Rosenfeld D, Panfil SL, Zur Y
School of Physics and Astronomy, Tel-Aviv University, Tel Aviv, 69978, Israel.
J Magn Reson. 1997 Dec;129(2):115-24. doi: 10.1006/jmre.1997.1263.
Adiabatic RF pulses play an important role in spin inversion due to their robust behavior in the presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of the Beff vector and the rate of motion along it. In this paper, we describe a method by which optimized modulation functions can be constructed to render insensitivity to B1 inhomogeneity over a predetermined B1 range and over a wide band of frequencies. This is accomplished by requiring that the optimized pulse fulfill the adiabatic condition over this range of B1 inhomogeneity and over the desired frequency band for the complete duration of the pulse. A trajectory similar to the well-known sech/tanh adiabatic pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along this trajectory. The optimized pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be traded off against the inverted bandwidth. Two design examples, including experimental results, demonstrate the superiority of the optimized pulses over the conventional sech/tanh pulse: in the first example, a large frequency band is to be inverted using a weak RF amplitude in a short time. In the second example, a pulse with a very sharp transition is required. Copyright 1997 Academic Press. Copyright 1997Academic Press
绝热射频脉冲在自旋反转中起着重要作用，因为它们在存在非均匀射频场时具有稳健的行为。这些脉冲的特征在于有效磁场（(B_{eff})）矢量尖端扫过的轨迹以及沿该轨迹的运动速率。在本文中，我们描述了一种方法，通过该方法可以构建优化的调制函数，以使在预定的(B_1)范围内和宽频带上对(B_1)不均匀性不敏感。这是通过要求优化后的脉冲在该(B_1)不均匀性范围内以及在脉冲的整个持续时间内所需的频带上满足绝热条件来实现的。使用了与众所周知的双曲正割/双曲正切绝热脉冲类似的轨迹，即半椭圆。优化过程通过优化沿此轨迹的运动速率来改善切片轮廓。优化后的脉冲可以根据特定的设计要求进行定制；特别是，可以在过渡锐度和反转带宽之间进行权衡。两个设计示例，包括实验结果，证明了优化脉冲相对于传统双曲正割/双曲正切脉冲的优越性：在第一个示例中，要在短时间内使用弱射频幅度反转一个大频带。在第二个示例中，需要一个具有非常尖锐过渡的脉冲。版权所有1997年学术出版社。版权所有1997年学术出版社
