I would venture the opinion that these days you actually get a PET/CT scan and not just a PET scan. If you are curious and look at the scans yourself you see that the PET itself had a poor resolution while the CT component is clear. (which is why they do them together).
I noted that the scanning speed has markedly improved since my first scan (newer equipment) and that may be part of the issue here. It might help to consider the following: A negative pet scan is of course good news it means that you do not have cancer OR that it is not big/active enough to be visible. In that respect it is a reflection of the past. Also, you normally get a full body PET/CT (including head/lung......). I make damn sure that I do everything possible to avoid a false positive. (a cold is a bad idea, so it talking and moving supposedly ... I am not going to try to see if this is so).
So far all my PET/CT scans have worked out, from the positive one initially, to the subsequent negative ones. (next one in 11 days)

Sandy re MRI this is not a "picture" of the cancer either, all you see is actually just water which is why MRIs are good for soft tissue.

M