A Positron Emission Tomography scan or PET Scan is a radiological procedure used for finding particular types of tissues in the body and studying its metabolism. It is often combined with a CT Scan or MRI. The patient is prepared by swallowing a tracer - a special type of radioactive isotope that both binds to living tissue and will emit a positron (a positively charged electron) when it decays. The tracer is added to a chemical that will bind to the tissue under investigation.