = stddev/mean) achieved by CS-RF shim versus MS-RF shim for transversal and coronal slices, and for volume shimming on the spine.
  
  
  RESULTS
  By MS-RF shim, mean deviation from nominal flip angle was reduced to less than 11% in all slices, all targets, and all subjects. Relative improvements in B1 (+) cv (MS-RF shim versus CS-RF) were up to 14%/39%/47% in transversal slices/coronal slices/ spine area.
  
  
  CONCLUSIONS
  Volumetric information about B1 (+) can be used to further improve the accuracy and homogeneity of the B1 (+) field yielding higher diagnostic confidence, and will also be of value for various quantitative methods which are sensitive to flip angle imperfections.
authors.affiliation: Department of Radiology, University of Bonn, Germany; Institute of Medical Engineering, Ruhr-University Bochum, Germany.
authors.forename: Alois M
authors.initials: AM
authors.lastname: Sprinkart
authors.forename: Kay
authors.initials: K
authors.lastname: Nehrke
authors.forename: Frank
authors.initials: F
authors.lastname: Träber
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Block
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gieseke
authors.forename: Georg
authors.initials: G
authors.lastname: Schmitz
authors.forename: Winfried A
authors.initials: WA
authors.lastname: Willinek
authors.forename: Hans
authors.initials: H
authors.lastname: Schild
authors.forename: Peter
authors.initials: P
authors.lastname: Börnert
mesh_terms: D000328:Adult; D000368:Aged; 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 / Q000379:methods*; D008875:Middle Aged; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D051598:Whole Body Imaging / Q000379:methods*; D055815:Young Adult
keywords: 3T; B1 mapping; RF shimming; body MRI; parallel transmit