2.4:
Positron Emission Tomography
Positron emission tomography, or PET, is a radiotracer-based medical imaging technique commonly used for tumor detection, determining the extent of metastases, evaluating heart ailments like impaired blood supply, and studying neural activity in the brain.
To perform a PET scan, the patient is most often injected with a radioactive tracer, a biological molecule bound to a positron-emitting isotope such as carbon-11, oxygen-15, or fluorine-18.
Once in the body, these radiotracers accumulate inside the tissue or the cells where they have higher affinity. For instance, fluorodeoxyglucose or FDG accumulates more in tumors due to their higher metabolic activity.
The unstable radioactive fluorine decays, emitting positrons – the positively charged antiparticle of electrons. The positrons combine with nearby electrons resulting in an annihilation reaction and emit two photons of 511 keV energy in opposite directions.
The PET detector collects millions of these annihilation events and, using complex computing algorithms, reconstructs the images of tracer distribution in the area under study.
2.4:
Positron Emission Tomography
Positron emission tomography (PET) is a medical imaging technique involving radiopharmaceuticals — substances that emit short-lived radiation. Although the first PET scanner was introduced in 1961, it took 15 more years before radiopharmaceuticals were combined with the technique and revolutionized its potential.
One of the main requirements of a PET scan is a positron-emitting radioisotope, which is produced in a cyclotron and then attached to a substance used by the part of the body being investigated. This "tagged" compound, or radiotracer, is then put into the patient (injected via IV or breathed in as a gas), and how the tissue uses it reveals how that organ or other area of the body functions. For example, F-18 is produced by proton bombardment of 18O and incorporated into a glucose analog called fludeoxyglucose (FDG). How the body uses FDG provides critical diagnostic information. For example, since cancers use glucose differently than normal tissues, FDG can reveal cancers. The 18F emits positrons that interact with nearby electrons, producing a burst of gamma radiation. This energy is detected by the scanner and converted into a detailed, three-dimensional color image that shows how that part of the patient's body functions. Different levels of gamma radiation produce different amounts of brightness and colors in the image, which a radiologist can then interpret to reveal what is going on. For instance, a radioactive form of iodine can be used to monitor the thyroid and radioactive gallium can be used for cancer imaging.
The main advantage is that PET can illustrate the physiologic activity—including nutrient metabolism and blood flow—of the targeted organ or organs. In contrast, CT and MRI scans can only show static images. PET is widely used to diagnose a multitude of conditions, such as heart disease, the spread of cancer, certain forms of infection, brain abnormalities, bone disease, and thyroid disease. PET can also locate regions in the brain that become active when a person carries out specific activities, such as speaking, closing his or her eyes, etc. PET scans are now usually performed in conjunction with a computed tomography or magnetic resonance imaging scan for better data visualization and interpretation.
This content is derived from Openstax, Anatomy and Physiology, Section 1.7: Medical Imaging and Openstax, Chemistry 2e, Section 21.3: Radioactive Decay and Openstax, Physics, Section 22.5: Medical Applications of Radioactivity: Diagnostic Imaging and Radiation