RADIOFREQUENCE (RF) PULSE DEVELOPMENT AND DESIGN FOR HIGH FIELD MRI

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来源获得了主要资金， 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。 概述：在高场强下改善MRI结果的承诺受到在高场强下遇到的困难的影响，包括：i）由于高场强下固有的不均匀B1场而导致的不均匀激励。 通常，非均匀激励产生非均匀组织对比度，尽管也可能产生其它有害影响。 ii）大的磁化率梯度，除非使用大的切片选择梯度，否则这会使切片位置失真。 然而，高场系统上可用的有限RF功率严重限制了可用于T2加权图像的梯度强度。具体目标提出了两种新的RF脉冲设计的进一步发展和改进，以改善这些有害影响。 此外，进一步开发的软件模拟MRI实验提出了这些新的RF脉冲到适当重新设计的MRI实验，以帮助有效实施。 具体目标1：对B1不均匀性具有免疫力的脉冲。 新的B1不敏感设计基于沿旋转坐标系中的沿着不同轴施加的矩形脉冲的优化级联，其中优化针对均匀尖端和对谐振偏移的免疫力。 该设计集中于激励脉冲，但包括将该方法扩展到自旋回波和反转脉冲。 具体目标2：降低峰值电压自旋回波频率选择性脉冲。 新的、降低的峰值电压设计方法由具有交替符号梯度的常规频率选择性脉冲的级联组成。该设计包括并入自旋回波脉冲中的扰流器梯度，以缩短脉冲的总长度。 这些脉冲在非均匀B1场的操作也被认为是。 具体目标3：进一步开发MRI模拟软件，包括“意外”磁化转移（MT）效应。 进一步的开发建立在已经为MP MRI实验开发的软件基础上，并将包括扩展相位图（EPG）算法，以覆盖广泛的MRI实验。 这些模拟将有助于有效地实现新的RF脉冲，并避免有害的MT效应。 预计这些模拟的进一步用途将用于优化4.0特斯拉的MRI序列。

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. Overview: The promise of improved MRI results at high field strength is compromised by the difficulties encountered at high field, including: i) Non-uniform excitation, due to the non-uniform B1 field inherent at high field. Typically, the non-uniform excitation produces non-uniform tissue contrast, although other deleterious effects can be produced as well. ii) Large susceptibility gradients, which can distort slice positions unless large slice-select gradients are used. However, the limited RF power available on high field systems severely limits the gradient strength that can be used for T2-weighted images. The specific aims propose the further development and refinement of two new RF pulse designs to ameliorate these deleterious effects. In addition, further development of software for simulating MRI experiments is proposed to aid in effective implementation of these new RF pulses into suitably re-designed MRI experiments. Specific aim 1: Pulses with immunity to B1 inhomogeneity. The new B1-insensitive design is based on optimized concatenations of rectangular pulses applied along different axes in the rotating frame, where the optimization is for both uniform tip and immunity to resonance offset. The design focuses on excitation pulses, but includes extension of the method to spin echo and inversion pulses. Specific aim 2: Lowered peak voltage spin echo frequency-selective pulses. The new, lowered peak voltage design method consists of concatenation of conventional, frequency-selective pulses with gradients of alternating sign. The design includes spoiler gradients incorporated into the spin echo pulse to shorten the overall length of the pulse. Operation of these pulses in inhomogeneous B1 fields is also considered. Specific aim 3: Further development of MRI simulation software with inclusion of "inadvertent" magnetization transfer (MT) effects. The further development builds on software already developed for MP RAGE MRI experiments, and will include extended phase graph (EPG) algorithms to cover a wide range of MRI experiments. These simulations will aid in effective implementation of the new RF pulses, and avoid deleterious MT effects. A further use of these simulations is expected to be in the optimization of MRI sequences for 4.0 Tesla.