let me try again: you don't find any traces of an artefact in your beta- and con-images. Did you look at the spmT-images? You can display them side-by-side with beta, con, RPV and ResMS images using CheckReg. If there is nothing suspicious in the spmT-images as well, then it is most likely an issue with MIP visualisation. If the spmT images look stripy in CheckReg, then it would most likely be visible as a pattern in the ResMS image as well. In that case, it would suggest that your model didn't fit every other slice.