Fig. 14.

Degree of correlation and similarity of tracts after reducing the number of seeds per voxel in probtrackx. A: Average over 27 tracts and 5 subjects of the correlation between 2 different runs of probtrackx; this is reduced when we reduce the number of seeds per voxel. X axis is the reduction in the number of seeds with respect to the original AutoPtx configuration. Y axis is the average correlation of the pairs of tracts. When we reach 0.1× seeds per voxel, the median correlation drops below 0.999 for some tracts. Right plot is a zoom of left plot. B: Worst instance (in terms of correlation) of probtrackx in the forceps major tract with a factor of 0.1× seeds per voxel. The results remain robust even using a reduced number of seeds per voxel. Correlation between the maps for these two runs was 0.929. Tracts were binarised for visualization using a threshold of 10% of the 99th percentile. The overlap of the 2 runs is shown in blue. The difference is shown in red. FMRIB58 FA atlas is shown in the background. C: Same tract (forceps major) from the same subject with a factor of 0.3× seeds per voxel. The results improve by increasing the number of seeds per voxel. Correlation between the maps for these two runs was 0.985. FMRIB58 FA atlas is shown in the background. The final decision was to use 0.3x.