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

超高场单次激发回波平面成像(EPI)可以实现高空间分辨率的快速脑成像。它非常有利于功能磁共振成像(FMRI)和弥散加权成像(DWI)的研究，用于测量脑功能和表征脑组织微结构的信息，并作为一种标准方法使用。然而，EPI的一个众所周知的问题是强烈的敏感性引起的几何失真，特别是在超高频，如7T。此外，在DWI中，EPI中的失真由于涡流而变化，这取决于扩散编码梯度的方向，并且成为一个主要的挑战。在这个项目的第一个资助期(2012-2015)，我们成功地实现了一个用于功能磁共振成像的在线失真校正程序包，并将该方法扩展到了DWI。由于EPI的空间分辨率有限，在大脑皮层区域出现强烈的失真和信号丢失。因此，到目前为止，弥散加权成像几乎只应用于白质的结构和病理研究。作为标准EPI的替代方案，我们开发了一种新的无失真技术，使活体人脑DWI具有前所未有的高空间分辨率和保真度。使用我们的新方法，虽然测量时间延长了，但灰质微结构变得非侵入性地可获得。在这个项目的继续中，我们的目标包括：i)全面开发一种全面的(即磁化率和涡流引起的)失真校正包及其标准DWI的实现，ii)进一步研究我们用于超高分辨率DWI的新技术，以通过抑制流动和运动引起的伪影来缩短扫描时间并提高图像保真度，以及iii)使用超高分辨率扩散图像详细描述灰质扩散的活体特征。这将首次对整个人脑在体内的灰质扩散进行全面的表征，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.