Hello. Here's a good page from the National Cancer Institute that explains the various scans http://imaging.cancer.gov/patientsandproviders/cancerimaging/nuclearimaging
It's really clever. Knowing that the cancer uses more sugar, they basically use radioactive sugar to track where it goes, and the areas where there are tumors light up like a christmas tree on the scan. If it wasn't so horrible, it was really interesting looking at the scans.