Nope, I don't want to refer to the b0 slices. I want to allow to have any slices used for creating the mask. Often the non-b0 volumes will create some really nice masks especially when b0s are crappy. 

So I could use 4d_idx or dwi_idx. Maybe 4d_idx is better because some people would say that the b0s are not dwis... Cool, thank you for looking into this.