Hi, Yuni-

I don't think editing masks is a common procedure. I am not sure of a way to remove voxels without possible biasing or changing of results, and without a heavy degree of arbitrariness.

Instead, as Daniel noted, you could intersect your large clusters with an anatomical map, to identify regions usefully.
-> as a supplement to this, you can also note where major peaks fall in the anatomical regions, but I wouldn't use this as the *only* information in your results: a peak is just one voxel, whereas your cluster is a much larger area. 3dExtrema points out local peaks (but what counts as a "peak" is also not always clear-- this also involves subjectivity, so that is why this should be descriptive and not definitive.)

Even when you threshold, it is good to consider the translucent thresholding in AFNI, described in this post:
[afni.nimh.nih.gov]
with links to both tutorials on the AFNI website and the AFNI Academy online Bootcamp lectures. Basically, all thresholds are arbitrary, and these viewing options help show more of your data.

--pt