Hello Cheng,

I beleive that what you are seeing is due to the 6mm FWHM spatial filter applied to the data. If you look carefully you will notice that the areas that are outside of the brain are very close to zero. If you apply a small threshold to the data they will go away.

At the edge of the brain the spatial filter will average some of the inbrain single into out-of-brain pixels.

cheers,
cameron