Dear colleagues,

I recently run pairwise longitudinal registration for two T1w images.
This generated an average image, the jacobian and divergence rate images and the deformation fields (y_*).

According to the spm manual the y_* volume should be a 4D image, containing three volumes one for each x,y,z coordinates. However, in my case, there is only one volume and it looks like a gray-white gradient.
Is this right?

I've tried this with two different types of datasets and the results are similar.
If this is correct, what information does this image contain?

I am interested in the deformations, because I then want to bring some other images (e.g. MT) to the average space to allow me to compare them directly (MT at time 1 vs time 2).

Colleagues suggested that I use the deformations tool.
If I understand correctly, the pullback function brings the warped image (in the case of longitudinal registration I assume this is the averaged image), back to space of the original image, e.g. either at time 1 or time 2.

In contrast, the pushforward warps an image, e.g. T1w at time 1, to the space of the averaged image.
If this is correct, then I can use the pushforward function to bring e.g. the MT at time 1 to the average space.
Can I use the y_* images as they are or do I need to use their inverse?

Thank you in advance for the help,
Marina