FDG, or fluorodeoxyglucose, is a radiopharmaceutical used in medical imaging, particularly in positron emission tomography (PET) scans. It is a glucose analog that helps visualize metabolic activity in tissues, making it useful for detecting cancer, monitoring treatment response, and assessing brain disorders. In a PET scan, FDG is injected into the body, where it accumulates in areas with high glucose metabolism, such as tumors. The scan then captures images of these areas, allowing healthcare professionals to identify abnormalities and make informed decisions about patient care.