Create a new coordinate system for an MRI, based on a defined plane


I have two MRIs of the lower jaw (“before” and “after” treatment), that I’ve successfully aligned and overlayed with the Registration and Transformation tools. I also added a common Plane on these MRIs.

What I need to do now is to measure the movement of the teeth, in the projection on this plane.

My plan was to make a new coordinate system, where X and Y would be within my plane and Z would be a perpendicular to this plane. Than I would take the coordinates of each point on the MRI “before” and “after” and mathematically calculate the vector of it’s movement.

The important part is to have the information of how they moved relative to the plane, that’s why I need a new coordinate system that would be based on my plane.

Does anyone here know how I could create this new coordinate system?

Thanks a lot in advance!!

I added some screenshots to make it more clear what I’m working with:
(1) the plane that I made
(2) the points, for which I need to measure their vector of movement
(3) an option that I saw, that gave me the idea to make a new coordinate system, so that I can get the coordinates of the points in the “local” coordinate system