Medical CT image alignment/registration

I have two DICOM images from two different CBCT scanners of the same dental anatomy. I can open these images in Horos.

How can I align/register these two images to each other using free (in any sense) software?

Hard to make specific recommendations without looking at the images, but OpenCV has a number of image registration algorithms (and a Julia binding, iirc?) Image alignment and registration with OpenCV - PyImageSearch

There’s also ImageFeatures.jl alongside ImageTransformations.jl for registering and aligning in pure Julia.

3D Slicer is the tool to use!