WHAT IS POSITRON EMISSION TOMOGRAPHY (PET) TEST?
Positron emission tomography (PET) is a noninvasive radiographic method for studying blood flow and metabolic changes occurring in specific organs or regions of the body tissues. It involves the injection or inhalation of gamma–ray–emitting, biologically compatible radioisotopes and the creation of images of radioisotope distribution in the body. As the radioisotopes disintegrate, they emit positrons, which are positively charged particles similar to electrons. As the positrons are captured by electrons, both are destroyed, resulting in the emission of two photons, which travel outward in opposite directions. The photons are detected simultaneously by the PET camera, an event known as a “coincidence.” The summation of these coincidences allows for the creation of a continuous map of the metabolic activity of the body. A computer then creates pictures of cross-sections of the body area studied, which show brighter areas according to the amount of radioisotope present.
Some examples of radioisotopes include oxygen-15, nitrogen-13, carbon-11, and fluorine-18, which are labelled onto substances such as water, carbon dioxide, or glucose. Because the radioisotopes are biologically compatible, they take the place of the body’s chemical elements (such as oxygen, nitrogen, or fluorine), and the resulting scan gives a true representation of the physiologic function of the body processes. The choice of radioisotope and material to be labelled is based on the body function to be studied. For example, blood flow is studied using 15O-labeled HÕ−, glucose metabolism is studied using 18F-labeled glucose, tissue perfusion is studied using 13N-labeled NH2, and anaerobic metabolism is studied using 11C-labeled acetate. Some conditions in which the use of PET has been studied include Alzheimer’s disease, asthma, brain tumours, cerebral atrophy, cerebrovascular disorders, chronic obstructive pulmonary disease, coronary artery disease, epilepsy, head trauma, Huntington’s disease, myocardial infarction, obsessive–compulsive disorder, pulmonary oedema, schizophrenia, and unstable angina.
The newest equipment, called “Dual Mode Imaging,” combines PET with structural imaging modalities such as Ultrafast CT or MRI for improved anatomical and malignant focus (bone, CNS, germ cell, lymphoma, neuroblastoma and soft-tissue tumours) imaging results. See Dual–Modality Imaging—Diagnostic.