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FIG 5. Schematic representation of diffusion anisotropy in brain white matter. Obliquely oriented cylinders represent axonal fibers and myelin sheaths, and small spheres represent water molecules. When the randomly chosen direction of diffusion is transverse to the fiber direction, the path of diffusion is blocked by hydrophobic myelin sheaths. When the randomly chosen direction of diffusion is parallel to the fiber direction, there are no barriers to free diffusion. Thus, the water molecules demonstrate a preference for diffusion along the direction of the fiber tract (12, 13, 16) (A). Schematic representations illustrate the situations in which axons are more tightly packed (B), the myelin sheaths are thicker (C), obliquely oriented axons are present (D), and the axonal diameter is reduced (E).
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