Fig. 1.

a) In grey matter, it is reasonable to assume image intensities are spatially correlated with neighbours isotropically for the purposes of smoothing and cluster formation. Illustrated in yellow is a voxel of interest with neighbouring voxels coloured red. b) White matter anatomy is oriented and extended in nature, therefore an isotopic neighbourhood is not appropriate. Shown is a fractional anisotropy map coloured by the direction of the primary tensor eigenvector (red: left–right, green: anterior–posterior, blue: inferior–superior). Not all voxels adjacent to the voxel of interest (yellow voxel within the optic radiation) are relevant for smoothing and cluster formation since neighbouring voxels contain different fibre tracts (e.g. tapetum of corpus callosum and arcuate fasciculus). In this example only the voxels anterior and posterior (shown in red) should be considered as neighbours for clustering and smoothing.