special subjects
Radionuclide Scanning
Introduction
Radionuclide scanning uses the radiation released by radionuclides (called nuclear decay) to produce images. A radionuclide is an unstable isotope that becomes more stable by releasing energy as radiation. This radiation can include gamma-ray photons or particulate emission (such as positrons, used in positron emission tomography). Radiation produced by radionuclides may be used for imaging or for treatment of certain disorders (eg, thyroid disorders). A radionuclide, usually technetium-99m, is combined with different stable, metabolically active compounds to form a radiopharmaceutical that localizes to a particular anatomic or diseased structure (target tissue). The radiopharmaceutical is given by mouth or by injection. After the radionuclide has had time to reach the target tissue, images are taken with a gamma camera. Gamma rays emitted by the radionuclide interact with scintillation crystals in the camera, creating light photons that are converted into electrical signals by photomultiplier tubes. A computer summarizes and analyzes the signals and integrates them into 2-dimensional images. However, only signals near the camera’s face can be accurately analyzed; thus, imaging is limited by the thickness of the tissue and the range of the camera. Portable gamma cameras can provide radionuclide imaging at bedside. Generally, radionuclide scanning is considered safe; it uses a relatively low dose of radiation and provides valuable information (eg, it enables clinicians to image the entire skeleton when they suspect cancer has metastasized to bone). 