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When an image is resliced, it is only possible to write out those parts of the 
resliced image for which there is data in the original image (ie, the voxel 
in the resliced image should map to some point within the field of view of 
the original).  Even very small movements can lead to loss of data at the 
edge of the FOV.

In voxels where there is missing data, some value needs to be used to fill in 
these unknown values.  Typically these values are set to zero.  In fact, when 
an fMRI time series is resliced, if a particular voxel position has an 
unknown value at any point in its time series, then the entire time series 
for that voxel is set to zero.  The reason for this is that because each 
volume of the time series moves in different ways, then there are likely to 
be different patterns of zeros in the resliced images.  Prior to doing the 
stats, it is typical to smooth the data, and this would have the effect of 
introducing movement effects into the data, which would arise because some 
voxels have data and others have zeros - depending on the subject motion.

Best regards,
-John

On Tuesday 24 June 2008 00:30, Vanessa Carmean wrote:
> Hi SPM'ers,
>    I am processing data using SPM5, and on a couple of subjects, the raw
> data looks fine, but after the realignment step, the images appear as if it
> they have been cut, removing the lower portion of the image.   The motion
> in these subjects doesn't seem to be significantly different from any of
> the other subjects, and nothing seems different between the steps we are
> doing for these subjects and the subjects who are going through the process
> correctly.  Is anyone familiear with what might be causing this?
>
>
> Thank you,
> Vanessa
>
> Vanessa Carmean, B.S.
> M.I.N.D. Institute
> University of California, Davis
> 2825 50th Street
> Sacramento, CA 95817
> (916) 703-0438