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Figure 3b. Diffusion within a single voxel. (a) Diagram shows the 3D diffusion probability density function in a voxel that contains spherical cells (top left) or randomly oriented tubular structures that intersect, such as axons (bottom left). This 3D displacement distribution, which is roughly bell shaped, results in a symmetric image (center), as there is no preferential direction of diffusion. The distribution is similar to that in unrestricted diffusion but narrower because there are barriers that hinder molecular displacement. The center of the image (origin of the r vector) codes for the proportion of molecules that were not displaced during the diffusion time interval. The color bar (right) shows the spectrum used in color coding to represent probability, from the lowest value, which is indicated by red, to the highest, which is indicated by blue. (b) Diagram shows the diffusion probability density function within a voxel in which all the axons are aligned in the same direction. The displacement distribution is cigar shaped and aligned with the axons. (c) Diagram shows the diffusion probability density function within a voxel that contains two populations of fibers intersecting at an angle of 90°. The molecular displacement distribution produces a cross shape.
