ANTs inverse transform is inaccurate

Hi everyone,

I am using niworkflows SpatialNorm module (version 1.5.4 with ANTs ; niworkflows.interfaces.norm module - niworkflows 1.5.4+17.g45ab13e1 documentation) to register T1w images to MNI space, which works perfectly fine:

Shape and affine of my T1w (we have a non-standard resolution after preprocessing)

(112, 112, 35)

array([[ 1.99832809e+00, 7.41248727e-02, 5.69230914e-02,
-1.17586777e+02],
[-7.08075762e-02, 1.99095488e+00, -2.90909529e-01,
-1.05518166e+02],
[-4.08772230e-02, 1.74940050e-01, 3.28665876e+00,
1.41945553e+00],
[ 0.00000000e+00, 0.00000000e+00, 0.00000000e+00,
1.00000000e+00]])

Shape and affine of MNI

(97, 115, 97)

array([[ 2. , 0. , 0. , -96.5],
[ 0. , 2. , 0. , -132.5],
[ 0. , 0. , 2. , -78.5],
[ 0. , 0. , 0. , 1. ]])

Resulting affine of normalized T1w

array([[ 2. , 0. , -0. , -96.5],
[ 0. , 2. , -0. , -132.5],
[ 0. , 0. , 2. , -78.5],
[ 0. , 0. , 0. , 1. ]])

However, when I use the inverse transform on the mni template, I get tiny inaccuracies in the resulting affine

(112, 112, 35)

array([[ 1.99832821e+00, 7.41248578e-02, 5.69230616e-02,
-1.17586777e+02],
[-7.08075911e-02, 1.99095488e+00, -2.90909529e-01,
-1.05518166e+02],
[-4.08772454e-02, 1.74940079e-01, 3.28665900e+00,
1.41945553e+00],
[ 0.00000000e+00, 0.00000000e+00, 0.00000000e+00,
1.00000000e+00]])

Comparing the native and MNI->native affine

array([[False, False, False, True],
[False, True, True, True],
[False, False, False, True],
[ True, True, True, True]])

The command I used for this example (via a jupyter notebook)

! antsApplyTransforms -d 3 -i {t1_native_path} -r {mni_path} -t {xfm_T1wToMni.path} -o {t1_mni_out}

I am out of ideas on why the forward transform works, but not the inverse, and why the inaccuracies are so tiny. I’d be grateful for any help and can of course supply more information.

Best,

André