the problem is with some hard-coded settings in spm_orthviews.m. This
leads to undersampling of small images in default display mode. If you
zoom in (to e.g. 20x20 mm), everything should be fine. If zooming is a
problem for you, then you would have took look at the "redraw"
subfunction in spm_orthviews to change the way how the sampling points
Also, if you want to use SPM spatial processing routines for your data,
you will want to go through all estimation and writing options very
carefully. There are some places where images are sampled at a coarse
resolution. These sampling distances are adjusted for human brains and
need to be reduced for your data.
Hope this helps,