Only a minor addition from me:
It’s important to distinguish between a reconstruction issue (e.g. erroneous tracks, very wavy tracks, etc.) and a visualisation issue (e.g. can’t see the structure of interest when streamlines are entirely opaque). The feedback above all relates to the former, which obviously takes precedence. However something like your first image can still conceivably be observed even with “appropriate” tracking, and is an inevitable consequence of rasterising 3D geometric data onto a finite 2D grid. E.g. If you have a very low resolution monitor, the issue will be far more pronounced, whereas if you have a very high resolution monitor / use something like multi-sampling anti-aliasing, the issue is reduced, as pixels from one streamline are less likely to obscure pixels of another streamline, and so the relative density of streamlines in different areas of the image becomes more apparent (you need to be able to see “empty space” in between streamlines). And there’s all sorts of tricks that can be done for visualisation, including the use of opacity (which is a “trick” in its own right). So, just wanted to be clear that the suggestions above are not a strict solution for the entirety of what you reported. 