FIGURE 1.

Nomenclature. Two bundles, the UF and IFOF, are used to highlight classifications of voxels (a–e), and fixels within the voxels. Voxels in a and b are examples of single‐fixel voxel and single‐bundle voxels and also single‐bundle fixel. Because the UF and IFOF diverge in Voxel c, this is an example of a multi‐fixel voxel and multi‐bundle voxel, with one fixel classified as a single‐bundle fixel and the other a multi‐bundle fixel. Voxel d highlights the fanning of the IFOF, which results in a multi‐fixel voxel and single‐bundle voxel, and both fixels are single‐bundle fixels. Finally, both the IFOF and UF pass through voxel E following the same orientation, thus Voxel e is a single‐orientation voxel but multi‐bundle voxel, and also a multi‐bundle fixel. This fixel, and thus also this voxel, represents a bottleneck for tractography