PET stands for Positron Emission Tomography. CT stands for Computerised Tomography.
PET/CT is a fusion of both techniques. CT images provide anatomical information while the PET study gives an indication of activity and function. Overlaying the PET image onto the CT image produces a fusion of anatomy with activity.

Positron emission.
The term applied, when an unstable nuclide, in this case fluorine 18 attached to a glucose-like compound (FDG), emits a positively charged particle called a positron. The positron travels only 1 mm in tissue before reacting with an electron, a negatively charged particle, to produce two 511keV gamma rays (photons) at 180° to each other.

Two photons are emitted simultaneously from within the patient.
Using a sophisticated PET/CT "camera", we are able to detect these events. The scanner surrounds the area of the patient being imaged. As the photons are emitted from the patient, they interact with a sensitive camera. The computer in the camera calculates where the positron must have originated from. By collecting millions of events, the PET scanner is able to create an image of the body that demonstrates where the greatest accumulation of labeled glucose is. This image is then superimposed onto the CT image

The PET/CT scan requires an injection of a radioactive form of sugar.
The injection is given via a small needle into an arm vein.
The main ingredient of the pharmaceutical is called FDG, an abbreviation for fluorodeoxyglucose. FDG is taken up by cells like normal glucose. More active cells, such as some cancer and inflammatory cells will take up more than the surrounding tissue. These areas will produce greater numbers of photons, acting like a “beacon”, to show up abnormal sites.
The injection is radioactive, but only for a short time, for more information see PET/CT Safety.
There are no known side affects to an injection of FDG, patients do not experience any sensation with the injection.

After the injection, patients are required to rest before the scan begins.
This resting phase is vital to obtaining high quality PET/CT images. Pathology is demonstrated because areas of disease use sugar more rapidly than the surrounding tissue. This means that the FDG injection will travel to areas of disease and demonstrate them on the scan.
Working muscles also use this form of sugar as an energy supply. The FDG injection will therefore travel to these muscles if they are being used.
By completely resting for a period of 1 hour after the injection, there is more chance of the FDG going to areas of disease, rather than normal tissues.