I am using 'applytopup' to unwarp my functional EPI images, but the process gets killed due to lack of memory. These are huge files (>60minutes of run time), so I have the same problem with other commands as well. I'm assuming the problem is that the whole 4D file is operated upon in memory storage, so I was thinking of just using fslsplit and running applytopup on each each volume. Is there any particular reason that commands operating only in spatial dimensions (i.e. smoothing, uwarping, etc.) aren't carried out in this way? Also, if there already exists a better solution it would be great to know.