Fig 2. FA measurement in a nonisotropic voxel. It was assumed that there are 3 fiber bundles traversing this voxel, each with an FA of 0.5. When all fibers are aligned in a single direction (A), the measured FA of the voxel is 0.5. When there is a crossing fiber within this voxel (B), then the measured FA of the voxel is <0.5.