Nuclear imaging of inflammation: homing-associated molecules as targets

Noninvasive imaging of inflammation could be a highly valuable tool as it could help diagnosing many inflammatory conditions, such as osteomyelitis, rheumatoid arthritis, sarcoidosis, inflammatory bowel disease, and fever of unknown origin [1,2]. Even though there are several imaging agents used for the imaging of inflammation by nuclear medicine, none of them are specific for inflammation [2]. Specific imaging of inflammatory processes is a demanding task. Unspecific accumulation, as a result of an increased blood flow and enhanced vascular permeability, increases the signal in the target tissue which makes specific detection difficult. Positron emission tomography (PET) imaging of inflammation offers a noninvasive, quantitative approach to diagnosis, therapy planning, and monitoring and to drug development. Nevertheless, clinical applications of this method are currently limited by the lack of specific imaging agents to visualize molecular events in vivo. 18F]FDG is the most widely used PET imaging agent in cancer and inflammation imaging, but it localizes general inflammation or cancer and does not define these specifically.Nuclear imaging modalities, such as single photon emission computed tomography (SPECT) and PET, offer functional and molecular information with high sensitivity. Of these two imaging modalities, PET is more sensitive and allows for quantitative imaging in a clinical setting [3]. The combination of molecular/functional and anatomic imaging is preferred in order to identify the exact location of the functional SPECT/PET signal.Most of the imaging agents for inflammation imaging are SPECT imaging agents. Currently, the golden standard in nuclear medicine imaging of inflammation is autologous radiolabeled leukocytes [1,2]. They accumulate in the inflamed tissue by specific leukocyte migration. First, the injected radiolabeled white blood cells (WBCs) adhere to the endothelium and further migrate into the tissue. The leukocytes are labeled either w