How do I run probtrackx(2) with an isotropic fiber orientation distribution? Do I set all volume fractions (i.e., in merged_f*samples.nii.gz) to zero, or do I also need to modify merged_ph*... and merged_th* so that the sampled diffusion directions are uniformly distributed around the unit sphere?
Based on some preliminary runs, it appears merged_ph*... and merged_th*... must be modified, along with setting all volume fractions to zero. I chose phi and theta so that when transformed to Cartesian coordinates, the direction vectors were uniformly distributed. That is, theta = acos(2v-1) and phi = [0,2i), where v is randomly sampled from (0,1).
Thanks for all of your help!!
-Kabi