What is PET? Positron-emission tomography (PET) is a medical-imaging technique that measures body function. Most medical images, such as X-rays, computed tomography scans, or magnetic resonance imaging, depict anatomy (physical structure), not function. Because PET images function, it is extremely valuable for assessing changes in organ performance that precede structural changes.
Radioactive tracers are the basis of PET. The PET camera detects positrons, or positive electrons. Positron release (positron emission) is a very rapid kind of radioactive decay. Positron-emitters include common elements (oxygen, carbon, nitrogen, fluorine, etc.) that can be easily incorporated into other chemical compounds. To trace a bodily process, a positron-emitting element is put into a chemical known to take part in that process. This positron-emitting chemical, either a normal body component, like oxygen, or a drug, is called a radiotracer. Glucose labeled with positron-emitting fluorine (18F) is a PET radiotracer commonly used to measure energy metabolism. Radio-labeling of drugs can reveal both the location of action ('receptor site') and the efficacy of interaction ('binding capacity'). The range of available PET tracers - and thus the number of bodily processes that can be imaged with PET - is currently quite broad, with still more in development.
A PET camera makes tens of thousands of measurements each second. Each measurement looks at a very small volume of tissue, only about 2 mm3. Taken together, these tens of thousands of highly precise measurements create a 3D measurement of an entire organ. Such 3D measurements can be viewed as images or analyzed in detail by sophisticated computer programs. The images along the margin of the reverse page are a series of PET slices.
How is PET used? The range of PET applications is limited only by the imagination of the user. PET has been used to study normal organ function and disease states. For example, PET research triggered a revolution in our understanding of energy metabolism during normal brain work. Current concepts of the neural systems of human cognition are being strongly influenced by PET. Even cell-to-cell communication systems are being explored by radio-labeling drugs that mimic the body's own hormones and neurotransmitters. In disease-related research, PET has many applications including diagnosis, outcome prediction, treatment assessment, and post-recovery. PET is not limited to any one organ and has already been applied to brain, heart, lung, liver, and bone. PET can be applied to disorders of many causes including congenital, developmental, infectious, neoplastic, traumatic, and degenerative diseases.
Well-accepted clinical uses for PET include the diagnosis of Alzheimer's disease, Huntington's Disease, Parkinsonism (and other movement disorders), the presurgical evaluation of focal epilepsy and of cardiac ischemia, and the post-treatment evaluation of brain tumors. New clinical applications being developed at several centers include the diagnosis of depressive disorders, head trauma, liver and bone cancer.
PET studies at the Research Imaging Center (RIC). Mapping human brain function is a major focus of PET studies at the RIC. PET explores the brain's organization for perception, action, language, attention, and emotion. We are combining PET studies with MRI anatomical images and electrical brain mapping to provide a comprehensive picture of the brain's organization. Brain mapping also has clinical applications; for example, in planning neurosurgery. Researchers at the RIC are also involved in investigating numerous diseases and disorders including depression, epilepsy, head trauma, stuttering, deafness, and cardiac ischemia.