title: Iterative multi-channel radio frequency pulse calibration with improving B1 field uniformity in high field MRI.
abstract: BACKGROUND
  In high field MRI capable of multi-channel radio frequency (RF) transmission, B 1 shimming is a time-consuming job because conventional B 1 shimming techniques require B 1 mapping for each channel. After acquiring the complex-numbered B 1 field maps, the optimal amplitude and phase of the driving RF pulse are determined for each channel to maximize the B 1 field uniformity in conventional B 1 shimming. However, time-consuming B 1 shimming procedures at the pre-scan may not be tolerated in the clinical imaging in which patient throughput is one of the important factors.
  
  
  METHODS
  To avoid the time-consuming B 1 mapping, the first spin echo and the stimulated echo were repeatedly acquired in the slice-selective stimulated echo sequence without imaging gradients. A cost function of the amplitudes and phases of the driving RF pulse for every channel was defined in a way that the ratio between the spin echo and stimulated echo amplitudes rapidly converged to √ 2. The amplitude and phase of the driving RF pulse were iteratively modified over the repeating RF pulse sequence so that the cost function was minimized.
  
  
  RESULTS
  From the finite-difference-time-domain (FDTD) electromagnetic field simulations with a human body model placed in a birdcage coil operating at 3 T, it was observed that the RF pulse calibration with iterative cost function minimization can give improvement of B1 field uniformity as well as flip-angle calibration. The experiments at 3 T also showed improvement of RF field uniformity in the phantom imaging studies.
  
  
  CONCLUSIONS
  Since the proposed RF pulse calibration is not based on B 1 mapping, the RF pulse calibration time could be much shorter than the B 1-mapping based methods. The proposed method is expected to be a practical substitute for the B 1-mapping-based B 1 shimming methods when long pre-scan time is not tolerable.
authors.affiliation: Department of Biomedical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, 446-701, Korea. theasdwmove@gmail.com.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hernandez
authors.affiliation: Department of Biomedical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, 446-701, Korea. mhcho@khu.ac.kr.
authors.forename: Min Hyoung
authors.initials: MH
authors.lastname: Cho
authors.affiliation: Department of Biomedical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do, 446-701, Korea. sylee01@khu.ac.kr.
authors.forename: Soo Yeol
authors.initials: SY
authors.lastname: Lee
mesh_terms: D000465:Algorithms; D002138:Calibration; D006801:Humans; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011846:Radio Waves