Hi Rick,

Thanks for your reply. Actually, both images have the same voxel size. The result of 3dinfo is as follows:

Original image dataset:
++ 3dinfo: AFNI version=AFNI_2008_07_18_1710 (Aug 5 2008) [32-bit]

Dataset File: t1_icbm_normal_1mm_pn0_rf0_ns.nii.gz
Identifier Code: AFN_wKQsyEeKmVbgWKuA3Id8Ww Creation Date: Fri Dec 7 16:50:50 2007
Dataset Type: Anat Bucket (-abuc)
Byte Order: LSB_FIRST {assumed} [this CPU native = LSB_FIRST]
Storage Mode: NIFTI file
Storage Space: 14218274 (14 million) bytes
Geometry String: "MATRIX(-1,0,0,90,0,-1,0,126,0,0,1,-72):181,217,181"
Data Axes Tilt: Plumb
Data Axes Orientation:
first (x) = Left-to-Right
second (y) = Posterior-to-Anterior
third (z) = Inferior-to-Superior [-orient LPI]
R-to-L extent: -90.000 [R] -to- 90.000 [L] -step- 1.000 mm [181 voxels]
A-to-P extent: -90.000 [A] -to- 126.000 [P] -step- 1.000 mm [217 voxels]
I-to-S extent: -72.000 -to- 108.000 [S] -step- 1.000 mm [181 voxels]
Number of values stored at each pixel = 1
-- At sub-brick #0 'MINC[0]' datum type is short: 0 to 3506


mask dataset:
++ 3dinfo: AFNI version=AFNI_2008_07_18_1710 (Aug 5 2008) [32-bit]

Dataset File: MNI152lin_T1_1mm_subbr_mask.nii.gz
Identifier Code: NII_j5mjOhvg9TERRRmtp_2EjA Creation Date: Mon Dec 1 17:20:05 2008
Dataset Type: Anat Bucket (-abuc)
Byte Order: LSB_FIRST {assumed} [this CPU native = LSB_FIRST]
Storage Mode: NIFTI file
Storage Space: 14442064 (14 million) bytes
Geometry String: "MATRIX(1,0,0,-90,0,-1,0,126,0,0,1,-72):182,218,182"
Data Axes Tilt: Plumb
Data Axes Orientation:
first (x) = Right-to-Left
second (y) = Posterior-to-Anterior
third (z) = Inferior-to-Superior [-orient RPI]
R-to-L extent: -90.000 [R] -to- 91.000 [L] -step- 1.000 mm [182 voxels]
A-to-P extent: -91.000 [A] -to- 126.000 [P] -step- 1.000 mm [218 voxels]
I-to-S extent: -72.000 -to- 109.000 [S] -step- 1.000 mm [182 voxels]
Number of values stored at each pixel = 1

However, I think 3dZeropad will do it by setting n to a negative value.

Best,
Siamak