Dear Zhiyuan Wang,

First, I'd say that the blank volumes need not be a problem for later processing in SPM, since you can keep ignoring them, selecting just the ones you want (e.g. the statistical modelling should be happy working with a subset of volumes from a 4D file).

However, if that seems ugly, and/or you need a tidier 4D file for use with another package outside of SPM, then you can easily use SPM's 3D to 4D conversion routine (don't be put off by the "3D" in the name; they can be 3D frames selected from within an existing 4D file!) to create a new 4D file containing just the volumes that you want. I've just added a little note about this to the SPM wikibook page here:

http://en.wikibooks.org/wiki/SPM/Working...

I've checked this in SPM8; I'm afraid I can't remember for certain whether similar functionality was in SPM5, but I think so.

Best wishes,
Ged