Hi Stephany,
cancer cells absorb sugar at twice the rate of normal cells. In a PET scan, they inject glucose, that has been irradiated by a cyclotron, leaving a radioactive tracer attached to the sugar molecules in the glucose. It is concentrated by the cancer sells in areas in the body where uptake in increased will show up as black spots on the films.

It is typically an almost whole body scan. When combined with a CT image, either through image fusion techniques or the next generation of PET, called PET/CT it can be very accurate.

It is just a piece of the total information required for an accurate diagnosis or can be used to tweak the treatment plan, pre-radiation. It can also be used to validate the effectiveness of the treatment plan.

Areas of intense healing and scarring can also show up as a dark spot as well as the bladder when the glucose is on its way out resulting in some "false positives".

PET is an emerging technology that is finding many other diagnostic capabilities all the time, such as early detection of Alzheimers disease as well as cancer.

It is the most expensive scan.

The PET scan that I had verified the tumor activity at the diagnosed location which was good. It showed no other cancerous activity (or as they like to call it) regions of interest (ROI).