PET scans work on the premise that a radioisotope marker is attached to the glucose molecules (via exposure of a bag of glucose in a cyclotron an hour or so prior to infusion).

It is then infused into the body by IV where subsequently the uptake of sugars are accelerated and concentrated by tumor activity (or sometimes erroneously by healing activity).

The scanner itself detects the concentration of the radioisotope (harmless by the way, with a very short half life) and converts this data into a film or digital image.

Introduction of excess sugar to the body prior to a PET scan may result in an erroneous reading by diluting the "marked" glucose molecules.

This could result in a concentration being below the threshold of detection resulting in a possible false negative result.