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。列出的机构是 中心，不一定是研究者的机构。 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。 目标1：脉冲对B1不均匀性具有免疫力。 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。 目的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。 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。 新的具体目标4. 从特定目标1产生的脉冲产生的SAR和MT效应（特定目标3）比它们所取代的脉冲多得多。 为了不延长MRI实验的采集时间，必须在每个RF脉冲之后收集更大量的k空间数据。 我们建议使用螺旋梯度读出来实现这一点。 然而，螺旋梯度波形需要针对梯度失真的校正（例如，涡流）和不正确的梯度定时以避免图像模糊。 此外，需要对样品谐振偏移进行校正。 具体目标4建议开发螺旋梯度模拟，能够添加梯度缺陷和共振偏移，以研究梯度缺陷和共振偏移需要校正的程度，以防止明显的图像模糊。 最初的模拟将针对3D MPWE实验。 新的具体目标5. 借助具体目标4的模拟，我们建议评估并可能扩展最近公布的方法，以改善梯度缺陷和共振偏移的影响。 这包括测量梯度缺陷和样品共振偏移的方法。 我们预计校正算法最终将不得不迁移到CIND的多处理器计算机，以使去模糊图像能够及时生成。

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. 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. 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. 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. New specific aim 4. The pulses generated from specific aim 1 generate considerably more SAR and MT effects (specific aim 3) than the pulses they replace. In order to not prolong the acquisition time for the MRI experiment, a greater amount of k-space data must be collected following each RF pulse. We propose to make use of spiral gradient readouts to accomplish this. However, spiral gradient waveforms require corrections for gradient infidelity (e.g., eddy currents) and for incorrect gradient timings to avoid image blurring. In addition, corrections for sample resonance offsets are required. Specific aim 4 proposes to develop simulations for spiral gradients, with the ability to add gradient imperfections and resonance offsets, to investigate the degree to which gradient imperfections and resonance offsets need to be corrected to prevent significant image blurring. The initial simulations will be aimed at 3D MPRAGE experiments. New specific aim 5. With the aid of the simulations of specific aim 4, we propose to evaluate and possibly extend recently published methods to ameliorate the effects of gradient imperfections and resonance offsets. This includes methods to measure gradient imperfections and sample resonance offsets. We expect the correction algorithms will eventually have to be migrated to the CIND's multiprocessor computer to enable de-blurred images to be generated in a timely manner.