afni.nimh.nih.gov

If you are drawing on a +acpc or +tlrc dataset, so that the midline of the brain is set to x=0, then you could use the program 3dLRflip to flip a mask dataset, producing a new one that is the mirror image of the first, then combine them with 3dcalc, as in
3dLRflip -prefix sivle elvis+tlrc
3dcalc -a elvis+tlrc -b sivle+tlrc -expr 'a+b' -nscale -prefix elvis_cloned
/bin/rm -f sivle+tlrc*

bob cox