The difference is explained here , about two-thirds of the way down the page.

PET scans can result in false positives -- what "lights up" on a PET scan may be a simple inflammation, rather than cancer. In addition, patients are often told not to exercise in the 24 hours before a PET scan for the same reason, as a muscle recovering from exercise will show a higher uptake of glucose than a muscle that is rested.

The main part of the Oral Cancer Foundation site has hundreds and hundreds of pages of useful information. The search link is at the very bottom of every message board page (in gold letters, next to Site Map).

Hope this helps.

All the best,
Leslie