New phase-based B1 mapping method using two-dimensional spin-echo imaging with hyperbolic secant pulses.
OBJECTIVE
To propose a new phase-based B1-mapping method that exploits phase information created by hyperbolic secant (HS) pulses in conventional 2D spin-echo imaging.


METHODS
In this B1-mapping method, HS pulses are used to accomplish π/2 excitation and π refocusing in standard multislice spin-echo imaging. When setting the ratio of pulse lengths of the π/2 and π HS pulses to 2:1, the spin-echo phase is independent of offset frequency and varies as a function of B1 strength. To eliminate undesired phase accumulations induced by unknown factors other than the B1 strength, two spin-echo images are acquired using HS pulses applied with opposite frequency-sweep directions, and the resulting phase images are subtracted from each other. To demonstrate the performance of the proposed method, phantom and in vivo experiments were performed using a surface coil and a volume coil.


RESULTS
The B1 maps obtained by using the proposed method were in accordance with the B1 maps obtained using previous methods in both phantom and in vivo experiments.


CONCLUSIONS
The proposed method is easy to implement without any sequence modification, is insensitive to B0 inhomogeneity and chemical shift, and is robust in a reasonably wide range of B1 field strength.
Department of Electrical Engineering, Korean Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea.
Yoojin
Y
Lee
Department of Electrical Engineering, Korean Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea.
Yeji
Y
Han
Department of Electrical Engineering, Korean Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea.
HyunWook
H
Park
Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Japan.
Hidehiro
H
Watanabe
The Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
Michael
M
Garwood
School of Biomedical Engineering, College of Biomedical and Health Science, Research Institute of Biomedical Engineering, Konkuk University, Chungju, Korea (ROK).
Jang-Yeon
JY
Park
D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D017352:Echo-Planar Imaging / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D009716:Numerical Analysis, Computer-Assisted; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D013113:Spin Labels
D013113:Spin Labels
B1 mapping; RF field mapping; flip-angle mapping; frequency-modulation pulse; hyperbolic secant pulse; spin-echo imaging