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

描述(由申请人提供)：该项目的目标是开发新的技术方法，以减轻严重不均匀和患者依赖的射频传输(B+)场的影响，这些场限制了超高场磁共振成像，特别是7特斯拉(T)的自旋1回波和快速自旋回波(FSE)神经成像。我们的方法包括针对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，用于7T高重要性自旋回波捕获。这些方法对于实现SNR和分辨率的提高以及利用7T MRI承诺的独特对比机制至关重要。它们将在较低的临床场强下提高自旋回波MRI的质量，并在实际应用中产生广泛的影响。从7T中获益最大的三种成像方法是FSE成像，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)功能磁共振成像(FMRI)。FSE序列是中枢神经系统成像病理学的关键，而DTI和BOLD功能磁共振成像已经彻底改变了结构和功能连接的研究。然而，在低场下，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时，FSE和DTI数据可以获得更高的SNR和空间分辨率，而Hahn自旋回波(HSE)BOLD fMRI可以获得更准确地定位神经激活的功能信号。不幸的是，由于患者依赖的B+场不均匀导致翻转角度不均匀，1目前在7T处的自旋回波采集质量受到严重限制。最近，为减轻B+不均质性的影响，已开发出为患者量身定制的单通道和并行激励方法，这些方法所需的基本硬件和软件正在成为7T扫描仪的标准。然而，对于自旋回波捕获所需的大尖角激发，这些方法的发展很少。该项目将解决发展中的这一差距。我们将开发用于大尖角切片选择激发的新的三维激励k空间轨迹，以及新的 使用这些轨迹设计激励和重新聚焦脉冲的脉冲设计算法。我们将改进目前占主导地位的辐条轨迹，由于辐条RF脉冲的高功率，该轨迹通常不适合大尖角切片选择性激励。我们假设存在另一种轨迹，它们可以提供改进的空间编码能力，具有更高的频谱带宽，受重塑的影响最小，以减少射频功率沉积，并在选择切片厚度和清晰度方面具有更高的灵活性。我们的脉冲设计方法将使激励脉冲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，还将增加其能力。我们将使用特定于FSE、DTI和HSE BOLD fMRI采集的数据和图像质量指标，评估患者定制脉冲在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.