Hi I googled with the issue you are asking about and I am sharing the link. Several links and such so I think you probably can find out what you are needing to know. Also may help you to know the questions you need to clarify with you Primary Doctor.
All the best to you. Hugs from Canada
https://www.google.com/search?q=Pet+scan+for+2.1+cm+focal+density&oq=Pet+scan+for+2.1+cm+focal+density&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIGCAEQRRg8MgYIAhBFGDzSAQgxNTk4ajBqNKgCALACAQ&sourceid=chrome&ie=UTF-8