Unraveling the structure and function of the brain requires a detailed knowledge about the neuronal connections, , the spatial} architecture of the nerve fibers. One of the most powerful histological methods to reconstruct the three-dimensional nerve fiber pathways is 3D polarized light imaging (3D-PLI). The technique measures the birefringence of histological brain sections and allows to} derive the spatial fiber orientations of whole human brain sections with micrometer resolution. However, the technique can only derive a single fiber orientation for each measured tissue voxel even if it is composed of fibers with different orientations, so that in-plane crossing fibers are misinterpreted as out-of-plane fibers. When generating a detailed model of the three-dimensional nerve fiber architecture in the brain, a correct} detection and interpretation of nerve fiber crossings is crucial.} Here, we show how light scattering in transmission microscopy measurements can be leveraged to identify nerve} fiber crossings in 3D-PLI data and demonstrate that measurements of the scattering pattern} can resolve the substructure of brain tissue like the crossing angle of the nerve fibers.} For this purpose, we developed a simulation framework that allows to study transmission microscopy measurements –} in particular light scattering –} on large-scale complex fiber structures like brain tissue}, using finite-difference time-domain (FDTD) simulations and high performance computing.} The simulations were not only used to model and explain experimental observations, but also to develop new analysis methods and measurement techniques.} We demonstrate in various} experimental studies on brain sections from different species (rodent, monkey, human) and in FDTD simulations} that the polarization-independent transmitted light intensity (transmittance) decreases significantly (by more than 50%) with increasing out-of-plane angle of the nerve fibers and that it is mostly independent of the in-plane crossing angle. This enables us to distinguish} regions with low fiber density and in-plane crossing fibers from regions with out-of-plane fibers,} solving a major problem in 3D-PLI and allowing for} a much better reconstruction of the complex nerve fiber architecture in the brain. Finally, we show that light scattering (oblique illumination)} in the visible spectrum reveals the underlying structure of brain tissue like the crossing angle of the nerve fibers with micrometer resolution}, enabling an even more detailed reconstruction of nerve fiber crossings in the brain and opening up new fields of research.

中文翻译：

---

使用光散射测量和时域有限差分模拟，以高分辨率重构大脑中的3D神经纤维结构和交叉。

弄清大脑的结构和功能需要有关神经元连接的详细知识，即神经纤维的空间结构。重建三维神经纤维通路的最强大的组织学方法之一是3D偏振光成像（3D-PLI）。该技术测量组织学脑部切片的双折射，并允许以微米分辨率推导整个人脑部切片的空间纤维取向。但是，即使该技术由具有不同方向的纤维组成，该技术也只能为每个测量的组织体素导出单个纤维方向，因此，面内交叉纤维会被误解为面外纤维。当生成大脑中三维神经纤维结构的详细模型时，}我们在来自不同物种（啮齿动物，猴子，人类）的大脑切片的各种实验研究中以及在FDTD模拟中证明，与偏振无关的透射光强度（透射率）随着出射光的增加而显着降低（降低了50％以上）。神经纤维的平面角，它主要与平面内交叉角无关。这使我们能够区分}纤维密度低和平面内交叉纤维的区域与面外纤维的区域，}解决了3D-PLI中的一个主要问题，并允许更好地重建复杂的神经纤维结构在大脑中。最后，我们证明可见光谱中的光散射（倾斜照明）揭示了脑组织的底层结构，例如神经纤维的交叉角与千分尺分辨率，