Dear community,
I have a methodological question that I hope some of you may be able to help me with. We have a small study where we performed probabilistic tractography using FSL probtrackx between a small subcortical region and a series of large cortical ROIs, deriving some metrics based on the waytotal and using them in downstream analyses (i.e., a handful of values to characterize one seed-to-ROI reconstructed path, not a voxelwise matrix or anything like that).
Now, as part of my analyses, I would really like to model the effect of the distance between seed and target, but am unsure how best to go about this. I am aware that using the -pd flag in FSL probtrackx outputs an additional map that tracks the average length of streamlines up to that point in each voxel, but ideally I would like one value such as a “mean tract length” to characterize one seed-target pair.
Does anyone have previous experience or an “established method”, either in deriving this from this paths lengths map from probtrackx, or using some other method? We’ve even thought about simply deriving distance between the centres of the seed and target ROI as the most crude approximation, but obviously white matter paths and their reconstructions do not follow a straight line, so it would be VERY rough indeed. I have heard that MRtrix may have more options, but as all other analyses were done in FSL, switching at this point is not really an option.
I would be very grateful for any input!