On the Steady-State Properties of Actual Flip Angle Imaging (AFI)
On the Steady-State Properties of Actual Flip Angle Imaging (AFI)
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DOI：
10.1002/mrm.21592
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发表时间：
2009-01-01
影响因子：
3.3
通讯作者：
Nehrke, Kay
中科院分区：
文献类型：
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作者：
Nehrke, Kay
AFI（实际翻转角成像）代表了一种通过测量有效翻转角的空间变化来映射B（1）发射场的有趣方法。然而，该技术的准确性依赖于横向磁化的充分破坏。在目前的工作中，配置理论被用来开发一个合适的RF和梯度扰流计划的AFI技术，使序列对非共振鲁棒，而不需要大的扰流梯度。此外，进行数值模拟来预测稳态信号，并因此预测作为序列和组织参数的函数的AFI技术的准确性。结果表明，该序列的破坏特性主要由射频脉冲的相移增量φ和序列梯度不平衡引起的扩散灵敏度决定。如果选择了一个有利的phi值，则可以在合理的组织参数范围内实现充分的破坏，并且对于中等破坏梯度可以实现翻转角。体模和在体头部成像实验显示出与理论预测的良好一致性，表明该方法的适当操作范围可以由理论可靠地预测。Magn Reson Med 61：84-92，2009. (C)2008 Wiley-Liss，Inc.
AFI (actual flip angle imaging) represents an interesting approach to map the B(1) transmit fields by measuring the spatial variations of the effective flip angle. However, the accuracy of the technique relies on the adequate spoiling of transverse magnetization. In the present work configuration theory was employed to develop a proper RF and gradient spoiling scheme for the AFI technique, making the sequence robust against off-resonance without the need of large spoiling gradients. Furthermore, numerical simulations were performed to predict the steady-state signals and, hence, the accuracy of the AFI technique as a function of the sequence and tissue parameters. It is shown that the spoiling properties of the sequence are mainly defined by the phase shift increment phi of the RF pulses and the diffusion sensitivity resulting from the unbalanced gradients of the sequence. Adequate spoiling may be achieved for a reasonable range of tissue parameters and flip angles for moderate spoiling gradients if a favorable value for phi is chosen. Phantom and in vivo head imaging experiments show an excellent agreement with the theoretical predictions, indicating that the proper operating range of the approach may be reliably predicted by the theory. Magn Reson Med 61:84-92, 2009. (C) 2008 Wiley-Liss, Inc.