You may find some nice images from the link as :http://www.med.harvard.edu/JPNM/chetan/

Fusion of CT and PET is much easier than MRI and PET as the high magnetic field in the later interferes the photon interaction and that requires Avalanche diodes that is the reason for more expensive PET-MR system rather than PET-CT (the later is less than 50% the cost of the former).

Check

https://wiki.nci.nih.gov/display/CIP/NBIA+at+CBIIT+Image+Collections

or https://wiki.cancerimagingarchive.net/display/Public/Collections

you may need to register first.

You may consider the following link as well.

http://medical.nema.org/

There is some Open Access MR, PET and CT brain data available here:

http://www.insight-journal.org/rire/

We recently published a database of 37 normal adult human brain [18F]FDG PET, T1 and FLAIR MRI, and CT images: https://www.biorxiv.org/content/10.1101/2020.12.15.422636v1

The database can be made available for research purposes on request.