Three-Dimensional Patient-Tailored RF Pulses for Spin Echo Neuroimaging at 7 T

描述（由申请人提供）：本项目的目标是开发新的技术方法，以缓解限制超高场MRI的严重不均匀和患者依赖性RF传输（B+）场的影响，特别强调7 T下的自旋1回波和快速自旋回波（FSE）神经成像。我们的方法包括患者定制的单通道和并行激励的新技术，包括轨迹设计和脉冲设计算法，在7 T下的自旋回波采集中非常重要。这些方法对于实现SNR和分辨率的改善以及利用7 T MRI承诺的独特对比机制至关重要。它们能够在较低的临床场强下提高自旋回波MRI的质量，并在实际应用中具有广泛的影响。 从7 T中受益匪浅的三种成像方法是FSE采集，例如液体衰减反转恢复（FLAIR）、扩散张量成像（DTI）和血氧水平依赖（BOLD）功能MRI（fMRI）。FSE序列是中枢神经系统成像病理学的关键，而DTI和BOLD fMRI则彻底改变了结构和功能连接研究。然而，在低场，这些序列受到有限的SNR和空间分辨率（FSE和DTI）以及血管内污染（BOLD）的阻碍。在7T下，FSE和DTI数据可以获得更高的SNR和空间分辨率，Hahn自旋回波（HSE）BOLD fMRI可以获得更精确定位神经激活的功能信号。不幸的是，由于患者依赖的B+场不均匀性导致翻转角不均匀性，1目前7 T下自旋回波采集的质量受到严重限制。最近，已开发出患者定制的单通道和并行激励方法，以减轻B+不均匀性的影响，这些方法所需的基本硬件和软件正在成为7 T扫描仪的标准。然而，对于自旋回波采集所需的大顶角激励，这些方法的发展很少。该项目将解决发展中的这一差距。 我们将开发新的三维激励k空间轨迹的大倾角切片选择性激励，和新的 脉冲设计算法，以使用这些轨迹来设计激励和重聚焦脉冲。我们将改进目前占主导地位的“辐条”轨迹，这是一般不适合大的尖端角度切片选择性激励由于高功率的辐条RF脉冲。我们假设存在替代轨迹，可以提供更高的频谱带宽，最小限度地受整形影响的空间编码能力，以减少RF功率沉积，并在选择切片厚度和锐度方面具有更高的灵活性。我们的脉冲设计方法将使激励和重聚焦脉冲的联合设计能够满足自旋回波成像序列的独特需求，不仅将提高脉冲的性能，而且还将增加其功能。我们将使用特定于这些模式的数据和图像质量指标，评估我们的患者定制脉冲在FSE、DTI和HSE BOLD fMRI采集中的性能。

DESCRIPTION (provided by applicant): The goal of this project is to develop new technical approaches to mitigate the effects of severely inhomogeneous and patient-dependent RF transmission (B+) fields that limit ultra-high field MRI, with particular emphasis on spin 1 echo and fast spin echo (FSE) neuroimaging at 7 Tesla (T). Our approaches comprise new techniques for patient- tailored single-channel and parallel excitation, including trajectory designs and pulse design algorithms, in spin echo acquisitions of high importance at 7 T. These methods will be essential to realizing the improvements in SNR and resolution, and exploiting the distinct contrast mechanisms that 7 T MRI promises. They stand to enhance the quality of spin echo MRI at lower clinical field strengths, and have broad impact in practical applications. Three imaging methods that stand to benefit greatly from 7 T are FSE acquisitions such as fluid-attenuated inversion recovery (FLAIR), diffusion tensor imaging (DTI), and blood oxygenation level-dependent (BOLD) functional MRI (fMRI). FSE sequences are key for imaging pathologies of the central nervous system, while DTI and BOLD fMRI have revolutionized structural and functional connectivity studies. However, at low field these sequences are hampered by limited SNR and spatial resolution (FSE and DTI), and intravascular contamination (BOLD). At 7 T, FSE and DTI data can be acquired with higher SNR and spatial resolution, and Hahn spin echo (HSE) BOLD fMRI can be used to obtain functional signals that can much more accurately localize neural activation. Unfortunately, because patient-dependent B+ field inhomogeneity causes flip angle inhomogeneity, 1 the quality of spin echo acquisitions at 7 T is currently severely limited. Recently, patient-tailored single-channel and parallel excitation methods have been developed to mitigate the effect of B+ inhomogeneity, and the basic 1 hardware and software required by these methods is becoming standard on 7 T scanners. However, there has been little development of these methods for the large-tip-angle excitations required by spin echo acquisitions. This project will address this gap in development. We will develop new three-dimensional excitation k-space trajectories for large-tip-angle slice-selective excitation, and new pulse design algorithms to design excitation and refocusing pulses using those trajectories. We will improve upon the current dominant 'spokes' trajectory, which is generally unsuitable for large-tip-angle slice-selective excitations due to the high power of spokes RF pulses. We hypothesize that alternative trajectories exist that can provide improved spatial encoding capabilities with higher spectral bandwidths that are minimally impacted by reshaping to reduce RF power deposition, and with improved flexibility in selecting slice thickness and sharpness. Our pulse design methods will enable the joint design of excitation and refocusing pulses to meet the unique demands of spin echo imaging sequences, and will not only improve the pulses' performance, but will also add to their capabilities. We will evaluate the performance of our patient-tailored pulses in FSE, DTI and HSE BOLD fMRI acquisitions, using data and image quality metrics specific to those modalities.