Hi Sally,

I believe that the linear interpolation method would do this,
providing that the sagittal slices have the same geometry.
Linear interpolation will interpolate each voxel's value from
the closest neighbors of the original, so if every other "new"
slice landed exactly on an "old" slice, the values should be
identical.

Nearest neighbor will never interpolate. It will give you the
exact value of the closest neighbor, which does not seem
to be what you want.

So try the linear interpolation method, and just check the
results of some of those slices. You could use 3dZcutup
and 3dcalc to grab a pair of sagittal slices and see if their
difference is (at least almost) identically zero.

- rick