Hi!

I’m working on a project where I use a `Versor3DRigidTransform`

for registering 3D MRI images, and I’m having some issues interpreting the output from the registration results. My end goal is to get a transformation which is equivalent to a centered rotation (as in the middle of the image) and a translation, this is because I’m applying image corrections in the frequency domain (i.e. MRI k-space data).

I’ve generated test data with a 3D Shepp Logan phantom with a 45 deg rotation between the fixed and moving image. The images are generated outside of ITK and I use `itk.image_from_array`

to create an image object, where I specify the centre of the image as the origin.

```
spacing = [1,1,1]
img = itk.image_from_array(np.abs(np.ascontiguousarray(img_array)))
img.SetSpacing([float(x) for x in spacing])
img.SetOrigin([-img_array.shape[i]/2*spacing[i] for i in range(3)])
```

I set up an initialiser with `.GeometryOn()`

to ensure the image origin is used for the transformation.

After running the registration I resample the moving image to the fixed space and I get a perfect overlap, but when I look at the estimated registration parameters there is a discrepancy in the rotation estimate between the versor rotation and the rotation matrix which I dont understand.

The final parameters from the registration are:

```
Rotation: (-0.04, 0.01, -21.93) deg
Translation: (-0.39, 0.94, 0.00) mm
```

But when I pull out the rotation matrix and the offset using `.GetMatrix()`

and `.GetOffset()`

I get

```
[-8.49832563e-04 -9.54532133e-04 -1.09447133e-05]
[[ 7.07122107e-01 7.07090904e-01 8.82676813e-04]
[-7.07091446e-01 7.07121348e-01 1.04216899e-03]
[ 1.12748595e-04 -1.36107396e-03 9.99999067e-01]]
```

The offset is basically 0, which I was hoping since I set the origin to be at iso-centre. But the rotation matrix equates to the following rotations (if my math is correct):

```
Rotation (x,y,z): -0.078, -0.00646, -45.0 degrees
```

And this is what I would expect.

Can someone explain why there is this difference in the outputs?

Please let me know if you need more information, I could go on and on about the details but tried to make it as brief as possible.

Thanks!