Mechanical Engineering and Materials Science, Washington University, St. Louis, Missouri 63130, USA.
Magn Reson Med. 2012 Mar;67(3):718-23. doi: 10.1002/mrm.23051. Epub 2011 Jun 28.
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.
我们提出了一种新的基于相位的方法，用于三维映射发射器线圈的射频场（B(1)）的幅度。该方法利用了正交轴旋转之间的非交换关系。我们在当前工作中实现这一原理的结果是最终磁化强度的相位与翻转角（FA）之间的简单关系。在这项研究中，我们关注的是小于 90°的 FA。与现有的 B(1)映射方案相比，我们的方法快速且易于实现。映射序列可以通过向常规的三维梯度回波序列添加具有相同 FA 但与激励射频脉冲相位正交的磁化准备射频脉冲来简单地获得。该方法能够在不超过 60°的 FA 下在 1 分钟内生成可靠的 B(1)场图。我们表明它对 T(1)、小化学位移和轻微的背景非均匀性具有鲁棒性。该方法特别适用于难以应用基于幅度的方法的情况（例如，长 T(1)和极化气体成像）。还提出了使用该方法对 FA 图进行噪声计算的方法。
