I can't speak to the clinical aspect of medical imaging but I've done some development with medical images. If you end up pursuing your diff concept here are a few things which might help point you in the right area or give you some additional keywords when searching:

1. CT images measure densities of tissue (in Hounsfield units).

2. The images will first need to be aligned/registered/fused. There are different ways to do this but it can be difficult if the two scans were done with different protocols/hardware as the image resolutions and dimensions can differ but also anatomy regularly changes (not as much in the head area).

3. Fusions are often described as having a primary (unchanged) and a secondary (the image that's modified to align to the primary).

4. Fusions can be very basic rigid alignments (like moving one photo on top of another) or be fancy by moving voxels independently (often called deformable registration, like cutting a photo into squares and moving them) however these are difficult to evaluate for correctness.

5. Fusions are often viewed as a transparency overlay of one image on top of another (in the case of deformable it's the deformed image over top the primary).

6. After doing a fusion a subtraction can be performed to find the difference in image values (the densities).

Best of luck and enjoy the field.