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

I hear of people going about every six months for a CT scan or MRI to see if their cancer has come back. But I don’t think they can pick up cancer cells, correct? I know they can see a large tumor but cancer cells are often microscopic, right? I also know they can pick up slipped discs; I don’t know how big discs are but cancerous cells? So many people are relieved at the “great news” that their cancer hasn’t come back after their scan shows no cancer. Set me straight.