black points on the Pictures For Checking Normalization

Hi everyone

I found there are many black points scattering on the pictures for checking normalization (see below). No such points were found in the FunImg data. I don't know these black points means voxels lost or some other meaning. The preprocessing steps include: slice timing, realign, bet, T1 coreg to FUN, new segment+DARTEL, Covariates Regression, Filter, Normalize by DARTEL.