Positron Emission Tomography (PET) is a specialised Nuclear Medicine procedure that uses positron emitting radiolabeled tracer molecules to measure biological activity. The commonest of these radiolabeled tracers is 18F-Fluorodeoxyglucose (18F-FDG), which is used to determine abnormal glucose metabolism in tumours and other sites. It has general applications in all areas where abnormal glucose metabolism may be present including circumstances such as differentiating tumour from scar tissue, evaluating the presence of tumour in the light of rising tumour markers and normal morphological imaging techniques and assessing response to therapy where other techniques are deemed to be unhelpful.