Institute of Neuroscience and Medicine (INM-1), Research Centre Jülich, Germany.
Institute of Neuroscience and Medicine (INM-1), Research Centre Jülich, Germany.
Neuroimage. 2015 May 1;111:464-75. doi: 10.1016/j.neuroimage.2015.02.020. Epub 2015 Feb 17.
3D Polarized Light Imaging (3D-PLI) is a neuroimaging technique that has opened up new avenues to study the complex architecture of nerve fibers in postmortem brains. The spatial orientations of the fibers are derived from birefringence measurements of unstained histological brain sections that are interpreted by a voxel-based analysis. This, however, implies that a single fiber orientation vector is obtained for each voxel and reflects the net effect of all comprised fibers. The mixture of various fiber orientations within an individual voxel is a priori not accessible by a standard 3D-PLI measurement. In order to better understand the effects of fiber mixture on the measured 3D-PLI signal and to improve the interpretation of real data, we have developed a simulation method referred to as SimPLI. By means of SimPLI, it is possible to reproduce the entire 3D-PLI analysis starting from synthetic fiber models in user-defined arrangements and ending with measurement-like tissue images. For the simulation, each synthetic fiber is considered as an optical retarder, i.e., multiple fibers within one voxel are described by multiple retarder elements. The investigation of different synthetic crossing fiber arrangements generated with SimPLI demonstrated that the derived fiber orientations are strongly influenced by the relative mixture of crossing fibers. In case of perpendicularly crossing fibers, for example, the derived fiber direction corresponds to the predominant fiber direction. The derived fiber inclination turned out to be not only influenced by myelin density but also systematically overestimated due to signal attenuation. Similar observations were made for synthetic models of optic chiasms of a human and a hooded seal which were opposed to experimental 3D-PLI data sets obtained from the chiasms of both species. Our study showed that SimPLI is a powerful method able to test hypotheses on the underlying fiber structure of brain tissue and, therefore, to improve the reliability of the extraction of nerve fiber orientations with 3D-PLI.
3D 偏振光成像（3D-PLI）是一种神经影像学技术，它为研究死后大脑中神经纤维的复杂结构开辟了新的途径。纤维的空间取向是从未经染色的组织脑切片的双折射测量中得出的，这些切片通过基于体素的分析进行解释。然而，这意味着为每个体素获得一个单一纤维取向向量，反映了所有包含纤维的净效应。单个体素内各种纤维取向的混合是标准 3D-PLI 测量无法预先获得的。为了更好地理解纤维混合物对测量的 3D-PLI 信号的影响，并改进对真实数据的解释，我们开发了一种称为 SimPLI 的模拟方法。通过 SimPLI，可以从用户定义排列的合成纤维模型开始，并以类似于测量的组织图像结束，从而重现整个 3D-PLI 分析。对于模拟，每个合成纤维都被视为一个光延迟器，即一个体素内的多个纤维由多个延迟器元件描述。SimPLI 生成的不同合成交叉纤维排列的研究表明，得出的纤维取向强烈受到交叉纤维相对混合的影响。例如，在垂直交叉的纤维情况下，得出的纤维方向对应于主要纤维方向。得出的纤维倾斜不仅受到髓鞘密度的影响，而且由于信号衰减而系统地高估。对人类和冠海豹视交叉的合成模型进行了类似的观察，这些模型与从两种物种的视交叉获得的实验 3D-PLI 数据集相反。我们的研究表明，SimPLI 是一种强大的方法，能够检验关于脑组织中纤维结构的假设，从而提高从 3D-PLI 提取神经纤维取向的可靠性。
