*6.2. Positron Emission Tomography*/*Computed Tomography*

PET is an imaging tool developed in the 1990s to visualize specific (patho)physiological processes of a particular area or of the whole body. The technique is based on injection of radioactive tracers, which are tracers attached to radionuclides that emit positrons (positively charged electrons) to become stable. Emitting positrons cannot exist freely and annihilate with antimatter (negatively charged electrons) by emitting two gamma-ray photons, each with the same energy (511 keV) in direct opposite directions. The PET camera system consists of a ring-shaped detector system which can detect the two photons when they arrive within a certain time frame. Recent developments in software of PET cameras have led to a correction method for the time these photons need to travel to the detector, the so-called Time-of-Flight technique. This improvement (since 2005) caused a higher efficacy in detecting photons. Since then, the use of PET imaging for clinical and research purposes increased considerably. Although PET was originally used in oncological diseases to detect malignancies, the usefulness of PET to image infection and inflammation has markedly increased during the last decade, and PET techniques evolved rapidly. Hybrid camera systems were developed to combine PET findings with CT or MRI to add anatomical information. These new camera systems provide better spatial resolution images (around 3–4 mm), decrease scan duration and radiation dose, and lead to increased diagnostic accuracy. Furthermore, the development of guidelines to standardize PET/CT techniques between centers by the European Association of Nuclear Medicine (EANM) enhances comparability of data and promotes multicenter research [99,100]. In addition, new and specific tracers to image infectious and inflammatory diseases are constantly being developed.
