定量磁化率成像及其在脑白质纤维重建中的应用

磁敏感性反映了物质在外加磁场作用下的磁化程度，是物质的固有属性，可以用磁化率来度量。大脑磁共振成像中，髓鞘中分子的各向异性排列导致脑白质磁化率的各向异性，而最小反磁性方向表征纤维走行的主要方向。定量磁化率成像(QSM)通过求解关于感应磁场分布的病态逆问题，由测得的相位图像导出潜在的组织体磁化率的定量图，使得对组织的磁化率进行定量分析成为可能。本项目旨在应用医学图像处理和分析方法，研究QSM的几个关键性问题：相位图像的解缠绕，不均匀背景场的抑制，定量磁化率图的重建。在此基础上，借鉴扩散张量成像(DTI)的研究思路，探索磁化率张量成像(STI)及其脑白质纤维束示踪应用。将DTI中提取出的纤维方向信息与磁化率各向异性相结合，发挥DTI高角度分辨率和STI高空间分辨率的优势，实现更好的脑白质纤维成像。使之成为活体组织纤维束非入侵式检测的新方法，为神经纤维成像研究提供新的对比机制。

Magnetic susceptibility is defined as the magnetic response of a substance when it is placed in an external magnetic field. It is an inherent property of the substance. In brain MRI, the white matter susceptibility anisotropy likely arises from the anisotropic alignment of molecules in the myelin sheath. White matter fiber pathways are reconstructed by navigating through the vector field defined by the least diamagnetic susceptibility eigenvector. Quantitative susceptibility mapping (QSM) solves the ill-posed inverse source problem of the induced magnetic field distribution, yielding quantitative maps of the underlying tissue bulk magnetic susceptibility from the measured phase images. Using the medical image processing and analyzing methods, the project aims to solve the key problems in QSM, including phase unwrapping, background field removal and susceptibility map reconstruction. On this basis, susceptibility tensor imaging (STI) is exploited to investigate tracking of fiber pathways refer to diffusion tensor imaging (DTI) technique. In order to obtain high resolution in white matter fiber tractography, the fiber direction information extracted from DTI is combined with the susceptibility anisotropy, which takes advantages of both high angular resolution of DTI and high spatial resolution of STI. It may provide a new way for in vivo and nondestructive characterization of the three dimensional fiber architecture of anatomical organs and may also be valuable for the ongoing validation of nerve fiber imaging.