## 22.8.2 PET Scanner A **PET scanner** is a powerful medical imaging device that detects signals emitted from within the body after a patient receives a radioactive tracer. Unlike X-ray or MRI, a PET scan shows not only the structure of organs but also how well they are functioning. ### Positron Emission Tomography (PET) An imaging technique used for detailed observation of metabolic and physiological processes within the body. This process relies heavily on the principles of particle physics and . ### Diagnostic Steps in PET Scanning 1. **Radionuclide Injection**: The patient is injected with a positron-emitting radionuclide, such as carbon-11 or Fluorine-18, which acts as a tracer. 2. **Tracer Accumulation**: The tracer travels through the bloodstream and accumulates in target tissues or organs. 3. **Radioactive Decay**: The radionuclide undergoes radioactive decay and emits a positron. 4. **Positron Definition**: A positron is the antiparticle of an electron, having the same mass as an electron but a positive charge. 5. **Annihilation**: The emitted positron travels a short distance, encounters an electron in surrounding tissues, and they annihilate each other. This is a practical application of concepts regarding matter and energy. 6. **Gamma Ray Production**: Electron-positron annihilation produces two gamma ray photons, each with an energy of 511 keV. 7. **Momentum Conservation**: These two gamma ray photons are emitted in opposite directions ($180^{\circ}$) to conserve momentum. 8. **Detection by PET Scanner**: * The PET scanner consists of a ring of detectors encircling the patient. * These detectors are sensitive to gamma rays. * When gamma rays strike the detectors, they are converted into electrical signals. 9. **Signal Processing**: * The scanner detects the simultaneous arrival of pairs of gamma rays traveling at $180^{\circ}$. * It records their time of arrival and spatial information. * Detected signals are processed by a computer to determine the exact location of the annihilation event within the body. 10. **Image Generation**: The computer creates an image from the detected photon pairs, providing insights into the body's physiological processes. ### Visual Aids