Hi. fslroi's required arguments are minimum index and size. The values in your roimask are probably 0 (outside) and 1 (included voxels), so i would try replacing the 0 with 1.
Good luck.
> Date: Fri, 29 Jun 2012 04:37:34 +0100
> From:
[log in to unmask]> Subject: [FSL] fslroi
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> Dear FSL experts,
> I want to extract ROI and use it as the reference volume.
> So I run the following commend:
> fslroi data1up.nii.gz 0 1 b0up1.nii.gz.
> The server returned an error:Image Exception : #21 :: Invalid ROI dimensions.
> Could you give me some suggestions about this?
> Thanks,
> Bing JI