I think there’s a number of experimental details that need to be taken into account when interpreting these data. I.e. What details are relevant in the determination of how many streamlines are present in the output file?
Firstly, if using
tckgen with default settings, and having all of your inclusion / exclusion regions included in the
tckgen call, then you will get an identical number of streamlines in every run. This is because the default termination criterion in
tckgen is based on a fixed number of streamlines in the output file. So the fact that this is not the case for your data means that there must be some additional relevant information. For instance: Are you generating a fixed number of streamlines from a seed region, and then post hoc selecting those streamlines that follow the entire trajectory of interest? Or are you instructing
tckgen to generate a fixed number of streamline seeds, and then counting the number of output streamlines?
In many of these cases, what ends up actually being quantified is “the fraction of streamlines that satisfy my criteria”; i.e. you generate some fixed number of streamlines in your seed region, and only some fraction of them reach your target region, and the number of streamlines in your output file reflect that fraction.
It entirely follows from this that increasing the streamline angle threshold could lead to a reduced number of streamlines in the output file. Your pathways of interest are reasonably straight, and a small angle threshold will prevent streamlines from deviating from the intended path. By increasing the angle threshold, streamlines are now free to latch on to crossing or adjacent fibre bundles, following unintended trajectories, and reducing the fraction of streamlines that satisfy your selection criteria.
So I would generally advise thinking carefully about exactly what parameter is being quantified, and what influences that parameter due to your experimental design. Even within the scope of the adage that “streamline count is not quantitative”, there are actually multiple branches in terms of different ways in which streamline count can be not quantitative.