Is it not true that MRI's and CT scans cannot pick up cancer cells?

No, they do not see individual cancer cells. They do, however, do a very good job of finding small tumors.

You would need a PET scan for cancer cells to show up.

MRIs and CTs looks for tumors, this is correct. If we could see/pick out individual cancers cells in your body it would be an amazing breakthrough. What those routine scans look for is the reappearance of full tumors. We can get pretty small now, but it’s still on the order of millimeters at the smallest.

However, the reason many people are relieved is because if the tumor doesn’t regrow, or the cancer metastasize elsewhere, it is essentially gone (for the time being). It’s the tumor that causes the symptoms. And the longer you go without either of those occurring, the greater the chance that the cancer was beaten. It’s all probabilities, sadly, you could be cancer free for decades and then it suddenly reappears, but the longer it’s gone the better.

And a PET scan is still also on the tumor stage. It can get more specific as to what is cancerous and what’s not through specific imaging agents, but it’s still a tumor-finding test. We cannot see individual cells in the body with an external scan like these.

You can’t see individual cells, but cancer cells show up as brighter spots on PET scans because they have a higher metabolism rate than do normal cells. Mayo Clinic

sorry, I forgot to include a link

Yeah, that’s what I meant by “It can get more specific as to what is cancerous and what’s not through specific imaging agents”. There are also other markers you can use with PET, it all depends on the molecule you choose to do the scan. Specific indicators can also be used with MRI, actually, but that’s a little harder to do and rather rare as a result.