The PET scan is a revision of the CAT scan demand, and thus supersedes the latter. Regarding the relaxed rule by NATO to let people own animals less violent than cats as pets, the demand was soon revised. The main change is the yearly tax collecting for pets other than cats for a less violent home environment.
Cats do the damage and intimidation that has to otherwise be done by NATO hired mercenaries. Other pets are also socially frowned upon because they're not holy. You can, should and MUST register your pets at your nearest civil registration office. Pets other than cats must also pay the service tax for the time you're wasting on the public harassment services. You can pay with money or children.
The scan is made every year along with the census to update your pets' files in order to know if you're being oppressed properly and if you must pay for not having a cat for each head living under the same roof. Cats should only die of natural causes as determined by necropsy.
Urban legend says that Phantom Limb's machine was a hard to swirl cat sausage machine he used on behalf of some mercenaries that robbed taxes for themseves by breaking into people's houses after Phantom did his job. This lasted until it became a bed-time brainwashing story that couldn't be covered anymore, and so he was punished as an example.