I was about to mention this as the cause. GIven that this is the case, I’m pretty sure it’ll be the cause as well. If the background of your FOD image is in fact zeros (not NaNs), you’re better off not providing the
-mask option, and allow interpolation (with the zero background) to essentially define the smooth edges of the bundle. The FOD amplitude threshold will still prevent streamlines from going all the way into the background. There’s a subtle effect at play that naturally helps here too: as interpolation will naturally provide FODs with smoothly diminishing amplitudes, the closer the tracking algorithm gets to the edge, the FOD threshold on such shrinking FODs (yet with a similar shape) will naturally result in a more narrow “cone” of angles making the (FOD) cut. So naturally, streamlines will be allowed to disperse less. While this is in a way artificially caused by a discrete FOD threshold, its nonetheless a useful effect in practice!
So long story short: try to go without the
-mask option for this particular setting, and see if that makes a significant difference. If that doesn’t help, you can always just increase the FOD threshold, or play around with the
-power parameter if you’re feeling adventurous. (higher power will result in less dispersion)
Note that a lot of these choices are quite arbitrary. But that is just the nature of tractography, I’m afraid. It’s easy for a software package to provide defaults that automatically result in less dispersion, but whether that’s appropriate for real anatomy (rather than a synthetic phantom) is another matter. As people tend to evaluate tractography on synthetic phantoms, I note there may be a bias to tune parameters to perform better on synthetic phantoms. All fun in theory, but of limited relevance for the real applications of tractography.