You can also use 3dLocalstat to make a kind of dilated blob. In this example, 3dLocalstat is used to look at a 2 voxel neighborhood in x and y for each voxel. A sum is computed at each voxel over its neighborhood, then any voxel with any neighbors at all in the neighborhood is given a value of 1 using 3dcalc.
3dLocalstat -nbhd 'RECT(-2,-2,0)' -stat 'sum' -prefix localstat_sum_am2 periphery_am2+orig
3dcalc -a localstat_sum_am2+orig. -expr 'step(a)' -prefix localstat_dilate2
The attached image shows the result of dilating a 1 voxel thick ROI with a 2 voxel neighborhood. This can be used in 3D and with spheres too.
