# Medical Tracer A **medical tracer** is a radioactive substance used in medical imaging to study the functions of tissues and organs. The tracer contains radioactive nuclei that emit gamma rays detectable outside the body. When introduced into the body, the tracer is absorbed by specific tissues and the emitted gamma rays are detected by imaging equipment, providing detailed information about biological processes. ## Definition A medical tracer is a substance containing **radioactive nuclei** used in medical imaging to study the functions of tissues and organs. ## Mechanism of Action Once introduced into the body, the tracer is **absorbed by specific tissues** due to its affinity for certain biological processes. The radioactive nuclei in the tracer **emit gamma rays**. These gamma rays are detected by specialized imaging equipment like **Positron Emission Tomography (PET)** or **Single Photon Emission Computed Tomography (SPECT)** scanners. The images provide detailed information about processes within the body, aiding in diagnosis and monitoring. ## Administration Methods Tracers can be introduced into the body via: * **Injection** into the bloodstream * **Ingestion** * **Inhalation** The method depends on the type of study and the area of interest. ## Applications * **Diagnosis and Monitoring**: Used to diagnose and monitor various conditions such as cancer, heart disease, and neurological disorders. * **Research**: Valuable in studying how diseases develop and progress, and exploring new treatments and diagnostic methods. ## Variety of Tracers Many different radioactive materials are used as medical tracers, chosen based on the specific study's needs and their properties such as half-life and type of emission. A short half-life is generally preferred to minimize the radiation dose to the patient. ## Common Medical Tracers | Element | Use | Half-life | | :--- | :--- | :--- | | Technetium-99m | Bone scan, cardiac stress, thyroid imaging | 6 hours | | Fluorodeoxyglucose (FDG) | Metabolic activity, cancerous tumors | 110 minutes | | Iodine-131 | Therapeutic applications, hyperthyroidism, thyroid cancer | 8 days | | Iodine-123 | Thyroid gland, brain, heart scan | 13 hours | | Gallium-67 | Infections, tumors, lymphoma, lung cancer | 78 hours | | Thallium-201 | Myocardial perfusion imaging, heart function, coronary artery disease | 73 hours | | Radium-223 | Bone metastases, prostate cancer | 11.4 days | | Carbon-11 | Neurodegenerative diseases, certain types of cancer | 20 minutes | | Oxygen-15 | Measure blood flow and metabolism in tissues | 2 minutes | ### Technetium-99m ($^{99m}\text{Tc}$) Technetium-99m is the most widely used radioisotope in nuclear medicine. The 'm' stands for **metastable**, meaning it stays in an excited state for a relatively long time before decaying by emitting a gamma photon.