diff --git a/post_xfibres b/post_xfibres
index 43ea191143ca038926888ee0ac55bf3b40b6b5da..7aa66856944c05234f8e6c5df9cf9e6917fb9c67 100755
--- a/post_xfibres
+++ b/post_xfibres
@@ -20,7 +20,8 @@ numfib=`${FSLDIR}/bin/imglob -oneperimage f*samples |wc -w`
 
 echo "$numfib fibres"
 fib=1;
-avwmaths S0samples -mul 0 numfibs
+avwroi S0samples numfibs 0 1
+avwmaths numfibs -mul 0 numfibs
 while [ $fib -le $numfib ];do
     echo $fib
     ${FSLDIR}/bin/avwmaths f${fib}samples -Tmean mean_f${fib}samples