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

这个子项目是利用资源的许多研究子项目之一。 由NIH/NCRR资助的中心拨款提供。对子项目的主要支持 子项目的首席调查员可能是由其他来源提供的， 包括美国国立卫生研究院的其他来源。为子项目列出的总成本可能 表示该子项目使用的中心基础设施的估计数量， 不是由NCRR赠款提供给次级项目或次级项目工作人员的直接资金。 概述：在高场强下改善MRI结果的前景受到高场下遇到的困难的影响，包括：i)由于高场下固有的非均匀B1场，导致激发不均匀。通常，不均匀的激发会产生不均匀的组织对比度，尽管也可能产生其他有害的影响。Ii)较大的磁化率梯度，除非使用较大的切片选择梯度，否则可能会扭曲切片位置。然而，高场系统可用的有限射频功率严重限制了可用于T2加权图像的梯度强度。具体目的是建议进一步开发和改进两种新的射频脉冲设计，以改善这些有害影响。此外，还建议进一步开发用于模拟MRI实验的软件，以帮助将这些新的RF脉冲有效地实施到适当重新设计的MRI实验中。 具体目标1：对B1不均质性免疫的脉冲。新的B1不敏感设计基于旋转框架中沿不同轴施加的矩形脉冲的优化串联，其中优化是为了实现均匀的尖端和对共振偏移的免疫力。该设计侧重于激励脉冲，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脉冲。这种新的、更低的峰值电压设计方法包括将传统的频率选择性脉冲与交流符号的梯度串联起来。该设计包括包含在自旋回波脉冲中的扰流梯度，以缩短脉冲的总长度。还考虑了这些脉冲在非均匀B1场中的操作。 具体目标3：进一步开发包含“无意”磁化传递(MT)效应的MRI模拟软件。进一步的开发建立在已经为MP RAGE MRI实验开发的软件的基础上，并将包括扩展相图(EPG)算法，以涵盖广泛的MRI实验。这些模拟将有助于有效地实施新的射频脉冲，并避免有害的MT效应。这些模拟的进一步应用预计将用于4.0特斯拉的MRI序列的优化。 新的特定目标4.从特定目标1产生的脉冲比它们所替换的脉冲产生更多的SAR和MT效果(特定目标3)。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数来实现这一点。然而，螺旋梯度波形需要校正梯度不保真度(例如，涡流)和不正确的梯度计时，以避免图像模糊。此外，还需要对样品共振偏移量进行校正。具体目标4建议开发具有添加梯度缺陷和共振偏移量的能力的螺旋梯度的模拟，以调查需要校正梯度缺陷和共振偏移量以防止图像显著模糊的程度。最初的模拟将针对3D MPRAGE实验。 新的特定目标5.借助于特定目标4的模拟，我们建议评估并可能扩展最近发表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量的方法。我们预计，校正算法最终将不得不移植到CIND的多处理器计算机上，以便能够及时生成去模糊图像。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. 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. 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.