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

Diffusion weighted magnetic resonance imaging (DWMRI) allows visualization of the orientation of the nervous fibers in the living brain. For compariso n, a method was developed to examine the orientation of fibers in histologi cal sections of the human brain. Serial sections through the entire human b rain were analyzed regarding fiber orientation using polarized light. Direc tion of fibers in the cutting plane was obtained by measuring the azimuth w ith the lowest intensity value at each point, and inclination of fibers in the section was evaluated using fuzzy logic approximations. Direction and i nclination of fibers revealing their three-dimensional orientation were vis ualized by colored arrows mapped into the images. Using this procedure, var ious fiber tracts were identified (pyramidal tract, radiatio optica, radiat io acustica, arcuate fascicle, and II more). Intermingled fibers could be s eparated from each other. The orientation of the fiber tracts derived from polarized light microscopy was validated by confocal laser scanning microsc opy in a defined volume of the internal capsule, where the fiber orientatio n was studied in four human brains. The polarization method visualizes the high degree of intermingled fiber bundles in the brain, so that distinct fi ber pathways cannot be understood as solid, compact tracts: Neighbouring bu ndles of fibers can belong to different systems of fibers distinguishable b y their orientation. (C) 2000 Wiley-Liss, Inc.