**1. Introduction**

Infection is a major cause of patient morbidity and mortality throughout the world. The diagnosis of infection can be challenging and imaging studies are often used for confirmation and localization. Radiological tests, such as x-rays, ultrasonography, computed tomography, and magnetic resonance imaging, reflect structural alterations in tissues and organs produced by a combination of the infection and the host's response to the infection. Structural changes take time to evolve and there is a delay between the molecular events of the disease process itself and the appearance of structural changes on radiologic imaging. Nuclear medicine imaging agents can be taken up directly by cells, tissues, and organs, or can be attached to native substances that then migrate to an inflammatory focus. These agents reflect physiological changes in the inflammatory process and can identify abnormalities before the development of structural changes [1]. For many years, the single photon emitting radiopharmaceuticals, gallium-67 citrate, and *in vitro* labeled leukocytes were the mainstay of nuclear medicine imaging of infection. Positron emission tomography (PET) has several advantages over single photon imaging. PET provides high-resolution three-dimensional images of the whole body facilitating precise localization of radiopharmaceutical uptake. Semiquantitative analysis could facilitate the differentiation of infectious from noninfectious conditions and could be useful for monitoring response to treatment. In view of the advantages of PET over single photon imaging as well as the proliferation of clinical PET over the past 25 years, it is not surprising that investigators have turned their attention to developing PET radiopharmaceuticals for diagnosing infection. The first and most extensively studied of these agents is fluorine-18 fluorodeoxyglucose (18F-FDG). Developed primarily for oncology, 18F-FDG uptake in inflammation was soon recognized. While such uptake could confound study interpretation in patients with tumors, the possibility of 18F-FDG for imaging infection was exploited [2]. The potential of human leukocytes labeled *in vitro* with 18F-FDG, copper-64 (64Cu), and zirconium-89 (89Zr) for imaging infection has also been investigated. Other PET agents that have been studied include gallium-68 (68Ga) citrate, iodine-124 (124I)-filauridine, fluorine-18 fluorodeoxysorbitol (18F-FDS), and 68Ga labeled peptides.
