Visualization of nerve fiber orientation in gross histological sections of the human brain

Diffusion weighted magnetic resonance imaging (DWMRI) allows visualization of the orientation of the nervous fibers in the living brain. For comparison, a method was developed to examine the orientation of fibers in histological sections of the human brain. Serial sections through the entire human brain were analyzed regarding fiber orientation using polarized light. Direction of fibers in the cutting plane was obtained by measuring the azimuth with the lowest intensity value at each point, and inclination of fibers in the section was evaluated using fuzzy logic approximations. Direction and inclination of fibers revealing their three-dimensional orientation were visualized by colored arrows mapped into the images. Using this procedure, various fiber tracts were identified (pyramidal tract, radiatio optica, radiatio acustica, arcuate fascicle, and 11 more). Intermingled fibers could be separated from each other. The orientation of the fiber tracts derived from polarized light microscopy was validated by confocal laser scanning microscopy in a defined volume of the internal capsule, where the fiber orientation was studied in four human brains. The polarization method visualizes the high degree of intermingled fiber bundles in the brain, so that distinct fiber pathways cannot be understood as solid, compact tracts: Neighbouring bundles of fibers can belong to different systems of fibers distinguishable by their orientation.