Rapid B 1 mapping using orthogonal, equal‐amplitude radio‐frequency pulses

We present a new phase‐based method for mapping the amplitude of the radio‐frequency field ( B 1 ) of a transmitter coil in three‐dimension. This method exploits the noncommutation relation between rotations about orthogonal axes. Our implementation of this principle in the current work results in a simple relation between the phase of the final magnetization and the flip angle (FA). In this study, we focus on FAs less than 90°. Our method is rapid and easy to implement compared with the existing B 1 mapping schemes. The mapping sequence can be simply obtained by adding to a regular three‐dimensional gradient‐echo sequence a magnetization preparation radio‐frequency pulse of the same FA but orthogonal in phase to the excitation radio‐frequency pulse. This method is demonstrated capable of generating reliable maps of the B 1 field within 1 min using FAs no larger than 60°. We show that it is robust against T 1 , small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B 1 mapping in situations (e.g., long‐ T 1 and hyperpolarized‐gas imaging) where magnitude‐based methods are not readily applicable. A noise calculation of the FA map using this method is also presented. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.