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的一个 由NIH/NCRR资助的中心赠款提供的资源。子项目和 研究者（PI）可能从另一个NIH来源获得主要资金， 因此可以在其他CRISP条目中表示。所列机构为 研究中心，而研究中心不一定是研究者所在的机构。 简介： 绝热脉冲是有用的，在实现均匀的激发分布在B1的不均匀性的存在。 BIR-4脉冲已被证明可以实现用户可选翻转角的绝热激发。 然而，这些脉冲是非选择性的。 BIR-4脉冲设计已经通过使用梯度调制技术来扩展，以创建切片选择性绝热激发脉冲。 不幸的是，这些技术需要高RF幅度，通常高于大多数商业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，实现绝热切片选择，显著降低RF峰值功率要求。 我们的切片选择性可调翻转AdiaBatic低峰值功率激发（稳定）脉冲由振荡梯度与BIR-4样的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来准确地采样。 在第一和第二段之间以及第三和第四段之间引入相位不连续性以产生90 °翻转角。 所得光谱绝热激发脉冲为21 ms长，光谱带宽约为80 Hz。 然后对脉冲进行二次采样，选择子波瓣的数量作为绝热性和最小切片厚度之间的权衡。最后的稳定脉冲由33个0.64 ms的子脉冲组成，由绝热包络缩放。 将STABLE脉冲集成到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。在低于标称峰值B1约45%时达到绝热阈值。在绝热阈值以上，激发截面在很大程度上保持不变。

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.