Hi Wind,
That is very peculiar!
How is the initial NIFTI dset created (e.g. do you specify ROI13_Mask.nii.gz with the SaveAs button in the drawing plugin)?
Are any other operations performed in the between creation and the 3dcalc command?
What is the output of:
nifti_tool -disp_nim -infiles ROI13_4.nii.gz
- rick
