Hi Shengwei,
I'm not sure if I understand.
The images we model in randomise are all scalars. Do you mean something
that is constant across space and/or constant across multiple subjects?
If yes, randomise can still be used. FWE results are valid, and if all voxels across space are identical, it will give the same result as the uncorrected (and both are fine then).
The spatial statistics
(cluster extent, cluster mass and TFCE) are, however, only meaningful for
actual imaging data. If these quantities that you define as scalars
aren't truly imaging data (i.e., not voxelwise), and were stored
as NIFTI just for some convenience, these statistics aren't meaningful.
Not sure if it helps. Maybe if you give more details.