Highly Accelerated Distortion-Free Diffusion-WeightedMR Imaging at Ultra High Field (7T): Gray Matter Characterization

超高场（UHF）下的单次激发回波平面成像（EPI）能够实现具有高空间分辨率的快速脑成像。功能性磁共振成像（fMRI）和弥散加权成像（DWI）研究非常有利于测量脑功能和表征有关脑组织微结构的信息，并用作标准方法。然而，EPI中的一个众所周知的问题是强磁化率引起的几何失真，特别是在UHF例如7 T处。此外，在DWI中，EPI中的失真由于取决于扩散编码梯度的方向的涡流而变化，并且成为主要挑战。在该项目的第一个资助期（2012-2015），我们成功地实现了一个在线失真校正包的功能磁共振成像和扩展的方法DWI。由于EPI有限的空间分辨率，在皮层区域出现强烈的失真和信号丢失。因此，扩散加权成像一直几乎完全应用于白色物质（WM）的结构和病理的调查到现在。作为标准EPI的替代方案，我们开发了一种新的无失真技术，可以实现前所未有的高空间分辨率和保真度的活体人脑DWI。使用我们的新方法，灰质微结构变得无创访问，虽然测量时间延长。在这个项目的继续，我们的目标包括i）全面发展一个全面的（即磁化率和涡流诱导的）失真校正包及其在标准DWI中的实现，ii）进一步研究我们用于极高分辨率DWI的新技术，以通过抑制流动和运动诱导的伪影来减少扫描时间并提高图像保真度，和iii）使用非常高分辨率的扩散图像的灰质扩散的详细体内表征。这将首次在体内对整个人脑中的灰质扩散进行全面表征，并为未来的科学和临床应用提供参考。

Single-shot echo-planar imaging (EPI) at ultra-high field (UHF) enables fast brain imaging with high spatial resolution. It is very beneficial for functional magnetic resonance imaging (fMRI) and diffusion weighted imaging (DWI) studies to measure brain function and to characterize information about brain tissue microstructure and used as a standard method. However, a well-known problem in EPI is strong susceptibility-induced geometric distortion, especially at UHF such as 7T. In DWI, in addition, distortions in EPI vary due to eddy-currents depending on the direction of diffusion encoding gradients and become a major challenge. During the first funding period of this project (2012-2015) we have successfully implemented an online distortion correction package for fMRI and extended the method to DWI. Due to limited spatial resolution of EPI, strong distortions, and signal loss appear in cortical regions. Therefore, DWI has been applied almost exclusively to the investigation of structure and pathology in white matter (WM) up to now. As an alternative to standard EPI, we developed a novel distortion-free technique that allows unprecedented high spatial resolution and fidelity of in vivo human brain DWI. With our new method, gray matter microstructure becomes accessible non-invasively although measurement times are prolonged. In the continuation of this project, our goal covers i) full development of a comprehensive (i.e. susceptibility- and eddy-current-induced) distortion correction package and its implementation for standard DWI, ii) further research on our new technique for very high-resolution DWI to reduce scan time and improve the image fidelity by suppressing flow- and motion-induced artifacts, and iii) detailed in vivo characterization of gray matter diffusion using very high-resolution diffusion images. This will allow comprehensive characterization of gray matter diffusion in the entire human brain in vivo for the first time and serve as a reference for future scientific and clinical applications.