SLICE-SELECTIVE TUNABLE-FLIP ADIABATIC LOW PEAK-POWER EXCITATION (STABLE) PULSE

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 导言： 在存在B1不均匀的情况下，绝热脉冲对于获得均匀的激发轮廓是有用的。BIR-4脉冲已被证明可以实现绝热激发，并具有用户可选择的翻转角度。然而，这些脉冲是非选择性的。BIR-4脉冲设计已通过使用梯度调制技术进行了扩展，以创建切片选择性绝热激励脉冲。不幸的是，这些技术需要较高的射频幅度，通常高于大多数商用扫描仪上可用的放大器的输出。在这项工作中，我们开发了一种替代方法，实现了绝热切片选择，并显著降低了射频峰值功率要求。我们的切片选择性可调谐翻转绝热低峰功率激励(稳定)脉冲由振荡梯度和类似BIR-4的RF包络组成，该包络由许多短空间子脉冲采样，以实现空间选择性。 方法和讨论： 使用SECH/TANH幅度/频率调制函数，使得幅度和相位变化足够慢，以通过所选数量的子脉冲进行准确采样。在第一和第二段之间以及第三和第四段之间引入了相位不连续，以产生90个翻转角。合成的光谱绝热激发脉冲长21ms，光谱带宽约为80赫兹。然后用作为绝热性和最小切片厚度之间权衡的子瓣数目对脉冲进行二次采样。最终稳定的脉冲由33，0.64毫秒的亚脉冲组成，按绝热包络进行缩放。将稳定的脉冲积分到GRE序列中，以将其与具有常规激励脉冲的标准序列进行比较。绝热阈值比标称峰值B1低45%左右。在绝热阈值以上，激发截面基本保持不变。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: Adiabatic pulses are useful in achieving uniform excitation profiles in the presence of B1 inhomogeneity. BIR-4 pulses have been shown to achieve adiabatic excitation with user-selectable flip angles. However, these pulses are nonselective. The BIR-4 pulse design has been extended through the use of gradient modulation techniques to create slice-selective adiabatic excitation pulses. Unfortunately, these techniques require high RF amplitude, typically above the output of the amplifiers available on most commercial scanners. In this work, we developed an alternative approach that achieves adiabatic slice selection with significantly lower RF peak power requirements. Our Slice-selective Tunable-flip AdiaBatic Low peak-power Excitation (STABLE) pulse consists of an oscillating gradient in conjunction with a BIR-4- like RF envelope that is sampled by many short spatial subpulses in order to achieve spatial selectivity. Methods and Discussion: A sech/tanh amplitude/frequency modulation function was used so that the amplitude and phase variations were sufficiently slow to be accurately sampled by the chosen number of subpulses. A phase discontinuity was introduced between the first and second segments and between the third and fourth segments to produce a 90 flip angle. The resultant spectral adiabatic excitation pulse was 21 ms long and had a spectral bandwidth of approximately 80 Hz. The pulse was then subsampled with the number of sublobes chosen as a trade-off between adiabaticity and minimum slice thickness. The final STABLE pulse was comprised of 33, 0.64 ms subpulses scaled by the adiabatic envelope. The STABLE pulse was integrated into a GRE sequence to compare it to a standard sequence with a conventional excitation pulse. Adiabatic threshold is reached at around 45% below nominal peak B1. Above the adiabatic threshold, the excited cross section remains largely invariant.