PET Radiopharmaceuticals

PET (Positron Emission Tomography) is a powerful imaging technique through which quantitative information on the distribution of positron emitter labeled radiopharmaceuticals (PET radiopharmaceuticals) in the body can be realized. Positrons (β +) are positively charged beta particles. They are emitted when the atom is proton enriched. A positron has only a transient existence. After losing all of its kinetic energy, it interacts with an electron and is annihilated. Both the mass of positron and electron are converted to energy during annihilation and two 511 KeV photons are emitted at a 180° angle to each other. The PET is based on the coincidence detection of the two aforementioned photons. Coincidence detection is a powerful method enhancing sensitivity and dynamic-imaging capabilities of PET. PET camera systems contain a ring of detectors that encircles the patient. The data collected over many angles around the body axis of the patient is used to reconstruct the image of the activity distribution in slice or tomographic form. PET studies, like all nuclear medicine radioisotope emission procedures, yield images that represent the distribution or the pattern of uptaking radiopharmaceutical depending on the physiologic, pharmacologic, and biochemical state of the individual's body. Thus, nuclear medicine procedures in general and PET procedures in particular are capable of providing information concerning how the body is functioning at a physiologic or biochemical level, whereas x-ray procedures such as computed tomography (CT) primarily depict human anatomy. At present, most of the new PET cameras are coupled to anatomical scanners. A PET/CT system allows the combination of functional and anatomic information and offers accurate matching of anatomic (CT) and functional (PET) images. Patients are imaged by both PET and CT in the same position of the patient, improving overall diagnostic utility of tomographic imaging methods. There are four positron emitting radioisotopes that are considered the biologic tracers, carbon-11, nitrogen-13, oxygen-15, and fluorine-18. 11 C (t 1/2 20.4 min), 15 O (t 1/2 2.1 min), and 13 N (t 1/2 10 min) are referred to as the essentials of life. They can be easily substituted directly into biomolecules without changing the properties of the molecule. 18 F (t 1/2 110 min) is not a normal constituent of biological molecules but can often be substituted for a hydroxyl group as in the case of deoxyglucose or can be substituted for a hydrogen atom in a molecule or placed in a position where its presence does …